BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

10. EVOLUTION

Our Earth is inhabited by a vast number of organisms exhibiting an immense variety of forms and lifestyles. In the plant and fungal kingdoms alone, approximately 360,000 species are currently known, and a considerably greater number likely remain undescribed. All this diversity has emerged through evolution following—most likely a single—origin of life more than 3.5 billion years ago, meaning that species changed over time and new species arose from existing ones. "Nothing in biology makes sense except in the light of evolution"—this frequently quoted statement by Theodosius Dobzhansky aptly highlights the pivotal Significance of the evolutionary process for biol

ogy. This chapter presents the most crucial aspects of the evolutionary process.

To put it very simply, the evolutionary process works as follows. By typically producing numerous offspring, each potential species is capable of increasing its population size in a geometric progression. For example, a single coast redwood tree (Sequoia sempervirens) produces 10⁹ — 10¹⁰ seeds over its long lifespan of several hundred years. Even an annual plant with a lifespan of just a few weeks, such as annual bluegrass (Poa annua), produces about 100 seeds by the end of its life. However, this potential geometric population growth is limited by biotic and abiotic environmental factors, so the number of individuals of a given species remains more or less constant over a relatively short geological timeframe. It follows that, statistically, across successive generations, a single parent individual is replaced by a single offspring. Which individuals survive and manage to reproduce usually depends not on blind chance, but on the specific traits of competing individuals. Offspring are never entirely identical because genetic and, consequently, heritable variation arises through Mutations as well as recombinations associated with parasexual and sexual processes. Depending on their genetic makeup, different individuals are more or less successful in a given environment. Greater "success" (fitness) translates into a relatively high probability of survival and reproduction. Natural Selection is precisely the result of this differential survivorship and reproductive success among various individuals. The combined action of mutations, recombination, and natural selection can alter the genetic composition of subsequent generations—in other words, drive evolution. Alongside these processes, random fluctuations in Gene frequencies also play an important role in evolution.

Nowadays, no scientist disputes the existence of evolution; it is substantiated by a multitude of observations. These include direct observations of species changing in nature over time (perhaps the most famous example being the shift in relative frequencies of the dark and light morphs of the peppered moth during the industrial revolution in Great Britain, which caused birch trunks to blacken with soot); the discovery of widespread intraspecific variation and nearly continuous transitions between many species; the observation of homologous similarities in morphological and molecular traits across very different organisms, as well as the hierarchical Structure of these similarities (i.e., decreasing similarity in the sequence: species of the same genus -> genera of the same family -> families of the same order, etc.); the gradual emergence of extant organisms as documented by the fossil record; and the numerous instances where humans have experimentally altered species through selective breeding.

Even before Charles Darwin published his epoch-making work *On THE ORIGIN OF Species by Means of Natural Selection...* in 1859, it had frequently been suggested that species are not constant but mutable, and that all past and present species descended from a single common ancestor. Darwin's primary merit lies in his successful proposal of a mechanism for evolutionary change that remains broadly accepted today. Although The phenomenon of evolution as a process of species modification was quickly embraced, natural selection was fiercely debated for a long time, right up to the works of August Weismann in the late 19th century and even beyond. Many accepted the important role in species transformation played by the "inner drive toward perfection" and the inheritance of acquired characteristics, as described by Jean-Baptiste Lamarck as early as 1809. Even Charles Darwin acknowledged The Significance of these mechanisms. The greatest shortcoming of Darwin's evolutionary theory was the lack of a convincing theory of heredity. Gregor Mendel's foundational insights into the mechanisms of inheritance, published in his *Experiments on Plant Hybridization* (1866), remained unknown to Darwin. It was only around the turn of the century, following the so-called rediscovery of Mendel's Laws of inheritance by Hugo de Vries, Carl Correns, and Erich von Tschermak (though regarding de Vries and Tschermak, there is serious doubt as to whether they independently rediscovered Mendel's laws without prior knowledge of his work), that the synthesis of evolutionary theory and genetics began. Initially, however, early geneticists—working primarily with sharply contrasting traits—rejected Darwin's model of gradual transformations, assuming instead that evolution proceeded in a saltatory manner via so-called macromutations. In 1918, Ronald Fisher first demonstrated that continuous variation could also result from Mendelian inheritance; subsequently, Fisher (1930), J.B.S. Haldane (1932), and Sewall Wright (1931) showed that natural selection could lead to genetic changes in accordance with the principles of Mendelian genetics. The "modern synthesis" proposed by these authors was further advanced chiefly through the works of Theodosius Dobzhansky (1937), Julian Huxley (1942), Ernst Mayr (1942), George Gaylord Simpson (1944), and in botany, G. Ledyard Stebbins' *Variation and Evolution in Plants* (1950), which made The concepts of synthetic evolution widely accessible to the scientific community. Since then, modern evolutionary theory has continually received fresh confirmation, refinement, and expansion.

10.1. Variation

Almost all plant traits—such as leaf shape and size (Fig. 10.1)—are, in most cases, far from identical even when compared within a single plant or between just two individuals; instead, they exhibit considerable variation, thereby demonstrating *Variability.* The mass of bean seeds from a single field (Fig. 10.2) varies *continuously,* meaning that virtually any mass value can be found within certain intervals. By contrast, the number of seeds in a legume pod varies *discontinuously* (or meristically). Here, we are always dealing with whole numbers rather than intermediate values.

Class="center">Fig. 10.1. Intra- and interpopulation variation in basal leaves among 6 populations of the Californian aster Layia gaillardioides. Left: populations from moist outer coastal mountains; right: populations from dry inner coastal mountains. All plants were cultivated under identical conditions; each leaf belongs to a separate individual

Fig. 10.2. Continuous variation curve for the mass of 712 bean seeds from several genetically identical individuals: A — mass in 0.1 g; B — number of seeds in each 0.05 g weight class; 1 — actual variations; 2 — theoretical normal distribution curve. Mean values occur significantly more frequently than extreme values

A special form of discontinuous variation is manifested as subtle, predominantly qualitative trait expressions within a single species (e.g., seeds with smooth or warted surfaces in corn spurry [Spergula arvensis], or individuals with white or red flowers in hollow larkspur [Corydalis cava]). When such variation satisfies specific genetic criteria, it is referred to as *polymorphism.*

Comparing traits and observing variation in this manner is possible at very different Levels of Organization. One can compare traits of a single individual *(intraorganismal variation) or of different individuals (interorganismal variation)* belonging to the same population—a group of indi-

viduals growing in the same Location and freely interbreeding with one another *(intrapopulation variation)*—or to different populations of the same species *(interpopulation variation)*, as well as to different species (interspecific variation), and so forth.

There are presumably three main causes of variation. The first factor is *ontogenetic variation.* In this case, homologous structures develop differently during the course of an individual's growth. An example is The change in leaf shape along a SHOOT (see Figs. 4.66, 4.68). The second factor lies in the interaction between an individual's genetic constitution (genotype) and its environment. As a result, depending on environmental conditions, a single genotype can manifest as various phenotypes. If a single specimen of yarrow (Achillea millefolium) is divided in half and cultivated, for instance, in the lowlands and in the mountains, these genetically identical plants will quite rapidly begin to look different (Fig. 10.3). This form of variation is termed *modification,* and the plants demonstrate phenotypic plasticity (see 12.2). Finally, observed variations can also be determined by the fact that different individuals are generally genetically distinct *(genetic variation).* While ontogenetic variations occur strictly within the Organism, and genetic differences only become apparent when comparing different individuals, some modifications represent intraorganismal variation, whereas others represent interorganismal variation.

Fig. 10.3. Experimentally induced modifications in Californian yarrow. Vegetatively propagated parts (clones) of 7 individuals from a single mountain population in the Sierra Nevada (Mather), grown in three experimental gardens: Stanford (30 m a.s.l.), Mather (1,400 m a.s.l.), and Timberline (3,050 m a.s.l.). Heritable differences among individuals, including the distinct reaction norm of each individual, at varying altitudes above sea level

10.1.1. Ontogenetic Variation and Phenotypic Plasticity

Ontogenetic variation is the result of differential development of homologous structures within a single organism in response to internal and/or external conditions. Striking Examples of ontogenetic variation are demonstrated by changes in mold fungus Morphology or The formation of diverse leaf shapes during the individual development of seed plants (see Fig. 4.68).

English ivy (Hedera helix) produces characteristic lobed leaves during its vegetative phase, but entire leaves from the onset of its reproductive phase. If a shoot section with entire leaves is used for vegetative propagation, no reversion to lobed leaves occurs.

Changes in form during the ALTERNATION OF GENERATIONS (gametophyte and sporophyte) can also be regarded as ontogenetic variation, although this involves not a single individual, but a succession of many genetically very similar individuals replacing one another. Morphological shifts during the alternation of generations have led, for example, to the gametophytes and sporophytes of certain brown Algae being initially described as separate genera.

Phenotypic plasticity means that a *genotype* (i.e., an individual of a specific genetic constitution¹) can produce different *phenotypes* depending on environmental conditions. Because modifications arise during the organism's development, it is evidently not always easy to separate phenotypic plasticity from ontogenetic variation. The latter occurs exclusively when ontogenetic variations result from the execution of a purely endogenously regulated developmental program. More frequently, however, ontogenetic variation also depends on the perception of environmental signals. The complex combined action of the genotype and environmental influences on the formation of a specific phenotype demonstrates that the genotype ultimately determines not rigid traits, but rather a range of potential phenotypic outcomes within certain boundaries—namely, the *reaction norm.*

¹Genotype refers to the hereditary information contained in DNA (rarely in RNA). It must by no means be equated with the individual itself. — *Editor's note.*

The crucial distinction between genotype and phenotype was established by the Danish botanist W.L. Johannsen. He worked with seeds obtained from inbred homozygous individuals of the common bean (Phaseolus vulgaris). From batches of seeds of varying weight produced by a single individual, he selected the lightest and heaviest ones for the next generation. Since the average seed weight of the resulting second-generation plants was identical, Johannsen concluded that variations in seed weight within a single individual must result from environmental influences. He attributed the observed differences in average seed weight among individuals derived from different parents to genetic differences between those individuals. Thus, the variability in seed weight within a single individual is purely modificational, whereas the variability in seed weight between genetically distinct individuals is genetic.

Phenotypic plasticity is easy to demonstrate experimentally if, for example, genetically identical individuals obtained through vegetative propagation are grown under different conditions (see Fig. 10.3). When vegetative propagation is impossible (as in many annual species), researchers have to work with genetically close individuals, such as sibling specimens. In such cases, allowance is naturally made for the possibility that observed differences may involve a (minor) genetic component.

Numerous experiments have led to the following General Conclusions regarding phenotypic plasticity.

✵ Modifications are non-heritable. If, for instance, the progeny of a very small plant grown under poor conditions are cultivated in a favorable environment, the descendants will not remain small; rather, they will grow large within the limits of their norm of reaction.

✵ Different traits of the same plant exhibit varying degrees of phenotypic plasticity and do not correlate with one another. In general, vegetative structures (such as plant height, leaf shape, and size) are more plastic than reproductive structures (such as flower size and seed mass). However, significant plasticity has also been documented in reproductive structures. Examples include the formation of cleistogamous flowers in sweet violet (Viola odorata) at the end of the growing period, or The production of brown or black seeds depending on day length in seablite species (Suaeda).

Phenotypic plasticity is not restricted to morphological and anatomical traits; it can also manifest in PHYSIOLOGICAL AND BIOCHEMICAL characteristics.

✵ The modifying Influence of the environment can cause differences between identical Organs of the same individual. In European beech (Fagus sylvatica), low light intensity leads to the formation of shade leaves (see Fig. 7.73). If lighting conditions change, sun leaves will subsequently develop on the same shoot beyond the shade leaves.

✵ Different modifications are driven by specific environmental influences. In cotton, for example, the number of internodes correlates with nitrogen availability and is independent of Water supply, whereas internode length depends on water availability.

