Phycology - Kostikov I.Yu. - 2009-2013

Chapter 8. Life Cycles

The changes in life forms that an individual undergoes throughout its ontogeny constitute its life cycle. A life cycle encompasses all Developmental Stages of an individual between identical life forms (for example, from zygote to zygote, gamete to gamete, or sporophyte to sporophyte).

Class="center">Types of Life Cycles

Life cycles are generally divided into two main categories: those of Algae lacking a sexual process and those featuring a sexual process. Algae lacking a sexual process are called agamontic, and their life cycles are referred to as cyclomorphosis. In algae with a sexual process—the so-called euhagamous species—three primary types of life cycles are typically distinguished: haplontic, diplontic, and haplodiplontic.

Cyclomorphosis. A classic example of cyclomorphosis is The life cycle of Chlorella. This alga exhibits only two life forms: vegetative Cells and aplanospores. Cyclomorphoses are often quite complex. For instance, in most Chlamydomonas species, motile vegetative cells produce zoospores that grow into new motile vegetative cells. Under unfavorable conditions, these cells transform into akinetes or enter a dormant state, frequently retaining The ability to reproduce via non-motile hemizoospores; when environmental conditions improve, these resting cells can regenerate flagella and return to the motile state (Fig. 8.1).

Fig. 8.1. Scheme of cyclomorphosis in the agamontic alga Chlamydomonas callunae: 1-3 - vegetative cells (1 - motile, 2 - hemimotile, 3 - coccoid); 4 - resting Cell (akinete); 5-7 - transformation of vegetative cells and akinetes into sporangia; 8-10 - sporangia; 11 - release of zoospores from a sporangium formed from a motile cell; 12, 13 - release of hemizoospores from sporangia formed from a coccoid cell or akinete; 14 - zoospore; 15 - hemizoospore.

Life Cycles of Euhagamous Algae. Nevertheless, when discussing life cycles, biologists typically refer to the developmental cycles of species possessing a sexual process. The life cycles of sexually reproducing algae invariably involve at least three stages: a vegetative stage, Gametes (or vegetative cells performing their function), and a zygote. Life cycles are primarily named According to the chromosome sets (ploidy level) of their vegetative developmental stages. If the vegetative stage has a haploid chromosome Complement, the life cycle is termed haplontic; if it has a diploid complement, it is diplontic. If the life cycle alternates between vegetative stages with both Haploid and Diploid chromosome complements, it is considered haplodiplontic. The sequence of ploidy changes throughout a life cycle is referred to as the alternation of nuclear phases.

Vegetative stages (generations) are defined as those phases of ontogeny during which algae actively vegetate and produce reproductive cells. Reproductive cells refer to specialized cells involved in both Selection/8.html">Asexual and sexual reproduction—namely, spores and gametes. Depending on the type of reproduction (asexual or sexual), vegetative generations include the sporophyte (the spore-producing generation), the gametophyte (the gamete-producing generation), and the gametosporophyte (a generation capable of producing both gametes and spores).

A life cycle may comprise only a single generation (e.g., exclusively a gametophyte or a gametosporophyte), or different generations may alternate irregularly or regularly (e.g., a sporophyte is succeeded by a gametophyte, which in turn gives rise to another sporophyte). Accordingly, life cycles are classified as having no Morphology/12.html">ALTERNATION OF GENERATIONS, irregular alternation, or regular alternation. If alternating generations are morphologically similar (e.g., the gametophyte is outwardly indistinguishable from the sporophyte), this alternation is called isomorphic; otherwise (e.g., when the sporophyte is macroscopic and the gametophyte microscopic), it is termed heteromorphic.

The ploidy of generations—and consequently the type of life cycle based on the alternation of nuclear phases—is determined by the stage of the life cycle at which meiotic (reductional) division occurs. Meiosis halves the chromosome number. Thus, all developmental stages from meiosis up to the sexual process possess a haploid chromosome set, whereas all stages from the sexual process following Fertilization up to the next meiosis exhibit a diploid set.

Types of Meiosis and Alternation of Nuclear Phases

Meiosis (reductional division) can occur at one of three stages in the life cycle: 1) during the germination of the zygote—so-called zygotic reduction; 2) during gamete formation (gametic reduction); or 3) during spore formation (sporic reduction) (Fig. 8.2). In addition, instances are known where meiosis takes place in vegetative (somatic) cells (somatic reduction).

