Phycology - Kostikov I.Yu. - 2009-2013

Chapter 7. Reproduction in Algae

The reproduction of organisms producing similar daughter offspring is called reproduction. Reproduction in Algae can occur either asexually or sexually. During asexual reproduction, daughter individuals inherit a genome identical to that of the parent Organism, except in cases where Gene or chromosomal Mutations occur. In sexual reproduction, the genomes of daughter organisms differ from the parental ones, because The formation of the daughter generation is usually preceded by processes of Genetic information exchange between different individuals As a result of Crossing-over, which takes place after the sexual process. The sexual process, along with the associated crossing-over and Meiosis, determines recombinant Variability and contributes to an increase in the level of genetic heterogeneity in populations. The level of recombinant variability can also increase as a result of genome segment exchange during parasexual processes (for example, conjugation or transformation in prokaryotes); however, parasexual processes are not directly related to The production of daughter individuals and therefore are not classified as modes of reproduction.

Asexual reproduction in algae is divided into two main types: a) vegetative, carried out through the division of vegetative Cells or via fragments of the vegetative body; b) reproduction by means of specialized cells – spores, sometimes referred to as true asexual reproduction or sporulation.

Vegetative Reproduction. In unicellular algae, vegetative reproduction occurs mainly through Cell Division into two. In multicellular and colonial representatives, vegetative reproduction is more frequently associated with the fragmentation of thalli and colonies. Fragmentation can be caused by mechanical factors (e.g., wind-driven wave impacts, currents), or it can occur as a result of the death of parts of thalli or colonies (for example, the death of individual intercalary cells in multicellular trichomes of cyanobacteria often leads to The breakdown of the trichome into reproductive fragments known as hormogonia). In a few algae, vegetative reproduction can be carried out using specialized PARTS OF THE body – propagules (the brown alga Sphacellaria) or brood tubers (green algae of the order Charales). A special variant of vegetative reproduction is reproduction via akinetes. Akinetes are cells formed from ordinary vegetative cells under deteriorating environmental conditions. Akinetes possess a thickened Cell wall, contain abundant nutrient reserves, and, unlike normal vegetative cells, are capable of entering a state of anabiosis or cryptobiosis under unfavorable conditions, thereby surviving adverse periods. Upon the improvement of vegetative conditions, akinetes germinate into new vegetative individuals.

Asexual reproduction via sporulation differs from vegetative reproduction in that it is carried out by specialized cells – spores, which are formed within sporangia. Sporangia in algae are unicellular, and the number of spores in a single sporangium can range from one (for example, in the filamentous green alga Oedogonium) to several dozen or even hundreds (in many brown algae).

In accordance with their capacity for active movement, spores are subdivided into motile flagellated ones (zoospores), non-motile ones with cell walls (hemizoospores, aplanospores, and autospores), and sluggish amoeboid ones lacking flagella (mono-, bi-, and tetraspores).

Zoospores always possess flagella, and often an eyespot (stigma) and contractile vacuoles. Zoospores can be naked or covered with cell walls.

Hemizoospores lack flagella, yet retain features of monad Organization – a stigma, contractile vacuoles, and basal bodies of flagella. Typically, hemizoospores are observed in amphibious algae, which produce zoospores in an aquatic environment, but halt The process of flagellar formation at the stage of their basal bodies when exposed to terrestrial conditions. If a hemizoosporangium during spore formation is transferred from a terrestrial environment back into Water, its development culminates in the formation of zoospores, meaning the hemizoosporangium transforms into a zoosporangium.

Aplanospores also lack flagella, and their initial Selection/3.html">Stages of development resemble the Cytology/cytology/16.html">Early stages of zoospore formation: in particular, basal bodies form within the protoplasts of future aplanospores. However, at the final stages of development, regardless of the Presence of water in the environment, the basal bodies either disappear or migrate deep into The Cell and transform into centrioles. Occasionally, contractile vacuoles can be observed in aplanospores, but stigmas never develop in them. Typically, mature aplanospores do not replicate the Morphology of the mother cells. Interestingly, almost all species in which hemizoospores and aplanospores have been discovered are also capable of forming sporangia with typical zoospores.

Autospores do not undergo even the Initial Stages of zoosporogenesis in their development. Autospores lack flagella, basal bodies, stigmas, and contractile vacuoles. Morphologically, autospores resemble mother cells, differing from them only in their smaller size. In many species that reproduce via autospores, zoospores are never formed. Hemizoospores, aplanospores, and autospores are usually enclosed within their own cell walls characteristic of the given division.

In addition to their reproductive function, aplanospores and autospores are often capable of fulfilling the function of surviving unfavorable environmental conditions.

Amoeboid non-flagellated spores – monospores, bispores, tetraspores – are known in red and some brown algae. The cell of such a spore represents a naked protoplast enclosed only by a Plasmalemma and capable of slow amoeboid movement. The formation of these spores is preceded in the vast majority of cases by the reductional division of the sporangial Cell Nucleus, which is why mono-, bi-, and tetraspores possess a haploid chromosome set. The names of these spore types reflect the quantity in which they are formed within the sporangium: one (monospores), two (bispores), or four (tetraspores).