✵ Individuals of the same species may differ in the degree of phenotypic plasticity for a given analyzed trait. Experimental cultivation of eight clones from 192 genotypes of soft brome (Bromus hordeaceus) under varying soil composition, fertilizer, and photoperiod conditions demonstrated that the degree of plasticity for traits such as developmental duration and inflorescence size follows a normal distribution. That is, relatively few of the 192 genotypes exhibit low or high plasticity for the observed traits, while the majority show moderate plasticity. Similarly, differences in the plasticity of homologous traits can be observed between different populations of the same species (and even more markedly between different species).

✵ The degree of trait plasticity is heritable and subject to selection.

These latter two findings confirm that the capacity for phenotypic plasticity has a genetic basis.

Phenotypic plasticity contributes to both intra-organismal and inter-organismal variability. For instance, the same water crowfoot plant (Ranunculus aquatilis) produces leaves of different shapes depending on day length and THE POSITION OF the leaf primordium above or below the water surface: deeply dissected leaves form underwater, whereas leaves floating On the surface or emerging above it are typically only lobed (see Fig. 4.68). When comparing leaves from two water crowfoot individuals occupying identical positions on the shoot, one plant may bear finely dissected leaves (having begun development underwater under the appropriate day length), while the other bears only lobed leaves.

10.1.2. Genetic Variation

Although ontogenetic variation and variability resulting from phenotypic plasticity are important for adapting the genotype¹ to the environment and therefore hold certain evolutionary significance, genetic variation is of the utmost importance for evolutionary transformations. Genetic variations in the phenotypes of individuals—within a population or species, for example—can be detected by growing them under identical conditions and comparing ontogenetically homologous structures or other properties. Only in this way is it possible to determine THE CONTRIBUTION OF genetic differences to observed variations. The primary sources of genetic variation are mutations and recombination.

¹ Organisms adapt to the environment—that is, genotypes realized within phenotypes rather than genotypes directly. — Ed. note.

Mutation, as a spontaneous (or experimentally induced) alteration of genetic material, can occur at various levels and in all genomes of a plant Cell. For instance, changes may affect the DNA nucleotide sequence of a single gene (gene mutation), Chromosome structure (chromosomal mutation), or the entire genome (genomic mutation). It is essential to recognize that all Mutations are random—meaning there is no way to predict their type or location—and that they are non-directional, bearing no relation to the selection pressures experienced by the individual.

One example of the non-directional nature of mutations is the evolution of herbicide resistance in plants. Many herbicides have been applied in various Regions of the world (such as Europe and North America) starting around the same time and at roughly similar concentrations. Despite these comparable selection pressures (at least regarding this factor), one can observe that the same species has developed herbicide-resistant genotypes in some regions but not in others. For example, the ubiquitous shepherd's purse (Capsella bursa-pastoris) acquired herbicide resistance in Poland in 1984, but nowhere else. In North America, barnyard grass (Echinochloa crus-galli) became herbicide-resistant through a plastid genome mutation, whereas in Europe, this occurred apparently via a nuclear genome mutation. This demonstrates that similar selection pressure does not necessarily lead to similar mutations.

10.1.2.1. Gene Mutations

Gene mutations can be point mutations, reading frame shifts, or result from The activity of Mobile Genetic Elements known as Transposons (Fig. 10.4). In a point mutation, a single nucleotide is replaced by another. If a purine nucleotide is replaced by another purine, or a pyrimidine by a pyrimidine, the change is termed a transition; if a purine is replaced by a pyrimidine or vice versa, the exchange is called a transversion. A reading frame shift occurs when one or more NUCLEOTIDES are inserted into the existing sequence (insertion) or nucleotides are lost (deletion). The consequence of an insertion or deletion is a shift in the triplets of the subsequent DNA region, resulting in a completely different readout. Such mutations are typically caused by random errors during METABOLISM/36.html">DNA Replication in the absence of DNA Repair.

Fig. 10.4. Point mutation (A) and reading frame shift (B). A — substitution of the highlighted nucleotide in the strand (G instead of A), leading to the incorporation of a different amino acid (Methionine [Met] instead of isoleucine [Ile]); B — loss (deletion) of the highlighted nucleotide, altering the reading frame of subsequent triplets and, consequently, the Amino Acid Sequence. The Emergence of a stop codon within the new reading frame causes premature termination of Protein Synthesis

Transitions can occur when rare tautomeric imino forms of adenine or cytosine incorporate into the sequence instead of their usual amino forms, or when even rarer enol forms of guanine and thymine replace their keto forms (Fig. 10.5). For instance, the imino form of adenine can pair with cytosine instead of thymine, while the enol form of guanine pairs with thymine instead of cytosine. Transversions can happen when gaps in The nucleotide sequence arise due to the loss of nucleotides (depurination, depyrimidination). When a gap is created by the loss of guanine, adenine is usually inserted in its place. If a gap in a guanine-depleted strand is filled with thymine—which is complementary to adenine—the result is a transversion from GC to AT (see Fig. 10.4, Fig. 10.5). Reading-error mutations accumulate in regions of the nucleotide chain where multiple identical nucleotides follow one another. DNA polymerase "errors" during DNA Synthesis can lead to the omission of a single nucleotide in the coding DNA strand. As a result, the corresponding nucleotide drops out during the next replication cycle, meaning a deletion occurs. Alternatively, an extra nucleotide may be incorporated into the newly synthesized DNA strand, resulting in an insertion.

Fig. 10.5. Amino and imino forms of adenine (A) and keto and enol forms of guanine (B). The rare imino form of adenine pairs with cytosine instead of thymine, whereas the rare enol form of guanine binds to thymine instead of cytosine

Transposons are genetic elements that replicate autonomously and are capable of changing their position within The Genome. This ability is achieved because transposons contain the Genetic information for an enzyme (transposase) that can recognize both a target sequence in the genome and the ends of the transposon in order to excise it. Furthermore, transposons can utilize cellular Enzymes required for replication. When a transposon inserts into a gene, it can disrupt its function and thus induce a mutation.

Gene mutations give rise to alleles, which can be defined as divergent forms of a gene originating from one another. If a diploid individual (possessing a pair of homologous Chromosomes) carries two identical alleles, it is homozygous for the gene in question. If the two alleles differ, it is heterozygous. While a diploid individual can have at most two alleles of a single gene, a population may harbor multiple alleles (multiple allelism). Alleles of the same gene can be completely dominant (determining the phenotype) or recessive (remaining phenotypically silent¹), and incomplete dominance is also possible (where both alleles contribute to the phenotype in varying proportions), with intermediate phenotypic traits representing a special case of incomplete dominance. Codominance (the full expression of both alleles in the phenotype) is most commonly observed at the protein level.

¹ A recessive allele is phenotypically unexpressed only in the heterozygous state. — Ed. note.

In older literature, genes and their alleles were frequently designated by letters, with uppercase letters denoting dominant alleles and lowercase letters representing recessive ones. For instance, the allele for dominant red flower color in snapdragons (Antirrhinum majus) was designated as R, and the recessive white allele as r. Modern literature more commonly employs a three-letter abbreviation for the gene and its alleles. Regardless of dominance or recessive status, wild-type alleles are denoted by uppercase letters, whereas mutant alleles are designated by lowercase. In either case, italics are consistently used for genes and alleles.

In eukaryotes, the average gene mutation rate is estimated at 1 mutation per gene per 10⁵ to 10⁶ Gametes. However, precise investigation of a specific trait can reveal variations in its mutation rate. In maize (Zea mays), the enzymes involved in anthocyanin Biosynthesis mutate at a frequency of 4.92 × 10^-4 gametes, whereas the frequency for producing wrinkled rather than smooth kernels is 1.2 × 10^-6. Furthermore, phenotypically observable mutations are not necessarily genetically homologous; consequently, they do not always involve the exact same region of DNA being altered in the same way. These mutation rates—values that depend heavily on the observational Methods employed—refer to so-called spontaneous mutations, for which no external causes can be identified. Higher mutation rates can be induced experimentally using, for example, ionizing radiation, ultraviolet light, and various mutagenic chemical agents.

The frequency of phenotypically expressed mutations can be determined by crossing an individual homozygous for a dominant gene with one homozygous for the corresponding recessive gene (AA × aa). In the absence of mutations, all hybrid progeny would be expected to have the genetic constitution Aa and match the homozygous dominant parent in phenotype. Should individuals exhibiting the phenotype of the homozygous recessive parent nevertheless appear, their occurrence must be attributed to the fusion of a mutant gamete (A → a) from the homozygous dominant parent with a gamete from the homozygous recessive parent. The frequency of such recessive phenotypes allows the mutation rate to be calculated.

Based on an average mutation rate of 1 × 10^-5 and considering that higher plants—following the best estimates for thale cress (Arabidopsis thaliana)—possess approximately 25,500 genes (see Box 7.1, Fig. 7.4), it can be inferred that about 20% of gametes carry mutations. No matter how widely this value may fluctuate across different genes, it clearly demonstrates that mutational genetic variability is a widespread phenomenon. The probability of mutation is not uniform across every region of DNA; certain zones mutate more frequently than others. This is likely because the mutation probability of a nucleotide is influenced by its local sequence context.

Since point mutations and reading frame shifts largely depend on The fidelity of DNA Replication and the efficiency of repair mechanisms, the mutation rate itself is under Genetic control. Mutations in DNA REPLICATION ENZYMES and repair enzymes can impact The rate of mutagenesis, and the genes encoding these enzymes are accordingly designated as Mutator Genes.

Mutation rates in plastid and Mitochondrial Genomes are lower than those in the nuclear genome. Nucleotide substitutions, measured as substitutions per site per year, occur at an average frequency of (5–30) × 10^-9 in the nuclear genome, (1–3) × 10^-9 in the plastid genome, and (0.2–1) × 10^-9 in the Mitochondrial Genome.

The functional impact of mutations on the gene product varies considerably. Silent mutations, owing to the degeneracy of The Genetic Code, do not result in an amino acid substitution and thus produce no phenotypic effect. The same applies to neutral mutations, where an amino acid change occurs but does not alter the function of the affected protein. By contrast, if an amino acid substitution affects the function of the gene product, it is termed a missense mutation. A drastic effect on the gene product occurs, for example, when an amino acid-coding triplet mutates into a stop codon (nonsense mutation) or when a reading frame shift arises, resulting in a completely different gene product. In both cases, a non-functional gene product is formed.

Whether mutations manifest in the plant's phenotype via their effect on the gene product (Fig. 10.6) depends on whether the mutant gene exists in a haploid or diploid state. In the diploid sporophytic generation, many mutations remain phenotypically silent because they are recessive. This is explained by the fact that in a diploid organism, every gene is represented twice. Following the mutation of one gene in such a pair, the unmutated allele can continue to produce the wild-type gene product.

Fig. 10.6. Gene mutants of the snapdragon (Antirrhinum majus). Overall plant development: A — normal; B — dwarf; C — early-flowering. Flower morphology: D — normal zygomorphic; E — radial; F — spurred

The Effect of a mutation also depends on the hierarchical role of the affected gene in metabolic or developmental pathways. If a gene exerts a high-level regulatory function, the phenotypic consequence of its mutation can be dramatic.

A well-studied example of such an effect, investigated in recent years primarily in thale cress (Arabidopsis thaliana) and snapdragon (Antirrhinum majus), involves genes that determine floral organ identity. Mutations in genes encoding Transcription factors—which thereby interfere with The regulation of downstream genes—can lead to abnormal floral organ sequences instead of the normal arrangement (sepals, petals, stamens, carpels), such as carpels, stamens, stamens, and carpels, or sepals, sepals, carpels, and carpels (see 7.4.3).

Just as the hierarchical ORGANIZATION OF THE genome—and the associated interconnected networks of gene action and biosynthesis—leads to the Distribution of a gene product across various plant structures, a single gene can influence multiple phenotypic traits (pleiotropy). Examples include the gene governing flower color in hoary stock (Matthiola incana), which also affects the plant's pubescence (homozygosity for the recessive allele, which blocks pigment biosynthesis, also results in the absence of trichomes), as well as the anthocyanin gene in peas, which influences the color of flowers, pods, seeds, and stipules. Conversely, phenotypic traits are frequently governed by multiple genes (polygeny). Finally, various types of interactions occur between non-homologous genes, which are collectively grouped under THE CONCEPT OF epistasis.