In zygotic reduction, the vegetative generation is always haploid, making the life cycle haplontic (e.g., in Chlamydomonas moewusii). Typically, the zygote undergoes a resting period prior to meiosis, during which preparation for reductional division takes place (Fig. 8.3: 1, 2).

In gametic reduction, the vegetative generation is always diploid, resulting in a diplontic life cycle (e.g., in species of the genus Fucus). In such cases, the zygote generally germinates without a resting period. Thus, the presence or absence of a resting period allows researchers to determine the type of meiosis and, consequently, the life cycle type. Exceptions to this rule are found exclusively within dinoflagellates (Fig. 8.3: 3, 4).

In sporic reduction, the diploid zygote germinates without a resting period into a diploid sporophyte. The sporophyte develops sporangia where meiosis occurs, subsequently forming haploid spores. These spores germinate into a haploid gametophyte that produces gametes. Syngamy of the gametes forms a diploid zygote, completing the cycle. Because the sporophyte is diploid and the gametophyte haploid in sporic reduction, the life cycle is haplodiplontic (Fig. 8.3: 5).

Thus, the type of reduction determines the life cycle Classification based on the nuclear Phases of the vegetative generations: haplontic, diplontic, or haplodiplontic.

Fig. 8.2. Possible types of meiosis (reductional division) and symbols for the MAIN STAGES OF the life cycle.

Fig. 8.3. Types of algal life cycles based on nuclear phase alternation and generation alternation: 1, 2 - haplontic cycle with zygotic reduction (1 - without alternation of generations, 2 - with alternation of generations); 3, 4 - diplontic cycle with gametic reduction (3 - without alternation of generations, 4 - with alternation of generations); 5, 6 - haplodiplontic cycle with sporic reduction (5 - alternating two generations: sporophyte and gametophyte; 6 - alternating three generations: gametophyte, sporophyte, and carposporophyte). Abbreviations: n - haploid chromosome set, 2n - diploid chromosome set, KS - carposporophyte, ksp - carpospores, ПС - resting period (PS). Other designations follow Fig. 8.2.

Alternation of Generations

Biological Prerequisites for the Evolution of Alternation of Generations. To perpetuate themselves in future generations, individuals of any species must fulfill several Functions: they must grow, colonize new territories, and maintain a certain level of genetic diversity. Accordingly, three main functions are distinguished: growth, dispersal, and recombination.

The growth function in algae is carried out primarily through Photosynthesis, whereas in colorless taxa, it is sustained by heterotrophic Nutrition. Dispersal is achieved through various pathways: in motile and amoeboid forms, via active locomotion of vegetative cells; in non-motile taxa, through passive transport of vegetative cells, multicellular thalli, or their fragments (via Water currents, wind, animals, etc.). However, in the vast majority of microscopic forms and a significant portion of macroscopic ones, the dispersal function is executed by specialized asexual reproductive cells—spores. The recombination function is accomplished primarily through the sexual process.

Gametosporophyte and life cycles without alternation of generations. In algae, scenarios where a single generation fulfills all three functions—growing, and producing both spores and gametes—are quite common. Such a "universal" generation is called a gametosporophyte (Fig. 8.4: 1).

Fig. 8.4. Evolution and complexity of life cycles with alternation of generations. 1 - cycle without alternation of generations featuring a gametosporophyte stage; 2, 3 - cycles with alternation of a gametosporophyte and a sporophyte; 4, 5 - cycles with irregular alternation of a gametophyte and a sporophyte (4 - monogenetic, 5 - digenetic); 6 - cycle with regular alternation of a gametophyte and a sporophyte; 7 - cycle with regular alternation of a gametophyte, a carposporophyte, and a sporophyte; 8, 9 - cycles without alternation of generations resulting from the disruption of cycles with alternation of generations. GS - gametosporophyte, G - gametophyte, S - sporophyte, KS - carposporophyte, Z - zygote.

For instance, this exact pattern can be observed in the unicellular green alga Chlorococcum, where zoospores are simultaneously capable of functioning as gametes. However, attempting to combine both dispersal and recombination functions within a single cell leads to a certain evolutionary trade-off: on the one hand, spores require the largest possible energy reserves, limiting their total number; on the other hand, gametes must primarily be numerous, and therefore cannot contain sufficient reserve assimilates. Averaging out the size and number of such "universal" reproductive cells ultimately reduces the efficiency of both dispersal and recombination.

