PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 6. PLANT REPRODUCTION

6.3. The Concept of Life Cycles and Alternation of Generations in Plants

METABOLISM/2.html">THE CONCEPT OF the Morphology/12.html">ALTERNATION OF GENERATIONS originated from the work of the Polish botanist M. Leszczyc-Sumiński (1820–1898), who established that the developmental cycle of ferns comprises two phases: sporiferous and sexual. On the prothallium of a fern—which he considered to be an intermediate yet fully independent entity between the spore and the young fern plant—he discovered sex Organs: antheridia containing spermatozoa and archegonia containing an egg Cell. This was a major scientific breakthrough. His book The Developmental History of Ferns was published in Berlin in 1848.

W. Hofmeister (1824–1877), a German botanist and professor at the universities of Heidelberg and later Tübingen, highly praised the discovery of M. Leszczyc-Sumiński. Inspired by the broad scientific Prospects opened up by this work, he embarked on comparative embryological studies of archegoniate plants.

In 1849, W. Hofmeister published his first work, in which he introduced the Concept of the alternation of generations in plants. Subsequently, W. Hofmeister made a monumental discovery: he concluded that bryophytes, pteridophytes, and gymnosperms undergo a regular succession (alternation) of two generations—sexual and asexual. These two generations differ from each other in shape, mode of Nutrition, and biological and morphological features. He presented his views on the Ontogeny of the studied taxa in his seminal work Comparative Researches on the Germination, Development, and Fructification of the Higher Cryptogamia and The formation of Seeds in the Coniferae, published in Leipzig in 1851.

In a series of papers published in the early 20th century (1911, 1927), Professor L. I. Kursanov of Moscow University described the alternation of generations in green and brown Algae.

Thus, a life cycle is a developmental sequence characteristic of various taxa, spanning from a specific stage to its repetition. For example, in seed plants, The life cycle extends from seed to seed; in lycophytes and other spore-bearing plants, from spore to spore. In the Life Cycle of higher plants, either the gametophyte (exclusively in bryophytes) or the sporophyte (in all higher spore-bearing plants, as well as gymnosperms and angiosperms) may be dominant.

A diagram of the life cycle typical of homosporous ferns is shown in Fig. 138. As seen from the diagram, the spore formation process in the life cycle is succeeded by the sexual process, and consequently, the asexual generation (sporophyte) is succeeded by the sexual generation (gametophyte).

Class="center">Fig. 138. Diagram of the life cycle in homosporous ferns (heterophasic heteromorphic alternation of generations)

In lycophytes, horsetails, and ferns, the sporophyte and gametophyte exist independently of each other, meaning they are spatially separated and represent two generations of the same species.

In bryophytes, the gametophyte is dominant. The sporophyte develops on the gametophyte and partially derives its nutrition from it. In gymnosperms and angiosperms, which are characterized by heterospory, the sporophyte is dominant. The gametophytes are formed on the maternal organisms, develop at the expense of the sporophyte, are strongly reduced, and never leave the maternal plant.

Thus, a generation should be understood as the aggregate of organic forms arising from a reproductive process and genetically linked as ancestors and descendants. The phenomenon associated with the succession of reproductive processes and resulting in the formation of morphologically, cytologically, and functionally distinct forms within the life cycle is called the alternation of generations, or succession of developmental phases.

6.3.1. GENERAL PATTERNS OF the Alternation of Generations (Succession of Developmental Phases)

The morphological features of sporophytes and gametophytes in representatives of various taxonomic groups, their interrelation, and the patterns governing the succession of developmental phases (alternation of generations) vary.

The alternation of generations can be facultative or obligate.

In primitive algae, the alternation of generations is facultative and is most commonly associated with environmental shifts (e.g., drops in Water Temperature, drying of a water body). For instance, the freshwater filamentous alga *Ulothrix* can repeatedly produce 4-flagellate zoospores which, after swimming in the water for some time, settle to the bottom, lose their flagella, and grow into new heterothallic filaments of *Ulothrix*. In autumn, isogametes are formed within the Cells (single-celled gametangia) of *Ulothrix*, and Fertilization results in the formation of a zygote. It develops a thick wall, settles to the bottom, and persists there until spring.

Schematically, the facultative alternation of generations in *Ulothrix* appears as follows: n... n.... n... 2n... n... n, etc.

However, in the vast majority of algae and in all higher plants without exception, the life cycle exhibits a strictly regular succession of spore formation and the sexual process, involving an obligate (mandatory) alternation of generations. Schematically, this can be represented as: n... 2n... n... 2n... n... 2n if the gametophyte is dominant in the life cycle; or 2n... n... 2n... n... 2n... n if the sporophyte is dominant.