A vast majority of mutations have negative effects, meaning they reduce the fitness of the mutants. This is entirely logical, as a gene's structure is the product of a long history of adaptive evolution, making the likelihood of improving it through random, undirected mutations very low.

A well-documented plastid genome mutation with a major phenotypic effect determines herbicide resistance mechanisms. Triazine herbicides affect plants by binding to the Photosystem II protein (Q_B), thereby disrupting photosynthetic electron transport. Herbicide resistance has emerged, for instance, in lambsquarters (Chenopodium album) and annual bluegrass (Poa annua) as a result of a point mutation in the plastid gene *psbA*, involving an amino acid substitution (Glycine instead of Serine) at position 264 of the Q_B protein, which drastically reduces herbicide binding.

Mitochondrial genome mutations can be responsible for cytoplasmic male sterility, a phenomenon frequently encountered in plants. In these cases, however, the cause is typically not localized gene changes, but large-scale structural rearrangements of the entire mitochondrial genome.

Transposon insertions are responsible for the differences between smooth and wrinkled seeds in the garden pea (Pisum sativum) that were originally observed and genetically analyzed by Gregor Mendel. In this case, gene function was disrupted by the integration of a transposon into a gene encoding a starch-branching enzyme, thereby affecting the water content of the seeds. The higher water content in the seeds of mutant individuals led to more pronounced desiccation and wrinkling of the ripening seed surface. Similarly, transposon mutations give rise to light-colored flowers with red sectors in snapdragons. The red sectors appear in areas of the petals where biosynthesis is restored due to the excision of the Tam3 transposon that had previously disrupted it. This latter example clearly demonstrates that transposon mutations can drive genetic mosaicism within the Tissues of a single individual, showing that even genetic uniformity within an organism is relative.

10.1.2.2. Chromosomal Mutations

Chromosomal mutations are caused by chromosome breaks that occur spontaneously or through transposon activity. As with gene mutations, the frequency of chromosomal mutations can be increased experimentally. Depending on the number of breaks and the subsequent behavior of the resulting chromosomal fragments, several types of chromosomal mutations are distinguished (Fig. 10.7).

Fig. 10.7. Chromosomal mutations: A — deletion: loss of a terminal chromosomal segment (from h to k); B — deficiency: loss of an interstitial chromosomal segment (from e to g). In heterozygous individuals, this deficiency manifests as a loop-like protrusion of the corresponding non-mutant chromosome region; C — duplication: doubling of an interstitial chromosomal segment (from g to i); in heterozygous organisms, the duplication is recognizable by a loop-like protrusion of the mutant chromosome segment; D — inversion: reversal of orientation of an interstitial chromosomal segment (from c to f); in heterozygotes, this leads to the formation of an inversion loop; E — translocation: reciprocal transfer of a terminal chromosomal segment (f and x–z, respectively) to non-homologous chromosomes. In individuals heterozygous for a reciprocal translocation, a cross-shaped configuration of pairing chromosomes is formed during Meiosis

A deletion refers to the loss of a terminal segment of a chromosome. The loss of the telomere leads to the fusion of the altered ends of sister chromatids produced after replication of the mutant chromosome. This ultimately generates a dicentric chromosome, which breaks during the subsequent Cell Division. The continuation of this breakage-fusion-bridge cycle means that deletions rarely result in stable structural changes to chromosomes.

• If two breaks occur within a chromosome and the intervening segment is lost, it is termed a deficiency. In an individual heterozygous for this mutation (possessing one mutant and one unmodified chromosome), a deficiency of a specific size can be identified in meiotic bivalents by the formation of a characteristic loop in one of the chromosomes. This loop contains the region of the normal chromosome that was lost in the mutant homolog and therefore lacks a homologous partner for pairing.

✵ If a chromosomal fragment resulting from two breaks is not lost, as in a deficiency, but instead is inserted into another chromosome at the site of its single break, a duplication is produced. In this process, the fragment may integrate into either a homologous or a non-homologous chromosome. If the duplicated regions in a homologous chromosome lie directly adjacent to one another, they may have either the same (tandem duplication) or opposite orientation (inverted duplication). Duplications can also be identified in heterozygous individuals (upon insertion into a homologous chromosome) during meiotic chromosome pairing through the formation of a loop-like protrusion.

• An inversion is a chromosomal mutation in which a chromosomal fragment resulting from two breaks is reinserted into the same site, but with reverse orientation. In the case of a pericentric inversion, the centromere is part of the inverted region; in a paracentric inversion, it is not. Inversion in a heterozygous individual manifests during meiosis as a characteristic inversion loop.

✵ In translocations, a chromosomal fragment is transferred to another, non-homologous chromosome. If the translocation is reciprocal—meaning that two non-homologous chromosomes have exchanged fragments—cross-shaped configurations formed by the pairing of four chromosomes appear in the meiosis of a heterozygous individual. A special case of translocation is the fusion of two acrocentric chromosomes (Robertsonian translocation).

The immediate phenotypic effects of chromosomal mutations can vary considerably. Depending on the function of the affected gene, deletions and deficiencies lead to the formation of either non-viable gametes or a non-viable diploid organism homozygous for the mutant chromosome. In the case of heterozygosity for the mutant chromosome, not only deletions and deficiencies, but also duplications of genetic information can lead to a disruption of the gene balance.

The levels of Gene Expression within a single genome are finely balanced against one another. Because The amount of a gene product is proportional to the number of gene copies and alleles, a decrease or increase in their number results in the disruption of this balance.

Finally, gene expression can also depend on genomic position. A change in position resulting from a chromosomal mutation can influence the phenotype as a position effect if gene expression is disrupted, for example, due to a new juxtaposition with heterochromatic regions of the chromosome.

Aside from their direct phenotypic manifestations in the mutant organism, chromosomal mutations also have far-reaching evolutionary consequences. Gene Duplication can lead to the emergence of gene families. On the one hand, gene families facilitate the synthesis of large amounts of a gene product; on the other hand, they enable the diversification of the Functions of the encoded Proteins.

An example of high product demand is seed storage proteins. In maize (Zea mays), nearly half of these proteins consist of zein. Zein is composed, among other things, of Polypeptides with molecular weights of 19,000 or 22,000. It is likely that at least 54 gene copies encode the smaller polypeptide and 24 gene copies encode the larger one. These genes are distributed across at least three chromosomes. The numerous genes comprising the family of proteins that bind chlorophyll a or b exhibit strong functional diversification in pigment binding. The divergence of nucleotide sequences within the coding regions of these genes can reach 55%.

Altering the spatial arrangement of genes through chromosomal mutations affects subsequent gene recombination in various ways. For instance, functionally related genes can be brought into close proximity by chromosomal mutations, which reduces the likelihood of their recombination. An example of this is the self-incompatibility locus in heterostylous primroses, which actually contains three genes (see 10.1.3.1). Recombinational opportunities are restricted, for example, when an individual is heterozygous for a pericentric inversion. If Crossing-over occurs within the region of the inversion loop in this case, one acentric and one dicentric chromosome are produced, both of which are subsequently lost during the course of meiosis. Because this pathway generates chromosomally unbalanced and presumably non-viable gametes, the inverted chromosomal segment is effectively protected against recombination (Fig. 10.8).

Fig. 10.8. Barrier effect of a chromosomal mutation (inversion): A — diagram of the altered chromosome pair (original form in black, mutant in gray) with indicated marker genes (a, b, c, d), break points, and the Rotation of the damaged chromosomal segment; B — meiosis in F₁: pairing of structurally different chromosomes and crossing-over within the inverted segment; C — formation in anaphase I of a bridge with two centromeres and an acentric fragment. Both structures are eliminated; only gametes with unaltered chromosomes of the original form or mutants are viable

Restriction of recombination through chromosomal mutations—specifically translocations—is also found in certain evening primroses (Oenothera): as a result of numerous translocations involving all chromosomes, chromosomes arise that do not form bivalents when heterozygous in meiosis, but instead arrange themselves in chains due to the Homology of the terminal segments of different chromosomes (Fig. 10.9). Consequently, gametes are produced carrying either exclusively maternal or exclusively paternal sets of chromosomes. This permanent complex heterozygosity is maintained because the fusion of gametes with identical chromosome configurations leads to non-viable zygotes as a result of lethal factors.

Fig. 10.9. Permanent complex heterozygosity. In certain evening primroses (Oenothera), numerous translocations spanning all chromosomes give rise to chromosomes that, when heterozygous in meiosis, do not form bivalents but instead form chains due to the homology of the terminal segments of different chromosomes. This produces gametes with either exclusively maternal or exclusively paternal chromosome sets

Finally, chromosomal mutations (particularly translocations) can cause changes in chromosome number (Fig. 10.10). This mechanism of altering chromosome number is also referred to as dysploidy.

Fig. 10.10. Changes in chromosome number via chromosomal mutation: A — haploid karyograms of two closely related Chaenactis species (Asteraceae) with 2n = 12 and 2n = 10; B — diagram of differentiating reciprocal translocation and fragment loss; C — meiotic chromosome pairing in F₁

The frequency of chromosomal mutations appears to vary considerably among different taxa. A comparison of established genetic linkage maps based on molecular markers shows that the chromosomes of wheat, barley, and rye are highly colinear—meaning that the linear arrangement of genes within them is largely unaltered and nearly identical. In contrast, two closely related sunflower species already differ by ten chromosomal mutations.

Chromosomal mutations are known not only in the nuclear genome, but also in organellar genomes. While they are relatively rare in plastid genomes and therefore typically serve as reliable taxonomic markers, rearrangements in the mitochondrial genome are extremely frequent. The reason for this is that, unlike the plastid genome, the mitochondrial genome contains numerous sequences in multiple copies. This facilitates pairing and, consequently, recombination within the mitochondrial genome.

10.1.2.3. Genomic mutations

Changes in chromosome number brought about by mechanisms other than the dysploidy described above are termed genomic mutations. Most commonly, they arise during mitotic or meiotic nuclear division when the distribution of chromatids or chromosomes to daughter Cells is impaired. If this affects not the entire genome, but only a single chromosome or a few chromosomes, it is referred to as aneuploidy. For example, if the chromatids of a single chromosome fail to separate during meiosis II (non-disjunction), the resulting haploid cell contains an extra chromosome that is absent In the second haploid cell (Fig. 10.10).

This mechanism can lead to changes in chromosome number within a species. To distinguish whether dysploidy or aneuploidy caused a change in chromosome number, it is necessary to examine meiotic chromosome pairing in a suitable hybrid individual.

If all chromosomes fail to separate during cell division, an euploid change in chromosome number occurs. The most common form of euploid genomic mutations is polyploidy.

The haploid chromosome number of an organism is conventionally designated as x. Accordingly, diploid individuals have 2x chromosomes, whereas polyploid individuals have 4x (tetraploids), 6x (hexaploids), 8x (octoploids), and so on. An even, proportional increase in chromosome number is called orthoploidy. Of course, it is also possible for an anorthoploid genome to arise with a chromosome number of 3x (triploids), 5x (pentaploids), etc. The genome of a tetraploid organism with a basic haploid chromosome number of x = 7 is usually designated as 2n = 4x = 28. This notation indicates that meiosis proceeds normally in this organism and that only bivalents are observed.

Somatic polyploidy occurs when chromosomes replicate during cell mitosis, but nuclear and cell division do not follow. This leads to the formation of restitution nuclei with a doubled chromosome number.

This phenomenon can also be induced experimentally by applying agents such as colchicine, an alkaloid extracted from the autumn crocus (Colchicum autumnale). Colchicine inhibits the Formation of the mitotic spindle without preventing chromosome duplication.

Somatic polyploidy can also lead to the formation of endopolyploid tissue in an individual. A classic example is the anther tapetum, which is typically endopolyploid, with its cells often containing numerous sets of chromosomes. If sister chromatids fail to separate during endoreduplication, so-called polytene (giant) chromosomes are formed. Such structures are typical of Diptera, for instance, and occasionally occur in the Cells of the plant embryo sac.

Somatic polyploidy assumes evolutionary significance when polyploidized tissues participate in The Development of reproductive organs, which can result in the formation of gametes with a doubled chromosome number.

