MODERN BOTANY - P. RAVEN - 1990

SECTION III. GENETICS

CHAPTER 9. EUKARYOTIC GENETICS

Mutations

The independent assortment described above is based on differences between Gene alleles. How do such differences arise? The first answer to this question was provided by the Dutch scientist Hugo de Vries (see Fig. 9-13).

Class="center">Fig. 9-13. A. Hugo de Vries standing near Amorphophallus titanum, a plant belonging to the same family as the calla lily. Native to the jungles of Sumatra, this plant has one of the largest inflorescences among angiosperms. The photograph was taken in the botanical garden of the Agricultural College in Wageningen, the Netherlands, in 1932. B. Oenothera glazioviana; by studying this species, de Vries formulated METABOLISM/2.html">THE CONCEPT OF Mutations

The Emergence of the Concept of Mutations

In 1901, de Vries began studying traits in evening primrose plants (Oenothera glazioviana), which had naturalized in large numbers on coastal dunes in the Netherlands. He discovered that although the inheritance pattern in this plant was generally clear and predictable, traits that had not been observed previously in either parental line would nevertheless occasionally appear in the offspring by chance. De Vries hypothesized that these new traits were the phenotypic expression of gene changes. Furthermore, according to his hypothesis, the altered gene could be transmitted to offspring just like other genes. De Vries called these hereditary changes in a single gene allele mutations, and the organisms carrying them mutants.

Ironically, only two of the approximately 2,000 changes observed by de Vries in the evening primrose can be considered mutations in the modern sense of the phenomenon. All others resulted from new gene combinations or the appearance of extra Chromosomes rather than drastic alterations of any single gene. However, even though most of de Vries's Examples do not fit modern Structure/97.html">Definitions, his formulated concept of mutations and his understanding of the role that mutations play in variation were correct and remain of fundamental importance to this day.

Selection/21.html">Types of mutations

Any change in an Organism's hereditary material is called a mutation. Mutations can be either Changes in the coding sequence of genes or in the way Genetic information is organized (Chapter 8). The MAIN TYPES OF mutations are listed below.

1) Point mutations. Point mutations affect only one or a few NUCLEOTIDES and can be caused by chemical or physical damage to DNA. Mutagens such as ionizing radiation, various chemical substances, and ultraviolet rays typically induce point mutations, which frequently lead to Cancer in humans and animals. Point mutations can also occur at a low frequency As a result of base-mismatching during DNA Replication.

2) Deletions. Small chromosomal fragments may be lost, for example, upon X-ray irradiation, which usually leads to changes in an organism's traits. In nature, many deletions occur as a result of Crossing-over between sister chromatids.

3) Position effect. Genes do not always occupy a fixed position on a chromosome but can move. In Bacteria, small ring-shaped DNA molecules independent of the main chromosome—known as Plasmids—can integrate into the chromosome and replicate along with it. (The consequences of this process for the evolution of bacteria and Viruses are discussed in Chapters 11 and 12, and its significance for Introduction/32.html">Genetic Engineering in Chapter 30.) In both bacteria and eukaryotes, genes can move as small mobile chromosomal fragments (Transposons). In either case, A change in THE POSITION OF genes incorporated into transposons can disrupt the expression of neighboring genes and lead to an effect that we define as a mutation. The proximity of a gene to a heterochromatic region or to other regions controlling Gene Expression can similarly be the cause of a mutation.

4) Inversions and translocations. During the evolution of certain groups of organisms, chromosomal fragments can break out and reinsert into the chromosome in an inverted orientation, resulting in a reversed gene sequence. Such a change in gene sequence is called an inversion. Certain plant groups are characterized by rearrangements of another order, in which a fragment of one chromosome attaches to another. Such a change is called a translocation; translocations are frequently reciprocal. In the case of inversions and translocations, genes located in the translocated segment may be expressed differently in their new environment, resulting in a mutation. This also alters the potential course of segregation, with important consequences for the plant.

5) Changes in chromosome number. A mutation-like effect can also be associated with changes in chromosome number, which occur spontaneously and quite frequently, though such mutations are usually eliminated immediately. Under certain circumstances, whole chromosomes may be gained or lost, and sometimes an entire set of chromosomes may be added (the phenomenon known as polyploidy). The Evolutionary Significance of polyploidy will be discussed in Chapter 28. Phenotypic changes may be observed in all such cases.