Depending on the type of nuclear division in the sporangial cell (mitotic or meiotic), spores are also frequently subdivided into mitospores and meiospores, respectively.

Sexual reproduction occurs as a result of the sexual process, which creates conditions for crossing-over. This, in turn, ensures a certain level of genetic heterogeneity in populations based on recombinant variability. Sexual processes in algae are divided into two main groups: somatogamy and gametogamy.

In somatogamous sexual processes, two vegetative (otherwise somatic) cells fuse. If such cells are represented by monad cells lacking cell walls, this type of somatogamy is called hologamy. A typical example of an alga with a hologamous sexual process is the genus Dunaliella.

If copulation occurs between vegetative cells that possess cell walls, this type of somatogamy is termed conjugation. During conjugation, the cells destined to copulate approach one another and become enveloped in a common mucus. Subsequently, each cell produces a copulatory outgrowth directed toward the other. When the copulatory outgrowths come into contact, The cell wall between them dissolves, forming a conjugation tube through which The Nucleus from one cell migrates into the other. Afterwards, the nuclei fuse to form a zygote. Examples of algae with a sexual process of the conjugation type include green zygnematalean and desmidal algae (Spirogyra, Zygnema, Cosmarium, etc.).

In gametogamy, copulation takes place between specialized cells of sexual reproduction – Gametes, which are formed within sexual Organs known as gametangia. In accordance with the morphology of the gametes, three MAIN TYPES OF gametogamy are distinguished: isogamy, heterogamy, and oogamy.

Isogamy refers to a Sexual process in which two morphologically identical monad gametes copulate. However, they differ physiologically and at the ultrastructural level: one of the gametes, which receives the Nucleus of the other gamete during copulation, is considered female and designated by the "+" sign. The second gamete, whose nucleus migrates into the Cytoplasm of the female gamete during copulation, is considered male and designated by the "-" sign.

In heterogamy (synonym – anisogamy), two monad gametes copulate that differ in size and degree of mobility. The male gamete is small and moves very rapidly. The female gamete is larger than the male one, and its mobility is limited.

Oogamy is a sexual process in which copulation occurs between a large, non-motile, flagellum-free female gamete – the egg cell (ovum) – and a small male gamete. Male gametes that possess flagella and are capable of active movement are called spermatozoa. If, however, the male gamete lacks flagella and is non-motile, it is called a spermatium. Gametangia in which egg cells are formed are called oogonia, whereas those producing spermatozoa or spermatia are called antheridia. In algae, oogonia and antheridia, unlike the gametangia of higher plants, are unicellular (the only exceptions being certain green algae from the Class Charophyceae).

Aside from the main types of gametogamy (i.e., isogamy, heterogamy, and oogamy), several additional specific types of sexual processes are known in algae. For example, in diatoms, a very common type of sexual process is autogamy. During autogamy, the diploid nucleus of the cell undergoes reductional division; subsequently, out of the four daughter haploid nuclei, two degenerate, while the other two fuse, giving rise to the diploid nucleus of the zygote. Thus, in autogamy, gametes are not formed, and the sexual process takes place without a sexual partner.

Another specific variant of the sexual process is ataktogamy, which is observed in certain green volvocalean algae (for example, in the genus Chlorogonium). Here, monad gametes of varying sizes are formed within the gametangia of representatives belonging even to the same population, and both morphologically identical and morphologically distinct gametes copulate. Moreover, the gamete that receives the nucleus from the other (i.e., the physiologically female gamete) may be equal in size to the male one, larger than it, or smaller.

Algae with isogamous sexual processes are distinguished into homo- and heterothallic species. In homothallic species, both "+" and "-" gametes are formed on the same thallus and are capable of copulating with each other, meaning such species are physiologically bisexual. In heterothallic species, "+" and "-" gametes develop on different thalli, and therefore heterothallic representatives are physiologically dioecious (unisexual).

Algae exhibiting heterogamous and oogamous types of sexual processes are also subdivided into monoecious and dioecious. In monoecious algae, antheridia and oogonia are located on the same thallus, meaning such species are both physiologically and morphologically bisexual. In dioecious species, female and male sexual organs are formed on different thalli, meaning physiological and morphological dioecism (unisexuality) takes place.

Following the fusion of the cytoplasms of vegetative cells or gametes involved in the sexual process (known as plasmogamy), nuclear fusion—or karyogamy—typically occurs, resulting in a diploid zygote nucleus. However, there are known exceptions to this rule among algae. For instance, in many chrysophyte algae, the nuclei do not fuse immediately after plasmogamy, resulting in a cell that contains two genetically distinct nuclei, known as a dikaryon. Such a dikaryotic cell subsequently develops into a dikaryotic zygotic cyst, which enters a dormant state. Karyogamy and the actual Formation of the zygote take place only after the dormancy period has ended.