This was first discovered in Primula x kewensis, a sterile hybrid between P. verticillata and P. floribunda. The spontaneous emergence of a fertile inflorescence on an otherwise sterile individual of this hybrid was directly caused by somatic polyploidization.

In the case of generative polyploidy, unreduced gametes fuse with one another. Due to meiotic irregularities, such gametes are diploid and occur in all plants at a low frequency (averaging 0.57% across studied species; the frequencies of diploid pollen grains and egg cells appear to be similar). In certain maize (Zea mays) cultivars, unreduced pollen grains have been observed at a frequency of at least 3.5%. The rate of diploid gamete formation is controlled both genetically and by environmental conditions. For instance, excessively high or low temperatures, as well as nutrient deficiency, lead to an increase in this frequency. The fusion of two unreduced gametes immediately gives rise to a tetraploid organism. Most often, due to the low frequency of unreduced gametes, generative polyploidization proceeds in two steps. In the first step, a triploid (3x) individual arises through the fusion of a normal reduced (haploid, x) gamete with an unreduced (2x) one, with the egg cell most frequently being the unreduced partner. If a triploid gamete from such an organism fuses with a normal reduced gamete, a tetraploid (4x) individual is produced.

The frequency of unreduced triploid gamete formation in triploid plants is significantly higher (around 5%) than that of unreduced gametes in diploid individuals. However, triploid plants also produce both Haploid and Diploid gametes, meaning that a large proportion of their gametes contain either more or less than half of the triploid chromosome Complement.

Several forms of polyploidy are distinguished based on the degree of homology between the combined genomes of a polyploid individual. If the combined genomes are homologous, it is termed autopolyploidy, whereas if the genomes are distinct, it is referred to as allopolyploidy (see 10.3.3.4).

Autopolyploidy and allopolyploidy represent the extreme ends of a continuous spectrum of genome similarity rather than rigid, objective categories. While the origin of polyploid progeny via somatic polyploidy, self-Fertilization, or the crossing of two individuals from the same population clearly falls under autopolyploidy, a certain degree of genome divergence is already observable when comparing individuals from different populations of the same species, and this divergence is even more pronounced between individuals of different subspecies, and so on. Given the impossibility of objectively demarcating auto- and allopolyploidy, it is practical to draw the distinction at species boundaries. Thus, polyploidy within a single species is considered autopolyploidy, whereas polyploidy resulting from the hybridization of different species is regarded as allopolyploidy. In reality, however, the problem remains unresolved, as species themselves are neither objectively definable nor biologically equivalent (see 10.3.1). The concept of "segmental allopolyploidy" is sometimes employed as an intermediate category bridging autopolyploidy and allopolyploidy.

The relative frequency of polyploid formation via the direct fusion of two unreduced gametes versus an intermediate triploid phase can vary between autopolyploidy and allopolyploidy. While the pathway to autopolyploidy most commonly proceeds via an intermediate triploid phase, the pathway to allopolyploidy is likely to be direct in most cases. This is typically attributed to the fact that the frequency of unreduced gamete formation in hybrids is significantly higher (averaging about 27%) than in non-hybrid individuals (0.57%).

In principle, not only genome multiplication but also haploidization is possible. This occurs when an egg cell develops parthenogenetically, i.e., without fertilization (see 10.1.3.3). Occasionally, haploid progeny arise from a diploid plant. If this process occurs in a polyploid plant, polyhaploid progeny are produced. Although the appearance of haploid or polyhaploid offspring is frequently observed in normally sexually reproducing species, the potential significance of such genomic mutations for plant evolution remains unclear.

The most immediate and commonly observed effect of polyploidization is an increase in the size of all plant organs. This so-called gigas effect occurs because an increase in chromosome number leads to a larger Cell Nucleus, and nuclear volume, in turn, correlates positively with cell size (Fig. 10.11). Consequently, polyploid plants typically possess thicker leaves and petals than their diploid ancestors, although they are often less branched and exhibit a delayed developmental schedule. Of major evolutionary significance is the high rate of meiotic irregularities in autopolyploid plants. These arise because homologous chromosomes are now present in quadruplicate rather than duplicate sets (e.g., in tetraploids). As a result, meiosis yields not only bivalents but also multivalents comprising several homologous chromosomes, while some chromosomes remain unpaired as univalents (Fig. 10.12). Consequently, aberrant chromosome segregation during meiosis produces gametes with extra or missing chromosomes, which often have reduced viability or are entirely sterile. Thus, the fertility of autopolyploid plants is most often diminished.

Fig. 10.11. The gigas effect. Polyploidization can lead to an enlargement of The Cell nucleus, individual cells, and the entire plant. Stomata, pollen grains, and chromosomes of diploid and tetraploid Antirrhinum majus

Fig. 10.12. Formation of tetravalents in the autotetraploid Nasturtium officinale

Because autopolyploid plants possess four homologous sets of chromosomes, the pattern of inheritance differs from that of diploid individuals. The detection of tetrasomic instead of disomic inheritance is frequently used as a diagnostic criterion to determine whether a species is an auto- or an allopolyploid.

The meiotic consequences of allopolyploidy are different. Here, diploid hybrids typically exhibit reduced fertility due to a lack of homologous chromosome pairing partners within the genome inherited from divergent parental species. Polyploidy, however, provides duplicate homologous chromosomes for every pairing partner, thereby restoring fertility (see 10.3.3.4).

Estimates of polyploidy frequency vary depending on the threshold chromosome number considered to represent polyploidy. If polyploids are defined strictly as those species

in which the chromosome number is a multiple of the lowest basic number known within a given genus, then 30–35% of angiosperm species are polyploid. If, however, one assumes that even the lowest chromosome number found within a genus may itself be polyploid, and considers all haploid numbers $x > 9$ to be polyploid, the proportion of polyploid flowering plants rises to 70–80%. In either case, it is evident that polyploidy is widespread among angiosperms and represents a major evolutionary driving force. Among ferns and their allies, the proportion of polyploids is estimated to reach as high as 95%, whereas among non-flowering seed plants, only about 5% are polyploid.

Although autopolyploid individuals presumably arise more frequently than allopolyploids, they are considerably more difficult to identify due to meiotic irregularities. Furthermore, owing to their hybrid origin, allopolyploids generally display greater genetic variability (see 10.3.3.4); consequently, it is widely accepted that the majority of polyploid plant taxa have originated via allopolyploidy and thus trace back to allopolyploid ancestors. On the other hand, autopolyploidy is well documented in species such as Plantago media, Dactylis glomerata, and species of the genus Heuchera.

10.1.2.4. Recombination

Genetic variability arises not only through mutations but also via the reassortment of genetic material from different individuals. In eukaryotic organisms, this process, termed recombination, is intimately linked to sexual reproduction. The recombination of parental genetic material results, on the one hand, from the random fusion of gametes (syngamy) and, on the other hand, from meiotic segregation during the formation of the next generation's gametes.

Despite the absence of conventional Sexual reproduction in Bacteria and archaea, these organisms possess mechanisms for Genetic Exchange and, consequently, for the recombination of hereditary material. DNA transfer can occur via direct cell-to-cell contact (conjugation), bacteriophage-mediated transfer (Transduction), or the uptake of naked extracellular DNA (transformation). These processes are collectively referred to as parasexuality.

Recombination processes can be tracked through the predictable Patterns of inheritance.

In 1866, Gregor Mendel published his pioneering quantitative formulation of the laws of heredity in Experiments on Plant Hybrids. Mendel's breakthroughs, which were of paramount importance not only for genetics but for evolutionary biology as a whole, went virtually unnoticed at the time. It was only after the rediscovery of his foundational rules of inheritance by Hugo de Vries, Carl Correns, and Erich von Tschermak in 1900 that genetics experienced explosive development. Mendel's primary research subject was the garden pea (Pisum sativum); the availability of numerous distinct cultivars differing clearly in multiple traits and rendered homozygous through continuous self-pollination enabled him to obtain clear-cut and quantitatively interpretable results.

Subsequent experiments led G. Mendel to postulate the well-known regularities now commonly referred to as Mendel's laws. When two individuals (P) differing in only a single trait are crossed (monohybrid cross), the first filial generation (F₁) turns out to be uniform. In the example with the four-o'clock flower (Mirabilis jalapa) shown in Fig. 10.13, the parent individuals have red or white flowers, whereas all F₁ offspring exhibit an intermediate state of the trait, bearing pink flowers. Whether F₁ will be intermediate for the studied trait or resemble one of the parents—as in the cross between two individuals of the stinging nettle (Urtica pilulifera) with serrate and entire leaves, respectively (Fig. 10.14)—depends on the expression of the alleles for this trait. In the case of allele dominance/recessiveness, F₁ will resemble the parent with the dominant allele, whereas in the case of incomplete dominance, F₁ may appear intermediate. The resemblance of all offspring in the first generation (F₁) is described by Mendel's first law, the law of uniformity of F₁. Admittedly, this can only be observed when the parent individuals are homozygous for the gene in question. When this prerequisite is met, the uniformity of F₁ manifests regardless of the direction of the cross: it does not matter which genotype is used as the maternal one and which as the paternal one¹. If two individuals from F₁ are subsequently crossed in the four-o'clock flower, a second filial generation (F₂) is obtained, in which individuals with white, pink, and red flowers occur in a 1:2:1 ratio. Crossing F₁ individuals of Urtica pilulifera among themselves yields an F₂ in which individuals with serrate and entire leaves occur in a 3:1 ratio. In both cases, a segregation of the trait is observed in F₂, which is why this phenomenon is known as Mendel's second law, the law of segregation in F₂. To explain the corresponding results from crossing various pea varieties—for instance, with smooth and wrinkled or yellow and green seeds—G. Mendel hypothesized that each studied trait is determined by two hereditary factors, which today are called the alleles of a single gene. In the case of the four-o'clock flower, the result of the cross RR (red flowers; each diploid individual contains two alleles of the same gene, haploid gametes receive one R allele) x rr (white flowers; haploid gametes contain the r allele) manifests in the uniformity of F₁ with the allelic composition Rr. Each F₁ individual produces an equal number of gametes with R and r. The random process of fertilization leads to the occurrence of three different genotypes (RR, Rr, rr) in F₂ in a 1:2:1 ratio. In Urtica, crossing ZZ x zz yields genotypes Zz in F₁, and genotypes ZZ, Zz, and zz in F₂ in a 1:2:1 ratio; however, due to the complete dominance of Z, the corresponding phenotypes appear in a ratio of 3 (serrate-edged leaves) to 1 (entire leaves). The occurrence of parental genotypes and phenotypes in F₂ clearly demonstrates that hereditary factors are discrete, i.e., although they combine in F₁, they do not blend.

¹ Strictly speaking, it is not genotypes that are crossed, but organisms with specific genotypes. — Ed. note.

Fig. 10.13. Disomic inheritance of flower color in the four-o'clock flower (Mirabilis jalapa). Monohybrid cross of parent plants (P) with white and red flowers; three generations of their offspring (F₁, F₂, F₃), showing heterozygous individuals with intermediate (pink) flower color. The allelic constitution (r — white, R — red) of the diploid plants and haploid gametes is indicated.

Fig. 10.14. Inheritance of leaf margin serration in Urtica pilulifera. Monohybrid cross of parent plants (P) with sharply serrate (pilulifera) or nearly smooth (dodartii) leaf margins; their offspring through three generations (F₁, F₂, F₃). The allelic constitution (Z — sharply serrate leaves; z — nearly entire leaves) of the diploid plants is shown.

The Discovery of the discreteness of hereditary factors marked a significant step forward compared to the views of C. Darwin, who assumed that hereditary factors blend («blending inheritance»). This was the weakest point in Darwin's theory of evolution, as even his contemporaries realized, since beneficial mutations could hardly be preserved under such conditions due to continuous «dilution» through crosses with non-mutant individuals.

In the aforementioned examples, the inheritance process was observed in the diploid sporophytic Generation of the studied object. Such inheritance is termed diplogenotypic. In contrast, for organisms like the green alga Chlamydomonas, in which mitoses, Vegetative Reproduction, and trait differentiation occur in the haploid phase while only the zygote is diploid, one speaks of haplogenotypic inheritance.