The transition from sporophyte to gametophyte occurs As a result of the reduction division (Meiosis) of sporocytes (spore mother cells), which takes place within sporangia and precedes spore formation. Spores give rise to the gametophyte, on which reproductive cells—Gametes—are formed. The transition from gametophyte to sporophyte is the result of the fertilization of an egg cell by a spermatozoid (in higher spore-bearing plants) or a sperm cell (in highly organized gymnosperms and angiosperms), resulting in the formation of a zygote, the restoration of the diploid chromosome set, and the subsequent Development of the sporophyte from the zygote.

The alternation of generations can be homophasic or heterophasic. These terms were introduced by P. Hartmann in 1939.

In homophasic alternation of generations, spore formation and the Sexual process in primitive haploid algae occur within the same nuclear phase; in other words, the succession of reproductive processes does not entail Changes in the nuclear phase. For example, zoospores and isogametes in *Ulothrix* are formed during different periods via mitosis on the same heterothallic haploid filaments (Fig. 139).

Fig. 139. Diagram of homophasic alternation of generations in *Ulothrix* (*Ulothrix sp.*)

In heterophasic alternation of generations, the shift in the reproductive process (or, rather inaccurately termed the mode of reproduction, as W. Hofmeister believed) is always accompanied by A change in the nuclear phase; that is, spore formation occurs on the sporophyte and is accompanied by a reduction in the chromosome number,

whereas gamete formation takes place on the gametophyte, and the chromosome number is restored as a result of fertilization (see Fig. 138).

Heterophasic alternation of generations can be either isomorphic or heteromorphic. In isomorphic alternation, the gametophyte and sporophyte are morphologically indistinguishable, though they differ in their cytological, physiological-biochemical, and functional properties (Fig. 140). Isomorphic alternation is quite rare and occurs exclusively in algae. It is characteristic of green algae (Ulva, Enteromorpha, Cladophora), as well as representatives of several orders of brown algae and most red algae.

Fig. 140. Diagram of heterophasic isomorphic alternation of generations in Ulva (Ulva sp.)

In highly organized algae (brown, some green, and red) and all vascular plants without exception, the alternation of generations is heterophasic and heteromorphic. This means that in most plant organisms, the gametophyte and sporophyte exhibit clear morphological differences In addition to cytological, physiological-biochemical, and functional ones (see Fig. 138).

The ratio of the sexual and asexual generations varies among different algae and higher plants. Depending on the level of development of the gametophyte and sporophyte, the alternation of generations can be haplogomophasic, diplogomophasic, haplodiplophasic, and diplohaplophasic.

In species with a haplogomophasic developmental cycle, the gametophyte is dominant, and only the zygote is diploid (primitive green algae). Such individuals are characterized by an initial, or zygotic, type of meiosis, meaning that only the zygote Nucleus undergoes reductional division, followed by cytokinesis and the formation of aplanospores.

In diplogomophasic alternation of generations, the sporophyte is dominant, and only the gametes are haploid (diatoms — Bacillariophyta; among brown algae — Representatives of the order Fucales; and siphonous green algae). In this case, meiosis precedes gamete formation. This type of meiosis is called terminal, or gametic. It is typical of animals and extremely rare in plants.

In haplodiplophasic alternation of generations, the gametophyte is dominant. Among algae, the sporophyte exists independently and may be morphologically identical to the gametophyte (Ulva), have a microscopic Structure (Acrosiphonia from Chlorophyta), or develop directly on the gametophyte (Phyllophora brodiei from Rhodophyta). Among higher plants, only bryophytes are characterized by a haplodiplophasic developmental cycle (Fig. 141). Here, the sporophyte, shaped like a capsule, always develops on the gametophyte and partially derives nutrition from it, although in the early Selection/3.html">Stages of development it is green and therefore capable of Photosynthesis. Bryophytes are homosporous plants; however, their gametophytes (i.e., the plants themselves) can be either monoecious or dioecious. In monoecious plants, antheridia and archegonia form on the same plant (Sphagnum squarrosum, S. compactum, Mnium cuspidatum). In dioecious plants, sex organs of a single type (either antheridia or archegonia) form on separate individuals (Marchantia polymorpha, Sphagnum magellanicum, Polytrichum commune).

Fig. 141. Diagram of haplodiplophasic alternation of generations in bryophytes

In the diplohaplophasic type of development, the sporophyte dominates the life cycle of higher plants. Gametophytes can exist independently of sporophytes (clubmosses, horsetails, ferns), or they develop entirely on the sporophytes (gymnosperms, angiosperms).

In both haplodiplophasic and diplohaplophasic life cycles, meiosis precedes spore formation. This type of meiosis is known as sporic, or intermediate. It is characteristic of all higher plants without exception and is also found in many algae.

Plants with haplodiplophasic and diplohaplophasic developmental cycles can be either homosporous or heterosporous.