Consequently, in most cases, the gametosporophyte produces two distinct types of reproductive cells that differ both functionally and morphologically: numerous gametes and large, relatively scarce spores. For example, in species of the genus Chlamydomonas, which exhibit an isogamous sexual process, the unicellular monad gametosporophyte can produce either 2–4 large zoospores or 8–16 smaller isogametes.

Sporophyte and The Emergence of alternation of generations. Further specialization, which enhances the efficiency of both dispersal and recombination functions, leads to the appearance of an alternation of generations: alongside the generation that continues to carry out both recombination and dispersal—the gametosporophyte—a new generation emerges, the sporophyte, which exclusively executes the outcomes of a successful fertilization event (Fig. 8.4: 2; 3).

The gametosporophyte expends a significant percentage of its energy and material reserves on gamete production. Yet, this yields only a small number of zygotes, as the vast majority of gametes fail to find a mating partner and perish. For example, in the green alga Volvox, given an approximately equal number of anteridia and oogonia, there are 64 spermatozoa per single egg cell. Naturally, 63 of these will not participate in fertilization and will die. In the genus Ulva, every successful sexual act is accompanied by several thousand uncopulated gametes; in the genus Chara, the number of spermatozoa formed to fertilize a single egg cell reaches about 40,000. Thus, each zygote bears an extremely high "cost of production." Therefore, a more advantageous strategy is not the direct germination of the zygote into a gametosporophyte, but rather into a sporophyte—a specialized "economizing" generation that avoids wasting limited resources on doomed gametes and instead merely multiplies the outcomes of a successful sexual process.

In the simplest cases, the sporophyte produces spores that subsequently develop into a gametosporophyte (e.g., in the genus Ulothrix). Such sporophytes are typically unicellular (Fig. 8.4: 2).

In more complex cases, the sporophyte produces spores that subsequently germinate back into a sporophyte—meaning There is a multiple reproduction of the consequences of a single successful fertilization event. Only under specific conditions (such as abrupt changes in salinity, Temperature, or the concentration of certain substances in the surrounding environment) does the sporophyte produce spores that germinate into a gametosporophyte (Fig. 8.4: 3).

Gametophyte. Cycles with the alternation of sporophyte and gametophyte. The next phase in the complexification of the life cycle is associated with the reduction in the gametosporophyte's ability to produce specialized asexual reproduction cells. This led to the emergence of a generation specialized solely in securing the recombination function—the gametophyte. Cycles featuring the alternation of sporophyte and gametophyte in algae can manifest as variants with either irregular (unstable) or regular alternation of generations.

Under irregular alternation of generations, the sporophyte and gametophyte succeed one another irregularly. For instance, in the brown alga Ectocarpus, the sporophyte produces zoospores in plurilocular sporangia that grow back into sporophytes, whereas unilocular sporangia yield zoospores that germinate into gametophytes. Thus, the sporophyte does not always give rise to a gametophyte, which accounts for the instability of the alternation of generations (Fig. 8.4: 4).

Irregular succession of generations can also be driven by the ability of gametes to germinate parthenogenetically into new gametophytes. This phenomenon is quite common among brown and siphonaceous green algae.

Furthermore, unstable generation shifts can result from the polyvariant developmental Pathways of the zygote. For example, in the marine alga Bryopsis, the zygote can germinate into either a sporophyte or a gametophyte (Fig. 8.4: 5). The developmental cycle observed in this case is known as a digenetic cycle. The Factors Determining the specific pathway of zygote germination remain poorly understood.

The most advanced cycles are considered to be those displaying complete functional specialization of generations, where sporophytes invariably give rise only to gametophytes, and vice versa—that is, cycles with regular alternation of generations. Interestingly, these exact cycles are characteristic of all Higher Plants and those algae currently considered evolutionarily most advanced (Fig. 8.4: 6).

Life cycles with the alternation of three generations. A distinctive type of life cycle with regular alternation of generations is found in red algae, where three vegetative generations—the gametophyte, carposporophyte, and sporophyte—regularly succeed one another rather than two (Fig. 8.3: 6; 8.4: 7). This cycle type is examined in detail in the chapter dedicated to red algae.

Disruption of cycles and abrupt speciation

It is now considered established that transformations of life cycles with irregular alternation of generations have, at least in some cases, led to abrupt (saltational) speciation. Such transformations most frequently occurred either As a result of the uncoupling of the life cycle due to the secondary loss of the capacity for sexual reproduction (Fig. 8.4: 8), or through the loss of the zygote's ability to germinate into a sporophyte within a digenetic developmental pathway (Fig. 8.4: 9).