The genotypic differences between phenotypically identical F₂ individuals of Urtica with serrated leaf margins (ZZ: Zz in a 1:2 ratio) can be identified by obtaining the next generation, F₃, from each of them via self-fertilization, or by crossing each F₂ individual with the homozygous recessive parent organism (zz)—that is, by performing a test cross (backcross). In the cross Zz x zz, the individuals of the reciprocal generation (R) with serrate (Zz) and entire (zz) leaves appear in a 1:1 ratio, whereas in the cross ZZ x zz, all individuals have serrate (Zz) leaves.

Individuals may differ not in one, but in two or more traits (dihybrid or polyhybrid crosses). In this case, another regularity can be observed. Crossing varieties of the snapdragon Antirrhinum majus with red and actinomorphic flowers (RRzz) and with white and zygomorphic flowers (rrZZ) (Fig. 10.15) also leads here to the manifestation of Mendel's first rule regarding the uniformity of F₁—all plants turn out to have red and zygomorphic flowers. If two F₁ individuals are crossed, the phenotypes encountered in F₂ are: red zygomorphic, red actinomorphic, white zygomorphic, and white actinomorphic flowers in a 9:3:3:1 ratio. These data can be explained by the fact that the red color (R) and the zygomorphic shape (Z) are dominant, and that the uniform first generation with the genotype RrZz produces four Different types of gametes. Random combinations of these four gamete types—RZ, Rz, rZ, and rz—form 16 possible combinations (the number of possible combinations can be calculated as the combinatorial number 4ⁿ, where n is the number of genes studied), comprising 9 genotypes (1 × RRZZ, 2 × RRZz, 2 × RrZZ, 4 × RrZz, 1 × RRzz, 2 × Rrzz, 1 × rrZZ, 2 × rrZz, 1 × rrzz). According to the dominance rule, in the studied example these nine genotypes fall into four phenotypic classes (RRZZ, RRZz, RrZZ, RrZz — red zygomorphic; RRzz, Rrzz — red actinomorphic; rrZZ, rrZz — white zygomorphic; rrzz — white actinomorphic). Notably, in this case, combinations of traits appear in the F₂ generation that were absent in both the parental generation and F₁. At the phenotypic level, these are red zygomorphic and white actinomorphic flowers, and at the genotypic level, all combinations differing from RRzz, rrZZ, and RrZz. Thus, it is evident that genetic recombination promotes the generation of genetic variability. The hereditary determinants of the two analyzed traits do not remain in the combinations characteristic of the parents, but combine independently with one another. These data correspond to Mendel's third law—the law of independent assortment of different hereditary determinants.

Fig. 10.15. Scheme of a dihybrid cross in Antirrhinum majus. Parent plants with red actinomorphic and white zygomorphic flowers; their descendants in F₁ and F₂; ♂ and ♀ gametes. The genes (each with two alleles, dominants denoted by uppercase letters, recessives by lowercase) determining flower color (R — red, r — white) and flower shape (Z — zygomorphic, z — actinomorphic) are located on different chromosomes (and are therefore unlinked). The 9 F₂ genotypes (RRZZ, RRZz, RrZZ, RrZz, RRzz, Rrzz, rrZZ, rrZz, rrzz) fall into 4 phenotypic classes (RRZZ, RRZz, RrZZ, RrZz — red zygomorphic; RRzz, Rrzz — red actinomorphic; rrZZ, rrZz — white zygomorphic; rrzz — white actinomorphic).

However, the independent assortment of hereditary determinants just described is not manifested in many cases. When crossing pea plants with straight green pods and curved wax-yellow pods, the expected 9:3:3:1 segregation ratio is not found in the second generation, and combinations of traits from the parental forms are observed significantly more often than new combinations. Thus, the two considered genes determining fruit shape and its color are not independent of each other, but are linked.

All the inheritance processes described so far, including this last deviation from Mendel's third law, can be explained by the processes of gamete fusion and meiosis, as well as by the organization of genes in the cell nucleus. The uniformity of the first generation of F₁ hybrids arises because, in a diploid organism, each chromosome is present in duplicate and, accordingly, each gene is represented by two alleles. The offspring (F₁) of homozygous parents turn out to be uniform—heterozygous. Since the chromosome number is halved during meiosis, the resulting gametes carry only a single allele. Their random combination upon the fusion of sex cells in a monohybrid cross leads, in the second generation (F₂), to the formation of three different genotypes in a 1:2:1 ratio, i.e., segregation is observed in F₂. The independent assortment of hereditary determinants described by Mendel's third law is revealed in cases where the analyzed genes are located on different chromosomes. Their independence is determined by the fact that, in the First Division of meiosis, the homologous parental chromosomes, arranged in pairs as bivalents, generally segregate randomly, so that not all chromosomes from one parent end up in a single daughter cell and those from the other parent in the other. Thus, thanks to the random orientation of bivalents, a mixing of parental chromosomes occurs. This process is also referred to as interchromosomal recombination, because although the mixing of parental chromosomes takes place, the chromosomes themselves remain intact. Deviations from the independent assortment of hereditary determinants occur when the analyzed genes are located on the same chromosome and are physically linked to each other. The observation that such genes do not always remain together, but are separated from each other with varying frequency, is explained by the fact that parental chromosomes can exchange segments during meiosis via crossing-over. This gives rise to chromosomes consisting of paternal and maternal parts. As a result, genes on the same chromosome may originate from the father and the mother, respectively. This second recombination process is called intrachromosomal recombination.

The scale of inter- and intrachromosomal recombinations strongly depends on the species-specific chromosome number and their size. A higher chromosome number increases the possibilities for combining maternal and paternal chromosomes, whereas the frequency of crossing-over is higher in larger chromosomes than in smaller ones simply due to spatial relationships.

The frequency with which genes located on the same chromosome separate from each other depends on the distance between them. A greater distance more frequently leads to Separation, since the probability of crossing-over between genes in such cases is high. Conversely, if two genes are located adjacent to each other, they are separated rarely, because crossing-over in a very short chromosomal region is highly unlikely.

Recombination frequency can be used to construct genetic linkage maps (Fig. 10.16), which represent the linear arrangement of genes on a chromosome. However, the recombination frequency of widely spaced genes is always lower than the value obtained by summing the recombination frequencies of the genes located between them. This can be explained by the fact that over very long distances, the recombination frequency drops again due to double or multiple crossing-over. The position of a gene on a genetic linkage map is called a locus.

Fig. 10.16. Position of certain genes (cp, ten, d, etc.) on chromosome V of the garden pea (Pisum sativum). Left: phenotype under normal or mutant gene conditions; partial manifestation occurs only in joint action with other genes (e.g., with A, the major gene for anthocyanin formation). Right: recombination frequencies.

Up to this point, examples have been used in which a single trait is coded by a single gene with two alleles. Examples where very many, and particularly continuously varying traits—such as plant height or leaf blade length—do not segregate in F₂ into discrete trait classes, but rather are characterized by a continuous variation range, lead us to the Conclusion that these traits are likely polygenic, i.e., coded by many genes. The recognition of discrete trait classes in a segregating generation can be further complicated by the fact that gene expression is influenced by the environment, which masks the existing discrete trait classes. The inheritance process of Quantitative Traits cannot be analyzed using the methods of Mendelian genetics; instead, it is investigated using the methods of quantitative genetics (see Fig. 10.16).

Recently, the availability of molecular methods has allowed for new approaches in the genetic analysis of quantitative traits. In this process, genetic linkage maps of molecular markers are first constructed. In the Second Stage, attempts are made to find a parallel segregation of molecular and phenotypic traits. If, for example, a phenotypic trait of individuals in a segregating second generation (F₂) correlates statistically significantly with a specific molecular marker (known as co-segregation), it is concluded that the gene determining the phenotype is located adjacent to the gene for the molecular marker. In this way, one can estimate the number of genes influencing a trait, their position in the genome, and their relative effect. These methods are known as QTL mapping (Quantitative Trait Loci).

Genetic recombination potentially leads to the emergence of an extremely large number of new genotypes. The number of genotypes (g) in the F₂ generation is calculated as g = 3ⁿ, where n is the number of independently segregating genes, each having two alleles. In a group of individuals possessing not two, but several alleles, the number of new genotypes for F₂ is calculated by the formula

g = (r(r + 1) / 2)ⁿ, (10.1)

where r is the number of alleles of a gene; n is the number of independently segregating genes. For just five genes with four alleles each, as many as 100,000 combinations are possible. Admittedly, in such calculations, one must take into account that not all genes can combine freely, as they are sometimes located on the same chromosome. Nevertheless, such a quantitative estimate demonstrates just how massive the generation of new genotypes is through recombination.

10.1.2.5. Extranuclear Inheritance

Plastids and Mitochondria (Organelles of plant cells), owing to their symbiotic origin, possess their own genomes—specifically, the plastid and mitochondrial genomes. The Inheritance of Traits encoded by these genomes does not follow Mendelian laws. The distinct nature of such non-nuclear (extrachromosomal) inheritance is explained by the fact that the zygote typically receives its organelles not from both parents, but exclusively from the maternal organism (maternal inheritance), and that the fusion of organelles—as a primary prerequisite for the recombination of genetic material—is a rare occurrence. The fusion of plastids has been observed in Chlamydomonas, and that of mitochondria in Yeasts.

A typical pattern of plastid inheritance can be observed in the four o'clock plant (Mirabilis jalapa) (Fig. 10.17). Here, as in many other species, individuals occur with normal green leaves as well as with variegated green-and-white leaves. White areas in the leaf tissue arise because the cells within them contain only colorless plastids due to a defect in chlorophyll synthesis. Crossing a normal green individual of the four o'clock plant with a variegated one leads, depending on the direction of the cross, to varying results that depart significantly from Mendelian rules. If the green plant is used as the maternal parent, all offspring are green. Conversely, if the variegated plant is used as the maternal parent, the majority of the offspring exhibit variegated leaves. These results are explained by the fact that in Mirabilis jalapa, plastids are inherited exclusively via the egg cell; consequently, the offspring display the plastid traits of the female parent alone. Instances where normal green offspring do appear—when using a variegated plant as the female parent or upon self-pollination of a variegated plant—are due either to an egg cell fortuitously containing only normal plastids or to a random distribution of plastids during embryonic development from a zygote containing both normal and defective plastids, such that only normal plastids end up in the leaf cells.

Fig. 10.17. Extrachromosomal inheritance of green-white variegation. Maternal inheritance in the four o'clock plant Mirabilis jalapa

In maternal inheritance, plastids are excluded from Male Germ Cells during pollen development, sperm maturation, or strictly at fertilization, or else the plastids contributed by the male germ cells degenerate. Although maternal plastid inheritance is the rule in flowering plants, biparental plastid inheritance is also known in some taxa, such as Pelargonium and Hypericum. Paternal plastid inheritance has been described for certain conifers, such as pines (Pinus) and larches (Larix), as well as for kiwifruit (Actinidia). Mitochondria are most frequently inherited maternally, though cases of biparental and paternal inheritance are also known for these organelles.

10.1.3. Recombinational System

It follows from formula (10.1) for calculating the number of possible recombinations as a function of the number of genes and alleles investigated (see 10.1.2.4) that in the absence of allelic variation (r = 1; homozygosity at every gene), no novel recombinations are generated. Although cellular Mechanisms of Recombination are still operative here, crossing genetically identical individuals does not lead to genetic novelties in the offspring. The extent of genetic recombination that drives genetic variation depends on the degree of genetic similarity between the crossed individuals. The genetic similarity of individuals within a reproductive community is determined by the mating system (self-fertilization vs. cross-fertilization and pollination), the breeding system (sexual vs. asexual),

life form, and the dispersal range of pollen and, consequently, seeds or fruits (gene flow). The aggregate of these factors can be termed the recombinational system of a species.

