PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 6. PLANT REPRODUCTION

6.2. The Concept of Sexual Reproduction and the Sexual Process. Types of the Sexual Process

Sexual reproduction is defined as a type of reproduction based exclusively on the sexual process (spore formation is absent). Unfortunately, METABOLISM/2.html">THE CONCEPT OF the "sexual process" is often narrowed down to "Fertilization," which is far from being the same thing.

The sexual process encompasses the entire sequence of events that precede its final stage—fertilization. These include the sexual differentiation of individuals (into male and female) or Tissues; The formation of gametangia (sex Organs)—antheridia and archegonia (very rarely absent); the formation of sex Cells, namely male and female Gametes (sometimes unformed, in which case their function is performed by single-celled organisms themselves or certain cells of Multicellular Organisms); the movement of gametes; and the fusion of sexually differentiated gametes, known as fertilization. Fertilization of the egg (female gamete) results in the formation of a zygote and subsequently the embryo of a new daughter Organism. Undoubtedly, fertilization is one of the most critical Stages of the sexual process, but it is by no means the only one.

Typically, in most Algae and all higher plants without exception, the sexual process involves the formation of sex cells, or gametes. Gametes are specialized haploid cells that generally develop within gametangia. (Exceptions among higher plants include gymnosperms and angiosperms: gymnosperms lack antheridia, whereas angiosperms lack both antheridia and archegonia.)

In plants, with the exception of Fucales algae, gamete formation is preceded by mitosis. Only in brown Cyclosporeae algae (Fucus) do gametes form As a result of Meiosis, similar to the process observed in animals.

A characteristic feature that distinguishes gametes from spores is that for further development and the formation of a new organism, two sexually differentiated gametes must fuse—that is, The process of fertilization must take place.

Depending on whether typical sex cells are formed during the sexual process or their function is fulfilled by other structures, all types of the sexual process can be divided into gametogamy and agametogamy (Fig. 137).

Class="center">Fig. 137. Types of the Sexual process in plants

Gametogamy is characterized by the formation of gametes. The morphological features of gametes vary across different plant species, with the greatest structural diversity observed in algae. Early evolutionary stages saw The Emergence of PHYSIOLOGICAL AND BIOCHEMICAL differentiation of gametes. These gametes are morphologically indistinguishable, share the same size, and are equally motile due to flagella, yet they differ in biochemical properties and are designated as "+" and "-" gametes. They are referred to as isogametes.

With the advancement of the sexual process, morphological differentiation of gametes emerged; they began to differ not only in physiological and biochemical features, but also in size, shape, and degree of motility. Thus, evolution led to the appearance of anisogametes: smaller ones became designated as male, and larger ones as female. Male gametes that possess flagella and can move independently are spermatozoa, while those lacking flagella are sperms. Female gametes with low motility are called heterogametes, and non-motile ones are termed eggs (oocytes).

There are Three types of gametogamy: isogamy, heterogamy, and oogamy.

In isogamy, the resulting isogametes leave the gametangia and fuse in an aquatic environment. Isogamy was first described by the German botanist N. Pringsheim (1843–1894) in 1858 in the green alga Pandorina. This type of gametogamy is characteristic of primitive unicellular (most Chlamydomonadales) and thalloid (most Ulotrichales) algae.

In heterogamy, motile male and female gametes are produced, but the female gamete is larger and less motile than the male. Upon maturation, the gametes leave the gametangia and fuse in Water. Heterogamy was first observed and described by I. N. Gorozhankin in the green alga Chlamydomonas braunii. Heterogamy is a relatively rare type of the sexual process, typical of certain Chlorococcales (Protococcophyceae), Ulotrichales (Ulotrichophyceae), most Siphonales (Siphonophyceae), and some brown algae.

Oogamy is characteristic of highly organized algae and all higher plants without exception. In oogamy, the egg completely loses motility, significantly exceeds the male gametes in size, and develops within an oogonium (in algae) or an archegonium (in higher plants), usually as a single Cell, with fertilization occurring inside the female sex organ. (As an exception, in some algae, the eggs escape from the oogonium and are fertilized in the surrounding water.)

Oogamy was documented by several botanists in various algae: by the French botanist G. Thuret (1817–1875) in 1853 in Fucus (Phaeophyta); by German botanists N. Pringsheim in 1856 in Oedogonium (Chlorophyta) and F. Cohn (1828–1898) in 1856 in Sphaeroplea (Chlorophyta); and by I. N. Gorozhankin in 1875 in Eudorina (Chlorophyta). Oogamy represents the pinnacle of the Evolution of the sexual process, yet even here, water remains essential as a medium for the locomotion of spermatozoa.

How oppositely sexed gametes recognize each other in an aquatic environment is not yet fully understood, although pheromones are known in brown algae and have been actively studied since the 1980s. Sex pheromones are soluble chemical messengers that coordinate The activity of sex cells during the sexual process. Brown algal pheromones may attract spermatozoa to the egg or, conversely, repel them, contributing to species isolation. In Fucus, specific Oligosaccharides and various Glycoproteins have been identified On the surface of both eggs and spermatozoa. Species-level recognition occurs only when the sperm glycoproteins are complementary to those of the egg.

In agametogamy, typical sex cells are not formed. Agametogamy occurs in the forms of hologamy and conjugation. In hologamy, two haploid individuals fuse (such as Dunaliella or Asteromonas from the class Volvocophyceae, division Chlorophyta), which apparently undergo specific physiological and biochemical changes by the time of fertilization, enabling these individuals to function as gametes. Their fusion results in a zygote.

During conjugation, the contents of haploid cells in unicellular or multicellular filamentous algae belonging to the class Conjugatophyceae (division Chlorophyta) fulfill the function of gametes. Conjugation in filamentous forms can be scalariform, when cells of two heterothallic individuals conjugate (Spirogyra), or lateral, when Cells of the same individual conjugate (Mougeotia). As a result of conjugation, the diploid chromosome number is restored in one of the cells (physiologically the "female" one), and a zygote is formed.

Thus, the types of the sexual process in the plant kingdom are diverse. All types of gametogamy and agametogamy are found among various algae. Higher plants are characterized by a single type of the sexual process—oogamy—while gymnosperms and angiosperms exhibit its specialized variant, siphonogamy. In siphonogamy, male sex cells—spermatozoa (in cycads and Ginkgo) and sperms (in all other gymnosperms and angiosperms)—are delivered to the egg via a pollen tube, eliminating the requirement for free liquid water.

As noted above, the sexual process in some form is intrinsic to all plants, which cannot be said of sexual reproduction. The concepts of "sexual reproduction" and "the "sexual process" must not be equated.

Sexual reproduction in plants must be understood as a type of reproduction in which a diploid individual (or different individuals) undergoes

reductional (!) division of maternal gamete mother cells (gametic or terminal meiosis), leading to The Development of male and female gametes, fertilization, zygote formation, and the subsequent development of daughter individuals of the same species from these zygotes. Thus, the entire process of sexual reproduction in such plants is analogous to that in animals.

To date, sexual reproduction is known in only a few algae. It has been described for brown algae of the class Cyclosporophyceae, charophyte algae (division Charophyta), green algae of the class Conjugatophyceae (Сonjugatophyceae), as well as certain green algae of the class Siphonales.

In all other plants, reproduction occurs through a strictly defined combination of asexual reproduction (spore formation) and sexual processes, which alternate in a specific sequence throughout the plant life cycle, resulting in the formation of two generations (or two developmental phases): the sexual phase (gametophyte, or prothallium, haploid, haplophase) and the asexual phase (sporophyte, or diploid, diplophase).