Cycle disruption via the loss of sexual reproduction. A classic example of this phenomenon can be observed in brown algae of the genus Cutleria. For instance, Cutleria adspersa is a fairly widespread species in the Mediterranean Sea. It possesses a diplohaplophasic developmental cycle with sporic reduction and an irregular alternation of sporophyte and gametophyte. The gametophytes are annual, typically forming small tufts up to 10–15 cm high. The sporophytes are mostly perennial, appearing as a thick crust spread over the substrate, and were previously described as an independent genus, "Algaozonia".

Cutleria adspersa is also found in the Black Sea, but exclusively in the form of gametophytes, propagating itself through the parthenogenetic germination of gametes without any sexual process taking place. Meanwhile, only Cutleria sporophytes inhabit the western Mediterranean. Experiments have established that they reproduce via zoospores, with meiosis failing to occur prior to zoospore formation.

Thus, within the Mediterranean basin, Cutleria adspersa exists as three disconnected populations: a Black Sea population represented solely by gametophytes (the "Cutleria" form), a western Mediterranean population represented exclusively by sporophytes (the "Algaozonia" form), and an eastern Mediterranean population predominantly represented by sporophytes ("Algaozonia") that occasionally still produce gametophyte stages ("Cutleria"). This fact is interpreted as an instance of speciation characterized by the abrupt emergence of two new species that are morphologically and biologically distinct yet closely related.

Saltational speciation resulting from the uncoupling of digenetic life cycles can be witnessed in Bryopsis. This genus is characterized by a diplophasic life cycle with gametic reduction. Notably, within the genus, there are species that exhibit an alternation of sporophyte and gametophyte alongside digenetic zygote germination, as well as species represented exclusively by gametophytes.

For instance, in Bryopsis halimeniae, the gametophyte resembles a system of pinnately branched "shoots," whereas the sporophyte is a system of nearly unbranched filaments. The sporophyte was previously described as an independent species of the genus Derbesia. The zygote formed following gamete copulation can germinate into either a sporophyte ("Derbesia") or a gametophyte ("Bryopsis"). The sporophyte produces zoospores that can likewise either regenerate the sporophyte generation or germinate into a gametophyte.

Concurrently, certain species of the genus Bryopsis lack the sporophyte stage altogether and reproduce exclusively via sexual pathways—meaning the zygote invariably germinates only into a gametophyte. Several species of the genus Derbesia are also known to lack sexual reproduction, propagating solely through asexual means via zoospores. It is believed that such morphologically and reproductively diverse representatives as monogenetic Bryopsis and true Derbesia are closely related, having originated from the loss of the zygote's ability to develop into a sporophyte, i.e., through the uncoupling of the digenetic cycle with alternation of generations.

Today, life cycle transformations provide an explanation for cases where molecular-phylogenetic reconstructions reveal close relationships among species, genera, and even families that differ fundamentally in morphology and reproduction, and were previously regarded by classical systems as unrelated or only distantly related.

Isomorphic and heteromorphic shifts in developmental forms

When there is an alternation of generations, the gametophyte (or gametosporophyte) and the sporophyte can be morphologically similar, known as isomorphic, or substantially different, known as heteromorphic. An example of a species with isomorphic generations is the green marine alga Ulva: both its sporophyte and gametophyte appear as thin, folded blades along the margins and are morphologically indistinguishable (Fig. 8.5: 1).

Fig. 8.5. Vegetative stages of Ulva and Laminaria. 1 - Ulva (the sporophyte and gametophyte are morphologically identical, both appearing as a macroscopic green blade); 2-4 - Laminaria (2 - macroscopic sporophyte, 3 - microscopic female gametophyte, 4 - microscopic male gametophyte).

A classic example of a species with heteromorphic generations is Laminaria. The sporophyte of kelp is macroscopic, shaped like a blade with a stem-like lower portion and rhizoids; the gametophyte is microscopic, resembling a short, branched filament (Fig. 8.5: 2-4).

There is currently no consensus as to whether isomorphic or heteromorphic developmental changes are evolutionarily primary. However, there is a general agreement that the heteromorphic alternation of generations, "honed" in the algal world, proved to be the most evolutionarily successful, and it is largely associated with macroevolution in the realm of higher plants.

Sexual Dimorphism

Sexual dimorphism in algae, as in other eukaryotic groups, consists of morphological differences between individuals of different sexes (e.g., male and female). Sexual differentiation in algae is a multifaceted phenomenon, observed both at the level of gametes (or vegetative cells that perform the functions of gametes in somatogamous sexual processes - see chapter **), and at the level of vegetative generations, including not only those that directly produce gametes, but even at the sporophyte level.