10.1.3.1. Mating System

Dioecy and Other Sexual Systems

The majority of flowering plants possess bisexual (hermaphroditic) flowers, whose structure inherently allows for self-pollination and self-fertilization. Continuous self-mating and subsequent self-fertilization increase the homozygosity of the offspring (Fig. 10.18), thereby reducing genetic variability. Elevated levels of homozygosity cause recessive alleles, once rendered homozygous, to express their frequently detrimental effects. The associated phenotypic traits that impair the physical fitness of the offspring are referred to as Inbreeding depression. The simplest way to prevent self-fertilization is dioecy, i.e., the development of unisexual flowers on separate individuals. While this form of sex segregation is widespread among animals, it is exceptionally rare in flowering plants, occurring in only about 5% of species. Dioecious species are more commonly found, for instance, among tropical woody plants and on oceanic islands.

Fig. 10.18. Inbreeding and homozygotization. Self-pollination and sib-mating lead to complete homozygosity over the course of several generations

Thus, in the Hawaiian Islands, about 15% of the species are dioecious. Sex Determination in dioecious flowering plants is primarily diplogenic, meaning that the genetic constitution of the sporophytes is the decisive factor for the formation of either exclusively male or exclusively female gametophytes within the flowers of a given individual. For example, in bladder campion (Silene latifolia), sex chromosomes are present (Fig. 10.19).

Fig. 10.19. Sex Chromosomes and diplogenic sex determination (A, B — 1,800×; from K. Belar; C — from W. Schumacher). Meiosis (metaphase I) in a pollen grain mother cell (A) and an embryo sac mother cell (B) in the dioecious plant Silene latifolia; C — diagram of diplogenic sex determination; X, Y — sex chromosomes

Sex chromosomes are distinguished as heterosomes from the remaining chromosomes of the genome, which are called autosomes. In this case, male plants have the chromosomal constitution XY and are classified as heterogametic because their gametes receive either an X or a Y chromosome, whereas female plants have the chromosomal constitution XX and, as homogametic individuals, produce gametes of only one type. In most plants, as in many animals, male organisms are heterogametic and female ones are homogametic. Upon crossing female and male individuals, this results in a 1:1 ratio of male to female offspring (see Fig. 10.19). However, in most plant species, deviations from this numerical ratio are observed. This indicates that sex determination can be governed not only genotypically, but also environmentally (phenotypically). Factors such as Temperature, day length, or water availability have been experimentally proven to influence Sex determination in dioecious plants. In hemp (Cannabis sativa), sex determination can be altered by the application of Abscisic acid, auxin, gibberellin, cytokinin, high concentrations of boron, carbon monoxide (CO), or by cultivation under long-day conditions combined with plant wounding.

Box 10.1. Recording and Analysis of Phenotypic and Genetic Variation

The genetic variation of a species can be observed at various levels using different methods. Phenotypic variation has both a genetic and an environmental component (see 10.1.1). Since the environmental component cannot be completely eliminated, determining the proportion of genetic variation relies on experimental results in which all analyzed genotypes are investigated under identical environmental conditions. For this purpose, plant material of diverse origins is grown from seeds in quantities sufficient for statistical analysis under uniform environmental conditions. In such a comparative cultivation experiment (often referred to as a common garden trial), it is possible to assess the extent to which Representatives of the same species differ genetically, because differences maintained under uniform cultural conditions can only be attributed to genetic variation. All phenotypic traits—such as morphological, anatomical, physiological, or ecological features—can be examined in this manner. It should be noted that due to phenotypic plasticity, genetic differences do not always manifest under specific cultivation conditions, and properties atypical of the species in its natural habitat may occasionally arise. Comparative cultivation under varied conditions and a comparison of experimental variation with that observed in nature help to identify such cases. The variation of phenotypic traits typically follows a normal distribution. Here, extreme low and high values of a trait are rare, while intermediate values predominate significantly (see Fig. 10.2). Important statistical measures, such as the mean, variance, and standard deviation, are used to describe this variation. The mean value x is calculated as the quotient of the sum Σ_x of all measured values divided by the number n of these values

Variance and standard deviation are measures used to describe data dispersion independently of the mean. Datasets from different distributions may have identical means (Fig. A). Variance s² is calculated as the quotient of the sum of squared deviations of each measured

value from the mean Σ (x - x)² divided by the number of measured values minus 1 (n - 1). Thus, the variance

Fig. A. Two populations (A and B) with different distributions of metric traits may have identical population means

Finally, the standard deviation s is the square ROOT of the variance In evolutionary biology studies, it is usually important to compare, for instance, the variation of a given trait between two populations to determine whether statistically significant differences are present or absent. Analysis of variance can be employed for this purpose. It tests whether the variance between populations is significantly greater than the variance within populations. If one wishes to investigate multiple traits rather than just a single one, multivariate statistical methods are used.

Biological Materials frequently exhibit deviations from a normal distribution—specifically, positively or negatively skewed distributions where high or low values of measured variables occur more frequently than intermediate ones.

Genetic variation can also be detected at the protein and DNA levels. To characterize intraspecific variation, researchers typically examine allo- or isozymes among proteins. Most commonly, this involves the standard enzymes of primary plant metabolism. Enzymes encoded by a single genetic locus with multiple alleles are termed alloenzymes. When multiple loci are available for an enzyme, they are referred to as isozymes. Alleles of the same locus or different loci within an enzyme system can be identified because they separate during Electrophoresis due to differences in their electrical charges and can subsequently be visualized using specific staining methods¹.

¹ Different alleles produce protein products with distinct electrical charges; it is these products, rather than the alleles themselves, that are separated and detected during electrophoresis. — Editor's note.

A major advantage of allo- and isozymes, collectively known as "isozymes," is that the two alleles of a single locus are usually expressed codominantly, meaning both produce proteins. This makes it possible to identify heterozygous individuals without performing cross-pollination experiments.

DNA analysis methods frequently employed for intraspecific studies can be grouped under the umbrella of fingerprinting techniques. Examples include RAPD (Random Amplified Polymorphic DNAs), AFLP (Amplified Fragment Length Polymorphism), ISSR (Inter Simple Sequence Repeats), and analyses of mini- and microsatellite DNA (VNTRs: Variable Number Tandem Repeats). These techniques are based on the Polymerase Chain Reaction (PCR: Polymerase Chain Reaction) and/or restriction fragment length polymorphism (RFLP: Restriction Fragment Length Polymorphism) analysis.

When describing genetic variation based on traits that allow the identification of gene loci and their alleles, quantitative metrics are generally used to compare species with differing characteristics, such as the percentage of polymorphic loci and the number of alleles per locus. A locus is considered polymorphic if more than one allele is detected in the studied material or if the frequency of the most common allele is less than 0.99. Another important metric is the mean level of heterozygosity. For an individual locus, the average heterozygosity $h$ is calculated using the formula

where x₁ is the frequency of the 1st allele; m is the number of alleles. The average heterozygosity H across all loci is the mean value of all h. In further analyses of the genetic structure of intraspecific variation, the total variability H_t can be divided into two components. Of these, H_s describes variation within populations, while C_st or F_st describes variation between populations.

Heterogametic female sex is known, for example, in Fragaria, Potentilla, and Cotula. Sex chromosomes harbor regulatory genes that influence the development of male and female floral organs. However, the genes responsible for The structure of these organs are also located on autosomes. The evolution of sex chromosomes was most likely driven by the chance suppression of the complete sterility that typically accompanies the recombination of morphologically distinct chromosomes. Sex chromosomes have been observed in only a fraction of dioecious species.

While The Role of dioecy in preventing self-fertilization is clear, its emergence as a result of selection for cross-pollination is frequently debated. An alternative hypothesis is that lacking male organs in a flower allows more resources to be allocated to seeds and fruits. This could explain why dioecy is often correlated with the production of fleshy and thus "costly" fruits.

Dioecy most likely originates via gynodioecy, where individual plants bear exclusively female flowers alongside others with bisexual flowers. Dioecy and gynodioecy are not the only ways sexual functions are distributed among different flowers and individuals. Additional systems include monoecy, gynomonoecy, andromonoecy, and potentially androdioecy and polygamy (Table 10.1).

Table 10.1. Approximate frequencies of various sexual systems in angiosperms

Among other seed plant groups, Ginkgo, all Cycadopsida, and the majority of Gnetopsida are dioecious. Sex chromosomes can also be found in some of their representatives. Within Coniferopsida, dioecy is quite rare. In the European flora, for example, yew (Taxus baccata) and common juniper (Juniperus communis) are dioecious. Among spore-producing plants, separate sexes are restricted to the gametophytes rather than the sporophytes. In ferns and their allies, unisexual gametophytes occur almost exclusively in heterosporous taxa. However, gametophytes of different sexes are always produced by the same sporophyte. In homosporous ferns, unisexual gametophytes can arise through environmental sex determination. Gametophytes of either male or female sex are known in bryophytes and algae, and the liverwort Sphaerocarpos also possesses sex chromosomes.

Incompatibility Systems

Self-incompatibility prevents self-fertilization in bisexual flowers (Fig. 10.19): fertilization by a flower's own pollen is inhibited. Regardless of the specific mechanisms involved, the underlying genetic principle is always the same. If the same self-incompatibility allele (S) is expressed in the pollen grain (or male gametophyte) and on the stigma, fertilization does not occur. Depending on whether the pollen's behavior is determined by the genotype of the male gametophyte itself or the sporophyte that produced it, as well as the floral morphology associated with the incompatibility system, three main self-incompatibility systems can be distinguished: homomorphic gametophytic (GSI), homomorphic sporomorphic (SSI), and heteromorphic.

In GSI, the pollen reaction is determined by the genotype of the individual pollen grain. Thus, pollen grains produced by a heterozygous individual fall into two reaction classes according to the S allele they carry, while the expression of the two S alleles in the style is codominant. When an individual with the allelic constitution is self-pollinated (or when two individuals sharing both alleles are crossed), neither S₁ nor S₂ pollen results in fertilization (Fig. 10.20), because both alleles are also expressed on the stigma. In a cross of S₁S₃ (female parent) × S₁S₂ (male parent), S₂ pollen successfully achieves fertilization because the S₂ allele is absent from the pistil. In a cross of S₃S₄ × S₁S₂, both types of pollen grains can effect fertilization.

GSI involving a single S gene is known in numerous plant families, such as Papaveraceae, Rosaceae, Solanaceae, and Scrophulariaceae. The number of distinct S alleles ranges from 20 to 70. GSI involving two genes (S and Z) has been documented in grasses (Poaceae). Here, the incompatibility reaction is triggered only when the alleles of both genes are identical in both pollen and pistil. Three self-incompatibility genes have been discovered in meadow buttercup (Ranunculus acris), and four in sugar beet (Beta vulgaris).

In SSI, the reaction of a pollen grain is determined not by its own allele, but by the genotype of the parent plant that produced it. Consequently, all pollen grains from a single individual exhibit the same reaction phenotype, even though they carry different alleles.

With SSI, considerable variability in allele expression is expected in both pollen and pistil. Most commonly (see Fig. 10.20), one allele is dominant in the pollen reaction, while two alleles are codominant in the pistil.

In a cross of S₁S₂ (female parent) × S₁S₃ (male parent) under SSI—assuming dominance of S₁—a complete self-incompatibility reaction occurs, contrasting with the GSI outcome (where S₃ pollen successfully fertilizes), because S₁ determines the reaction of both pollen and pistil under codominant expression (see Fig. 10.20). If dominance lies within the pollen, the self-incompatibility reaction does not occur at all, and pollen carrying the S₁ allele can effect fertilization because the pollen reaction is entirely governed by the S₃ allele.

Fig. 10.20. Pollination and Fertilization incompatibility in angiosperms. S-alleles (S₁, S₂, S₃, S₄) of pollen grains on the stigma (top) and within the maternal tissue of the style and Ovary (bottom), showing pollen grains and pollen tubes (indicated by light lines). In gametophytic self-incompatibility, the reaction depends on the genotype of the haploid pollen grains. In sporomorphic (sporophytic) self-incompatibility, the genotype of the pollen-producing parent is decisive (in parentheses). (The genotypes S₁S₃ and S₃S₄ are expressed in the pistil, not in the pollen. — Translator's Note.)

SSI is best understood in Asteraceae and Brassicaceae, and is known across a total of eight families. The number of S gene alleles is roughly comparable to that in GSI.