In heterosporous higher plants (such as Selaginellaceae and Salviniaceae), the prothalli are heavily reduced compared to homosporous ones (Fig. 142). Nevertheless, this phenomenon is of great biological significance.

Fig. 142. Sporophyte, sporangia, spores, and gametophytes of Selaginella (Selaginella sp.): A — general view of the sporophyte; B — microsporangium with microspores; C — microspore; D — male gametophyte; E — megasporangium with megaspores; F — megaspore; G — female gametophyte: 1 — microspore wall; 2 — megaspore wall

In Selaginellaceae and Salviniaceae, the prothalli develop almost entirely within the spore wall; consequently, they are reliably protected from the external environment (adverse conditions) and form rapidly during ontogeny, which is highly significant from an evolutionary perspective.

In seed plants (which are characterized by heterospory), the extreme reduction of the male gametophyte (pollen grains, pollen) is accompanied by an expansion of its Functions. Along with the typical function of gamete formation (sexual function), the male gametophyte assumes a transport function. Male gametes are delivered to the egg cell (into the archegonium in gymnosperms or the embryo sac in angiosperms) via a pollen tube, which is formed by the male gametophyte.

In the developmental biology of heterosporous plants, three features deserve special attention:

1) physiological and morphological differentiation of spores, with the megaspore performing the function of species dispersal in higher heterosporous plants;

2) reliable protection of gametophytes from adverse conditions;

3) reduction of gametophytes and, as a consequence, their rapid development during ontogeny.

In the course of evolution in seed plants, the megaspore loses its dispersal function, which is now carried out by seeds. They exhibit more pronounced adaptations to terrestrial existence: extreme reduction of gametophytes, especially male ones; the appearance of sperm cells (spermatozoids in Cycas, Ginkgo) alongside non-motile sperms (in conifers and all angiosperms); and the formation of a pollen tube during male gametophyte germination, leading to siphonogamy, which renders the fertilization process independent of liquid water.

The ultimate result of The Emergence of heterospory is a shortening of the plant Organism's life cycle.

6.3.2. Concepts of the Evolution of Plant Life Cycles

While various taxa exhibit certain specific features in their life cycles, the general main directions in the evolution of plant life cycles emerge quite clearly.

1. In the Cytology/cytology/16.html">Early stages of evolution, all individuals performed both vegetative and reproductive functions. Sexual and asexual processes were of equal importance; the same individual could potentially produce both spores and gametes (see Fig. 139).

2. Individuals gradually differentiated in their reproductive modes into sporophytes and gametophytes, though they remained morphologically indistinguishable (see Fig. 140).

3. Morphological differences between the gametophyte and sporophyte began to appear, but the gametophyte remained dominant, with the sporophyte developing upon it (see Fig. 141).

4. The sporophyte became independent of the gametophyte; that is, the sporophyte overcame its dependence on the gametophyte (see Figs. 134, 135, 136).

5. The dominant position of the sporophyte became established in the life cycle. Initially, it was characterized by isospory (homospory), with the spore serving the function of dispersing the species.

6. Heterospory emerged, leading to the sexual differentiation of gametophytes. The function of species dispersal was now carried out exclusively by the macrospore (megaspore) (see Fig. 142).

7. Associated with heterospory was a reduction in gametophytes, which accelerated their development. At the same time, the Functions of the male gametophyte expanded: in addition to forming gametes and performing sexual functions, it acquired a transport function (delivering male gametes to the egg cell).

8. A strict sequence of alternation between spore formation and the sexual process was gradually established.

9. In highly organized vascular plants (gymnosperms and angiosperms), the gametophytes reached an extreme degree of reduction and developed entirely on the sporophytes. The macrospore lost its function of species dispersal, never leaving the parent plant.

10. Based on the combination of spore formation and the sexual process, the seed arose—a complex structure in which the rudiment of a new organism (a new sporophyte) formed while retaining remnants of the maternal sporophyte. In angiosperms, these remnants comprise the seed coat and, in some plants, a nutritive tissue called perisperm (such as in Caryophyllaceae and Chenopodiaceae). In gymnosperms, the seed also contains a well-developed female gametophyte transformed into trophic tissue (the primary endosperm).

The alternation of generations holds great evolutionary and biological significance.

The major Evolutionary Significance of the alternation of generations lies in the fact that the shift in reproductive processes led to the dominance of the sporophyte in the life cycle—an organism of highly complex Organization, optimally adapted to life in diverse environments.

The Biological Significance of the alternation of generations is that, progressively, with the increasing morphophysiological complexity of the sporophyte and the reduction of the gametophyte, the sexual process was perfected. Its final stage—fertilization—became independent of liquid water droplets, which was crucial for existence in terrestrial environments.