Species in which the same individual is capable of producing gametes of both sexes are called monoecious or hermaphroditic. For example, in the water net (Hydrodictyon), the same cell produces both "male" and "female" gametes, making it bisexual (monoecious). Another example of a monoecious plant is water felt (Vaucheria): in Vaucheria, both spermatozoa and egg cells are produced by the exact same plant.

Species in which male and female gametes are produced on different individuals are called dioecious (gonochoric). An example is Laminaria - in species of this genus, male gametes (spermatozoa) are produced on a filamentous male gametophyte consisting of small cells, while female gametes (egg cells) are formed on a female gametophyte composed of relatively large cells (Fig. 8.5: 3, 4).

When male and female plants differ noticeably in appearance, this phenomenon is termed sexual dimorphism. For instance, sexual dimorphism is expressed to some extent in Laminaria, given that the Cells of the female and male gametophytes differ in size.

In algae, there are known instances where female and male individuals are entirely dissimilar, making sexual dimorphism extremely pronounced. In particular, this phenomenon can be observed in a significant portion of Oedogoniales - among species belonging to the so-called nannandrous forms (from the Greek "nanos" meaning dwarf, and "andros" meaning male) (Fig. 8.6).

In these algae, sexual dimorphism and dioecy manifest in the generations that produce gametes (the female gametosporophyte and the male gametophyte, or nannandrum). Furthermore, dioecism without sexual dimorphism occurs in the spore-producing generations (the female gametosporophyte and the male sporophyte) as well as in the zoospores formed from the zygote.

Fig. 8.6. Life Cycle of nannandrous species of the genus Oedogonium: zoospores (zsp) and oogonia are formed on the filamentous female gametosporophyte (GS); zoospores and special male spores, known as androspores (ansp), are produced on the male sporophytes (S). Upon release from the sporangium, the androspores settle on the female gametosporophyte near the oogonium and germinate into a dwarf male gametophyte, the nannandrum (G). Two spermatozoa are formed in the apical cell of the nannandrum, one of which penetrates the egg cell through a rupture in the oogonial wall and fertilizes it. The zygote (Z) enters a resting stage. Following the rest period, meiosis occurs within the zygote, forming four zoospores, two of which subsequently germinate into the female gametosporophyte, while the other two develop into the male sporophyte.

Diversity of Life Cycles

The Diversity of types of sexual reproduction, reduction division, alternation of generations, developmental shifts, sexual dimorphism, and numerous instances of parthenogenesis give rise to a vast variety of life cycles in algae. The complexification of life cycles played a crucial role in algal evolution at the medium and low taxonomic ranks, and within different divisions, the primary trends in life cycle evolution were generally distinct.

Thus, in certain algal divisions, life cycles are quite uniform. For example, in Rhaphidophyta, Eustigmatophyta, Dictyochophyta, and Glaucocystophyta, sexual reproduction has not been observed, and the life cycle represents a cyclomorphosis, albeit sometimes quite intricate.

Among eugamic forms across several divisions, the life cycle proceeds without alternation of generations and with a fixed type of chromosome reduction. For instance, in eugamic Euglenophyta, Xanthophyta, and Chrysophyta, reduction is zygotic and the life cycle is haplontic. A specific feature of the life cycle in the latter division is that the gametic nuclei do not fuse immediately after copulation; instead of a typical zygote, a specialized binucleate zygotic cyst is formed, which enters a resting stage.

Nuclear fusion occurs only after the resting period concludes. Meiosis takes place immediately within the newly formed zygote Nucleus. In Bacillariophyta, the life cycle is always diplontic, featuring gametic reduction and a specialized stage known as the auxospore.

In Dinophyta, an interesting feature of the developmental cycle is that the zygote, even within the same species, can germinate either with or without a reductional division; in other words, the type of reduction is not rigidly fixed, unlike in representatives of other divisions.

Complex cycles with the alternation of two generations are found in only four algal divisions: Phaeophyta, Haptophyta, Chlorophyta, and the primitive Rhodophyta. In evolutionarily advanced red algae of the class Florideophyceae, the life cycle involves the alternation of three generations: the sporophyte, the gametophyte, and the carposporophyte. Interestingly, with the exception of Haptophyta, all of these divisions contain numerous species with complex morphological structures.