Naturally, as the male gametophyte of seed plants, pollen possesses traits controlled by its own haploid genome. Instances where certain pollen characteristics depend on the genotype of the parent plant that produced it can be attributed to the participation of the anther tapetum in forming the pollen grain wall. A striking and early-studied example is pollen grain shape in pea (Pisum sativum). When an individual with elongated pollen grains (LL) is crossed with one having round pollen grains (ll), the heterozygous (Ll) F₁ generation produces exclusively elongated pollen grains, even though half of the pollen carries the allele for round grains (l). Conversely, the presence or absence of starch in maize (Zea mays) pollen is a purely gametophytic trait. Heterozygous individuals derived from crosses between "starchy" and "starchless" plants produce starchy and starchless pollen grains in a 1 : 1 ratio.

In most cases, homomorphic SI and SSI correlate with certain other traits. With GSI, pollen at the time of pollination is usually bicellular, the stigma cuticle is ruptured, and the stigma is wet, whereas self-fertilization is prevented by the arrest of pollen tube growth within the style. In contrast, with SSI, pollen at the time of pollination is typically tricellular, the stigma cuticle is continuous, and the stigma is dry, meaning that the pollen tube simply cannot penetrate the stigma. An exception is provided by grass GSI, which morphologically corresponds to SSI.

In heteromorphic self-incompatibility, genetically determined mating types are also manifested in morphology. An example is the primrose (Primula). In most species of this genus, two flower morphs are found (Fig. 10.21). Thus, there are individuals with a long style and low-set anthers

within the corolla tube, and individuals with short styles and high-set anthers. These two forms also differ in the size of the stigma papillae and pollen grains. This phenomenon is termed distyly or, more generally, heterostyly.

Fig. 10.21. Heterostyly in Primula sinensis. Stigmas (G) and anthers (S) are located at different heights: A — flower with a long style and large papillae on the stigma (N) and small pollen grains (p); B — flower with a short style, small stigma papillae (n), and large pollen grains (P)

Similar heteromorphy is known in approximately 25 families of flowering plants and 155 genera. Rubiaceae alone contains 91 heteromorphic genera. In addition or as an alternative to the aforementioned traits, the morphological differentiation of flower types may also include style pubescence (e.g., in Oxalis) and coloration (e.g., in Eichhornia), anther size (e.g., in Lithospermum, Pulmonaria), or exine sculpture (e.g., in Armeria, Limonium, Linum). Not only two, but three flower forms (tristyly) are found, for instance, in purple loosestrife (Lythrum salicaria), the genus Eichhornia (Pontederiaceae), and Narcissus (see Fig. 10.21).

Heterostyly is associated with genetic self-incompatibility. In Primula, short-styled individuals are heterozygous for Ss, whereas long-styled forms are homozygous for ss. With the dominance of S and a sporophytic self-incompatibility reaction, successful fertilization is possible only between these two floral morphs. Fertilization within a single flower or between flowers of the same morphotype does not occur.

If S were not dominant but codominant with s, pollen from long-styled individuals would not be able to germinate on the stigma of a short-styled flower (Ss), since both plants would contain the same allele. If the self-incompatibility reaction were gametophytic, S-dominance would allow self-fertilization in short-styled individuals, because half of their pollen carries the s allele, which is not expressed in the stigma.

In contrast to GSI and homomorphic SSI, the S gene in heteromorphic systems has only two alleles. Accordingly, in a population, statistically only every second mating is successful, given that ideally both flower forms occur in a 1:1 ratio. With a very large number of alleles in GSI and SSI systems, the percentage of successful matings in the population is significantly higher.

The function of heteromorphy, supplemental to operating genetic self-incompatibility, is presumably to reduce the frequency of illegitimate pollination (within a single flower or between two flowers of identical structure). In Primula, pollen adheres to the abdomen of a pollinator insect when it visits a short-styled flower. This pollen is much more likely to land on the stigma of a long-styled flower than on that of a short-styled one. In the event of illegitimate pollination, the stigma may become blocked for legitimate pollen, or the illegitimate pollen may delay the germination of the legitimate one. The question regarding the evolutionary sequence of the emergence of heteromorphy and the genetic incompatibility system remains controversial.

The S gene in Primula is actually not a single gene, but a cluster of three very tightly linked genes. The first gene (G/g) encodes style length, stigma papilla size, and style incompatibility response; the second (P/p) encodes pollen grain size and its incompatibility response; the third (A/a) encodes anther position. Rare recombination within this gene complex in heterozygous short-styled individuals can lead, in certain crosses, to progeny with low-set anthers and short styles, or high-set anthers and long styles. Both of these homomorphic recombinants are self-compatible and can easily self-pollinate due to the relative positioning of their anthers and stigmas. This gene complex in Primula exemplifies how tight linkage of three genes—likely arising via chromosomal mutations—can lead to suppressed genetic recombination. In this example, the frequency of crossovers within the functional gene complex is thus markedly reduced.

In addition to the self-incompatibility systems already described, a distinct delayed self-incompatibility system is sometimes recognized. Despite various differences in detail, what these phenomena share is that the incompatibility reaction manifests only after the pollen tube has grown through the style and is most commonly expressed as ovule abortion.

The biochemical processes underlying self-incompatibility reactions must be fundamentally different across various systems. While in GSI, Ribonuclease-active Glycoproteins perhaps recognize allele-specific pollen RNA and destroy it, in Brassicaceae SSI it is known that the reaction involves the interaction between pollen-derived glycoproteins and receptor protein Kinases.

Of all investigated flowering plants, approximately 50% are self-incompatible. The distribution of various self-incompatibility systems across taxa, as well as a comparison of their functions at the biochemical level, suggest that the three described systems evolved independently of one another. Furthermore, it can be assumed that GSI and heteromorphic sporophytic self-incompatibility arose multiple times in parallel, most frequently perhaps from self-compatible ancestors. This implies that the sporophytic pollen reaction in heteromorphic taxa is not homologous to the sporophytic pollen reaction in GSI. Whether the widespread GSI system evolved multiple times remains an open question. A comparison of nucleotide sequences of ribonuclease-active glycoproteins operating in this system has not yet provided a definitive answer. Contrary to earlier assumptions, the prevailing view today is that the earliest angiosperms were self-compatible. However, a secondary transition from self-incompatibility to self-compatibility is also frequent (see 10.1.3.2).

In plant breeding, a species' compatibility system is of great practical importance. For instance, crossing self-compatible and highly self-pollinating species requires specialized techniques, whereas in self-incompatible species, generating "pure" inbred lines is costly.

10.1.3.2. Pollination

Self-compatible hermaphroditic flowers are not necessarily self-fertilizing. Preventing self-fertilization (autogamy) and promoting fertilization by foreign gametes (allogamy) can be achieved by hindering or completely excluding self-pollination. This is accomplished either through the temporal separation of stamens and carpels or through their spatial separation within the flower. In temporal separation (dichogamy; Fig. 10.22), either the androecium matures before the gynoecium (protandry, e.g., in Asteraceae), or the gynoecium matures before the androecium (see Fig. 10.22) (protogyny, e.g., in many Ranunculaceae).

While the rarer protogyny is usually associated with self-compatibility, protandry is frequently found in self-incompatible flowers. Here, the function of protandry is presumably to prevent the stigma from being blocked by its own pollen or to reduce interactions with self-pollen that would be disadvantageous to foreign pollen.

In spatial separation (herkogamy), the stamens and carpels are arranged within the flower in such a way that self-pollination does not occur.

An example is found in the flowers of iris (Iris; Fig. 10.22), where each stigma, positioned above the stamen, is sheltered by an outgrowth of the petaloid style lobe. As a pollen-laden insect leaves the flower, this lobe presses against The surface of the style and covers the stigma. When the insect visits the next flower, a similar lobe brushes pollen off its body, which can then be transferred to the pistil's stigma.

Fig. 10.22. Dichogamy (A, B) and herkogamy (C – E). Protandry in Epilobium angustifolium. Flower in male (A) and female (B) developmental stages (life-size). In Iris pseudacorus flowers, anthers and stigmas are spatially separated, and the stigma (n) is shielded by a petal-like stigma lobe (C — general view; D — Cytology/practical/54.html">Longitudinal section of the flower; E — schematic cross-section in the region of the anthers)

Although dichogamy and herkogamy can effectively prevent self-pollination within an individual flower, they cannot rule out pollination between flowers of the same inflorescence. Pollination between flowers of the same individual (geitonogamy) is genetically identical to self-pollination.

If The sequence of flowering within an inflorescence (e.g., from bottom to top) and the pollinator's movement across it (e.g., from bottom to top) are strictly determined, the pollinator moves primarily from functionally female flowers to functionally male flowers. As a result, protandrous dichogamy can prevent geitonogamy.

Since the majority of flowering plant species possess hermaphroditic flowers, and a genetic self-incompatibility system is characteristic of only a few, while floral biology mechanisms cannot always prevent self-pollination or pollination between flowers of the same individual, it must be acknowledged that self-pollination and self-fertilization are common phenomena in plants. It is estimated that around 40% of flowering plants worldwide are capable of self-pollination and self-fertilization. Some sources even indicate that in temperate floras (such as the British Isles), about two-thirds of the species have this capacity. Because the long-term effects of continuous self-fertilization are negative (loss of genetic diversity, or inbreeding depression), an explanation is required for the remarkably high frequency of self-fertilizing species. The loss of genetic variation due to increasing homozygosity is mitigated by the fact that populations of self-fertilizing species typically consist not only of progeny from a single individual but also contain numerous distinct self-fertilizing genotypes¹. Furthermore, even a low frequency of cross-fertilization effectively prevents complete homogenization. If, for instance, only every tenth fertilization is cross-fertilized, allele A—originally occurring in the population at a frequency of 0.5 (50% of all gene alleles in the population belonged to A)—is nevertheless maintained at an equilibrium frequency of ~0.1 (10% A) after several self-fertilizing generations. Inbreeding depression can be overcome because genotypes carrying homozygous recessive alleles with deleterious effects are eliminated by selection over a few generations. A potential advantage of self-fertilization lies in enhanced reproductive efficiency and accelerated development.

¹ Plants can self-fertilize, not genotypes. — Ed. note.

For example, in the genus Leavenworthia (Brassicaceae), self-incompatible populations develop seeds from 52 — 62 % of their ovules, whereas in self-compatible

populations this figure is 73 — 90 %. Across a broad range of species, these values average about 22 % for self-incompatible and 75 % for self-compatible populations.

Reproductive success can be impaired simply by the absence of a pollinator or mating partner. Pollinator scarcity typically occurs under extreme conditions, such as in persistently cold and wet habitats. For species that flower only once in their lifetime (semelparous species), There is a risk that pollinators will be scarce or entirely absent during this usually brief flowering window. Colonizing species face The problem of finding a mating partner with particular frequency. When a fallow field is colonized, for instance, the local populations of invading species typically originate from a single founding individual. Colonization success is guaranteed if such an individual can self-fertilize. Consequently, many weeds that colonize human-disturbed habitats are predominantly self-fertilizing plants. Because routine self-fertilization and the lack of need to attract pollinators lead to reduced flower size, diminished nectar production, and lower pollen output in many weeds, their development is accelerated by shortening the reproductive phase of their life cycle. This enhances their colonization success, largely because they can produce multiple generations within a single growing season. However, an increased developmental rate may also be required under conditions of a very short growing season.

10.1.3.3. Breeding Systems

Like many other organisms, plants are capable of not only sexual reproduction but also asexual reproduction. Reproduction refers broadly to any process by which the number of individuals increases across successive generations. GROWTH AND REPRODUCTION are almost invariably intertwined. Because asexual reproduction

yields offspring that are genetically identical to their parents, the genetic diversity of a species or population depends largely on the relative frequency of asexual reproduction. Asexual reproduction, also known as apomixis, can occur either vegetatively or via seeds (through agamospermy — the formation of seeds without a sexual process).