Evolutionary Effects of Mutations

When mutations occur in haploid organisms, such as the fungus Neurospora or bacteria, the phenotypic changes caused by them immediately come under environmental control. If they are beneficial, natural selection will increase the number of individuals with such a phenotype in the population; if they are harmful, they will typically be quickly eliminated from the population. Other mutations may be more or less neutral and persist by chance; but overall, their effect on the organism is either positive or negative. The situation is different in a diploid organism. Each chromosome and all genes are present in duplicate, and a mutation in one of the homologs, even if it is unfavorable in a double dose, may have a much lesser effect or even be beneficial when present in a single copy. For this reason, such mutations can persist in a population. Mutant genes may even change their function, or selection within a population may modify them so that they become beneficial.

Although most mutations are harmful, the capacity for mutation is extremely important because it enables individuals and species to change and adapt to altering environmental conditions. Thus, mutations are The basis of evolutionary change. In eukaryotes, spontaneous mutations occur at a frequency of about 5x10^-6 per locus per Cell Division (i.e., one mutant gene at a given locus per 200,000 cell divisions); together with recombination, mutations create the vast store of variation necessary for evolutionary transformation through natural selection.

Gene Organization

Contrary to views that persisted for many years and in contrast to what has been found in bacteria, eukaryotic genes are typically present in multiple copies. Some genes are present in a single copy, but many have hundreds or even thousands of copies, which are sometimes clustered and sometimes scattered across one or more chromosomes. The main groups of duplicated genes are called satellite sequences, tandem clusters, transposons, and multigene families.

Satellite DNA consists of short sequences, such as ATAAT or ATATAAT, repeated a million times and making up approximately one-third of all Nucleic Acids in an organism. Almost all satellite DNA is clustered near centromeres and at the ends of chromosomes—in regions that remain tightly coiled and thus stain well at all Stages of the Cell Cycle. Satellite DNA apparently plays a structural rather than an informational role.

Tandem clusters are repeating sequences of genes encoding Proteins that The Cell requires in large amounts, as well as rRNA genes, which in most eukaryotes are represented by several hundred copies. Genes responsible for histone synthesis have from ten to several hundred copies. High-copy genes are located together in one or more clusters.

Transposons are gene sequences that are repeated thousands of times. They have a normal A-T and G-C base pair ratio and therefore a normal density. Transposons possess The ability to spontaneously change their Location on a chromosome while carrying along other genetic material; such shifts can have a substantial impact on gene expression. Such genetic changes in maize, reported by Barbara McClintock starting in the late 1940s (Fig. 9-14), allowed her to first postulate the existence of transposons. McClintock was awarded the Nobel Prize in 1984 for her research. Transposons have recently been discovered in both bacteria and eukaryotes.

Fig. 9-14. Barbara McClintock, who was awarded the Nobel Prize in 1984 for her pioneering work in genetics. She is holding an ear of corn of the type she used in her research on transposons

Multigene families — groups of genes found in most eukaryotes. They apparently arise through the duplication, rearrangement, and specialization of pre-existing genes and are of paramount importance in eukaryotic evolution.

Phenotype Determination

The outward appearance of any organism—its phenotype—is the result of a vast number of complex, interacting biochemical processes. Some of these occur only during the organism's development, while others continue throughout all stages of its life. A distinctive feature of plants is that these interactions never cease. Simple changes at the DNA level can profoundly affect an organism's appearance in ways that are both complex and unpredictable. When the science of genetics was still relatively young, genes were described by their most common phenotypic expressions, such as red petal color, pubescence, and so on. Later, however, it was discovered that a single gene can control an entire suite of traits—a phenomenon known as pleiotropy—and that most traits are controlled jointly by many genes, a phenomenon known as epistasis. In epistatic interaction, one gene alters the phenotypic expression of another, nonallelic gene. Essentially, no gene acts in isolation; their effects are always modified by the internal environment created by the interaction of thousands of genes.

We have come a long way in understanding the mechanisms of heredity and The Development of organismal traits. However, much remains to be learned; this field of science will remain central to botany for many years to come.