Vegetative reproduction involves the formation of offspring from somatic tissues in the complete absence of any sexual processes. The progeny arise exclusively through mitotic cell divisions and thus without any change in nuclear phase. This is a common phenomenon among Fungi, algae, mosses, ferns, and angiosperms. Among gymnosperms, vegetative reproduction is rare. Studies of the flowering plant flora of the British Isles indicate that about 46 % of species possess the capacity for vegetative reproduction. These are most frequently perennial herbs, but occasionally also shrubs. Annual and biennial plants cannot reproduce vegetatively, and among trees this capacity is uncommon. Examples of vegetative reproduction in trees include poplars (Populus), elms (Ulmus), and members of the genus Prunus. The capacity for vegetative reproduction is particularly pronounced in grasses and aquatic plants. Individuals that have originated via vegetative reproduction and are no longer physically connected to the parent plant are termed ramets. The collective group of ramets derived from a single genetically distinct individual (a genet) constitutes a clone.

In vascular plants, the organs of vegetative reproduction are most commonly shoots or modified shoots. This may involve simple fragmentation of ordinary above-ground shoots (such as root-pruned shoot tips in Rubus) or aquatic shoots (e.g., in Elodea), as well as runners (stolons, e.g., in Fragaria) and rhizomes (in many grasses). Alternatively, specialized multicellular propagules may be formed. Examples include vegetative buds (bulbils in terrestrial plants such as Allium and Polygonum viviparum; turions as overwintering buds in aquatic plants such as Elodea and Hydrocharis), root tubers (e.g., in Ranunculus ficaria), tubers on subterranean stolons (e.g., in the potato), small bulbs (e.g., in Galanthus), or bulbils replacing spikelets (e.g., in Poa alpina and P. bulbosa). Propagules may also be produced by algae, Lichens, and mosses. Shoot formation on roots followed by root fragmentation is a very common mechanism of vegetative reproduction in trees (root suckers). In some cases, leaves can also form plantlets for vegetative reproduction (e.g., in Asplenium and Kalanchoe; see Fig. 4.32). In algae, lichens, and mosses, the primary mechanisms of vegetative reproduction include cell division and budding (e.g., in yeasts), dissociation into single cells or thallus fragmentation, and the formation of specialized propagules.

The contribution of vegetative reproduction to population maintenance can be substantial. For instance, it has been estimated that more than 99 % of the individuals in a population of creeping buttercup (Ranunculus repens) originated through vegetative reproduction. Through vegetative reproduction, genetic individuals — genets — can attain remarkable size and occasionally achieve a surprisingly great age.

The Application of Molecular genetic techniques to determine genetic identity in studies of, for example, the North American aspen (Populus tremuloides) revealed that a single clone occupied 43 hectares and comprised approximately 47,000 ramets. Similarly, in herbaceous plants such as red fescue (Festuca rubra), individuals of the same clone have been found growing up to 220 m apart. Based on the current growth rates of this species, it can be calculated that such a clone reaches an age of several hundred to approximately 1,000 years. These findings demonstrate that the number of genetic individuals in a population can be vastly smaller than the number of physically independent individuals. Obviously, this profoundly affects the scale of genetic variation within the population.

Agamospermy refers to the formation of seeds without the participation of a sexual process. This phenomenon is known in representatives of about 34 families and is particularly frequent in the Asteraceae, Poaceae, and Rosaceae. Because opinions on the delimitation of agamospermous species vary widely, it is impossible to draw definitive conclusions regarding their exact number. The mechanisms of agamospermous seed formation are highly diverse. In representatives of the genera Citrus, Opuntia, or Nigritella, agamospermous embryos arise within the ovule tissue without the formation of an embryo sac. In this so-called adventive embryony (= sporophytic agamospermy), embryo formation is not precluded by a sexual process. Thus, both sexual and agamospermous embryos can occur on the same plant, sometimes even within the same seed. In this case, agamospermy is facultative. However, despite the absence of a sexual process for seed formation, pollination and fertilization are generally required (pseudogamy) to trigger the Development of the endosperm. In gametophytic agamospermy, an embryo sac with a sporophytic, i.e., unreduced, chromosome number is formed within the ovule. If this embryo sac arises independently of the normal haploid embryo sac, the phenomenon is termed apospory. If, however, such an embryo sac replaces the normal one, it is called diplospory. Both of these agamospermy mechanisms are most frequently associated with polyploidy. In both cases, the embryo may develop either from an unreduced, unfertilized egg cell (parthenogenesis) or from another cell of the unreduced embryo sac (apogamy)¹. Aposporous species such as Kentucky bluegrass (Poa pratensis), Neilreich's cinquefoil (Potentilla neumanniana), or goldilocks buttercup (Ranunculus auricomus), like species with adventive embryony, are facultatively agamospermous because they retain the capacity to form a typical embryo sac. Here, too, pollination and fertilization are usually required for endosperm development. In contrast, diplosporous species such as hawkweeds (Hieracium), dandelions (Taraxacum), reed grasses (Calamagrostis), or matgrass (Nardus) are most commonly obligately agamospermous, because the tissue from which embryos develop asexually replaces the tissue that would otherwise give rise to the embryo sac in sexual species. In diplospory, pseudogamy is generally not required for endosperm development.

¹ In Russian literature, the term "apogamy" (or more precisely "apogamety") is commonly used. — Ed. note.

The different mechanisms of agamospermy can be clearly distinguished based on their cytological processes. However, because various mechanisms may occur within closely related forms (e.g., Hieracium subg. Hieracium: diplospory; subg. Pilosella: apospory), it can be inferred that they are closely related in their evolutionary origin.

Methodologically, it is difficult to elucidate the GENETIC BASIS OF agamospermy. Agamospermous species sometimes retain residual sexuality, and they often coexist with closely related sexual species of the same ploidy level with which they can hybridize. Adventive embryony appears to be regulated by a single genetic locus. In the case of apospory and diplospory, at least two genes are reliably involved. In Taraxacum, for instance, one gene causes the suppression of meiosis during embryo sac development, whereas another gene induces parthenogenetic development of the egg cell. In various grasses (e.g., genera Panicum, Brachiaria, Tripsacum), gametophytic agamospermy is regulated by a single gene. However, Genome Mapping has shown that this "gene" may actually conceal a larger chromosomal region characterized by severely reduced or completely absent recombination. It remains unclear how many genes are actually contained within this region and participate in the regulation of agamospermy.

The expected complete absence of genetic variation in agamospermous species does not actually occur. The genetic variation observed in these plants can stem from several causes:

✵ Agamospermy is rarely strictly obligate, and at least occasional sexual reproduction can generate variation.

✵ The formation of the unreduced embryo sac in diplosporous species may initiate meiosis but fail to complete it, resulting in the formation of an unreduced so-called restitution nucleus.

✵ Somatic mutations may accumulate across successive generations.

✵ Somatic recombination, in the form of transposon-induced chromosomal rearrangements in somatic cells, likely occurs frequently in agamospermous species.

Despite these mechanisms generating genetic variation, its overall level in agamospermous species is greatly reduced compared to their sexually reproducing close relatives.

Agamospermy, combined with typically reduced genetic variation, causes even the most minor morphological differences to remain more or less constant across generations. Because phenotypic constancy serves as the basis for species delimitation under the morphological species concept (see 10.3.1), a vast number of agamospermous forms can be recognized as agamospecies (micro-species). For example, in the Central European flora, according to some authors, the genera Taraxacum and Hieracium encompass approximately 250 and 190 such micro-species, respectively.

10.1.3.4. Gene Flow and Life Form

The genetic similarity between mating individuals depends on the distance over which pollen and diaspores (spores, seeds, fruits) are dispersed. If pollen and diaspores were typically dispersed over only very short distances, the probability of mating between parents and offspring, or between offspring of the same individual—i.e., between genetically very similar individuals—would be extremely high. Consequently, the effect of genetic recombination would be minimal. As the transport distance of pollen and diaspores increases, the likelihood of mating between genetically distinct individuals rises. Because hereditary material is transported via pollen and diaspores, both phenomena can be grouped under the umbrella term "gene flow," provided that pollination leads to fertilization and that diaspores germinate and the resulting plants subsequently mate with other individuals. Given The Diversity of pollination and dispersal mechanisms (see 11.2), it is difficult to arrive at realistic estimates of gene flow distances.

Regarding pollination, it is well established that the frequency of pollination by a given individual's pollen decreases exponentially with distance from the source (Fig. 10.23), and that pollination most frequently occurs between individuals separated by distances ranging from a few decimeters to several tens of meters, and considerably less often over several hundred meters. Nevertheless, pollinators have occasionally been observed covering substantially greater distances. Some South American bees make foraging flights of up to 23 km while carrying out pollination. Fig wasps pollinating tropical figs regularly fly distances of 6 to 14 km, and vertebrate pollinators (such as birds and bats) are generally capable of covering greater average distances than insects.

Pollen transfer can be quantified directly—for example, in zoophily, by observing the movement patterns of pollinating animals, or in wind pollination (anemophily), by measuring the amount of deposited pollen at varying distances from its source. However, direct observation does not provide conclusive data on whether the observed pollination events actually result in fertilization. This is best determined by genetically comparing parents with their offspring, provided the genetic makeup of other potential parents is known. Experimentally established populations of monkeyflowers (Mimulus) (see Fig. 10.23), in which the genetic constitution of every individual was known, made it possible to identify the paternity of every seed produced within the population and thus analyze pollen-mediated gene flow in utmost detail. Studies on this species revealed that in animal-pollinated plants, the distance of gene flow is typically greater than the foraging flight distance of pollinators between two individuals. This can be explained by the fact that not all pollen gathered on a single flower is necessarily deposited on the next one; a portion of it may be carried significantly further. This phenomenon is known as “carry over.” Conversely, in wind-pollinated plants, the gene flow distance is usually shorter than the pollen dispersal distance. Here, too, the explanation lies in the observation methodology, because at a great distance from

the pollen source, its atmospheric density becomes very low, thereby drastically reducing the probability of pollination and fertilization.

Fig. 10.23. Distribution of bumblebee (Bombus sp.) visits to Primula veris in Northumberland (England) as a function of distance

Detailed knowledge of pollen dispersal distances is critical when assessing the potential risks associated with genetically modified crops grown in proximity to closely related, interbreeding wild species.

Genetically dissimilar individuals can become neighboring mating partners because diaspores are capable of dispersal over considerable distances. In principle, everything stated regarding pollen transfer also applies to diaspore transport. As the distance from the source individual increases, the density of produced diaspores decreases exponentially (Fig. 10.24), with seeds and fruits of flowering plants typically traveling distances ranging from a few decimeters to several hundred meters. Nevertheless, certain exceptions exist that hold evolutionary and biological significance.

Fig. 10.24. Abundance distribution of Verbascum thapsus seeds as a function of dispersal distance

For instance, it is well documented that volcanic oceanic islands (such as Krakatoa or Surtsey) were relatively rapidly colonized by plants. Extensive relatedness analyses combined with molecular clock dating based on genetic distances suggest that long-distance diaspore transport—even between continents and islands of the Southern Hemisphere—is a frequent occurrence on a geological timescale. Long-distance dispersal, for example between Australia/New Zealand and South America, can be considered proven for Taraxacum and Gentianella.

In the debate over self-fertilization versus cross-fertilization, it has been demonstrated that even occasional outcrossing can significantly contribute to the maintenance of genetic variation. The same applies to the potential role of occasional long-distance gene flow via pollen and diaspore transfer in maintaining or enhancing genetic diversity. Consequently, these rare and difficult-to-observe dispersal events are of profound importance.

Another facet of the interplay between gene flow and genetic variation within populations is the life form of the species under investigation. Lifespan, as a key component of the life form, is crucial because the exchange of individuals between populations is much more likely in perennials

than in populations of annuals. This leads to a substantial increase in genetic diversity among perennial species.

Based on their lifespan, plants are classified into annuals, biennials, and perennials. Among temperate annuals, a distinction is made between winter annuals and summer annuals (spring or non-wintering annuals). Whereas The life cycle of winter annuals typically begins in autumn and concludes in spring or early summer (spanning two calendar years yet lasting less than a full year), summer annuals complete their entire life cycle within a single growing season. The lifespan of perennials varies widely, ranging from just a few years to several millennia. In addition to lifespan, reproductive traits also shape different life forms. Plants that reproduce only once in their lifetime, regardless of how long they live, are termed monocarpic, whereas plants capable of repeated reproduction are called polycarpic.