Evolution of sex in algae : An overview

Evolution of sex in algae : An overview 

In plants, asexual and sexual reproduction are the two main modes of reproduction. Asexual reproduction does not entail sex and does not involve the fusing of any kind of cell or gamete.

Spores are a unique form of cell that reproduces in this manner. Every one of these spores has the ability to grow into a new plant.

In contrast, when two cells, or gametes, combine during sexual reproduction, a zygote is created that eventually grows into a new plant. Without fusing, individual gametes are normally unable to create new plants.

Origin of sex: 

The most primitive algae, such as those belonging to the Cyanophyceae (Myxophyceae) family, such as Gloeocapsa and Chroococcus, reproduce by fusion, whereas Oscillatoria, Nostoc, and Anabaena, among others, reproduce vegetatively through the division of a small group of cells known as hormogonia, which subsequently gives rise to new plants through additional division.

Certain Cyanophyceae members use arthrospores for reproduction. This demonstrates that all blue-green algae reproduce asexually, with no sexual reproduction occurring at all. Both asexual and sexual reproduction are common in the remaining algae, whether they are filamentous or unicellular.

It is believed that the gametes most likely started from zoospores, which are motile asexual spores. With a few exceptions, sexual reproduction has been added as a supplemental technique rather than taking the place of asexual reproduction. Except for size, zoospores typically resemble gametes.

For instance, it is believed that the zoospores in algae like Chlamydomonas, Ulothrix, Cladophora, etc. were most likely formed prior to the creation of the gametes. In above mentioned and many other filamentous forms, the asexual reproduction takes place by means of uninucleate and biflagellate zoospores which are formed by repeated division contents of the ordinary vegetative cell.

The zoospores are generated under favorable circumstances. Under unfavorable circumstances, gametes develop in vegetative cells that previously generated zoospores under favorable circumstances. With the exception of size and behavior, gametes and zoospores are fairly identical. In forms like Chlamydomonas debaryanum the zoospores and gametes are quite similar.

The two forms of zoospores that are created in Ulothrix are the macro- and microzoospores, and the gametes are produced in the same way that the zoospores are. The gametes and microzoospores are fairly similar. The cells start to make gametes at the end of the season when the plants are struggling with a lack of food and other favorable conditions. On the other hand, Cholnosky suggests that Ulothrix variabilis gametes originate from viable zoospores that are unable to escape and divide further to produce gametes. He claims that the only differences between the gametes are their size and number of flagella. With the exception of size, gametes and zoospores in Oedogonium are fairly identical.

We can conclude that unfavorable conditions and cell aging lead to the formation of gametes. One may argue that gametes are zoospores on a smaller scale. These smaller zoospores are unable to procreate asexually. Only as a result of fusing and zygote creation can they produce new offspring. Each of these instances demonstrates that the gametes originated by reduction from zoospores.

Evolution of sex: 

There is no indication of sexuality in any Myxophyceae member. Other classes of algae have demonstrated sexuality. In order to track the development of sex in algae, it is now essential to understand the original characteristics of the gametes. It is believed that the gametes were identical in size and form at first.

These gametes had the same morphology but differed physiologically. For example, in some species of Chlamydomonas and Ulothrix, the gametes are of the plus and minus strains. We call this isogamy. After that, anisogamy emerged. In certain cases, such as in Chlamydomonas braunii and Pandorina, the two joining gametes in this process are of different sizes. The quantity of divisions in a single cell under these circumstances is around

The highly developed form of heterogamy is called oogamy. In oogamy, a single cell functions as a female cell or egg by becoming larger, accumulating enough nourishment, and not dividing at all. The other cell repeatedly divides to produce motile, tiny cells that function as male gametes. The male gametes that are produced in Polysiphonia, however, are not mobile. Oedogonium, Volvox, and Chlamydomonas coccifera all have less evolved oogamy.

No single evolutionary line has been responsible for the emergence of sex in algae. This has happened in multiple separate directions. In Volvocales, Gonium is isogamous, Pandorina is slightly anisogalnous, Eudorina and Pleodorina are marked by anisogamy, and Volvox is oogamous. 

These species serve as examples of this assertion. Progressive somatic differentiation is also shown in the Gonium-Pandorina-Eudorina-Pleodorina-Volvox series. It has been suggested that it has anything to do with the progression of anisogamy from isogamy to oogamy. On the other hand, species of the Volvocales genus Chlamydomonas exhibit a range of behaviors, from isogamy to oogamy. For example, Chlamydomonas snowiae has isogamous behavior, C. braunii exhibits anisogamous behavior, and C. coccifera exhibits oogamous behavior.

A significantly more evolved state has been reached in almost all brown and red algae as well as in a few green algae, including Spirogyra, Vaucheria, and Chara. The two joining gametes in Spirogyra are created inside the common vegetative cells called gametangia. There is no distinction between the male and female structures of gametangia.

It is expected that the people who get the contents will be feminine, and the people from whom the contents are removed will be male. Gametes in Vaucheria are carried in well-differentiated gametangia, or oogonia and antheridia. The antheridia produce antherozoids and the oogonium contains an egg.

There is further progress in Chara. Here, the sex organs are restricted to nodes on the branches with restricted growth rather than being formed throughout the plant body. The male and female sex organs are highly developed and specialized organs known as globule and nucule respectively. Within the globule, the spermatogenous filaments grow, and the nucule contains an egg.

Fucus and Sargassum are two types of brown algae that have fertile conceptacles on their receptacles that house the male and female sex organs. The fertilization process occurs in the water, and the oogonia may be released even prior to fertilization. In contrast to Sargassum, when all seven defective eggs bar one remain, Fucus has eight eggs in its oogonium. Red algae have demonstrated the highest level of sexual evolution among algae.

Male spermatia in Polysiphonia are nonmotile. Water currents carry the freed spermatia to the female after they have been released. These examples make it abundantly evident that the emergence of sex in algae did not follow any particular evolutionary sequence. It is thought that it developed and evolved independently in green, brown and red algae.

In summary, 

we can state that gametes were created from zoospores by reduction and that sex evolved in reaction to environmental factors. Through anisogamy, isogamy gave rise to oogamy. The male sex organ produces many male gametes that guarantee fertilization during oogamy, whereas the female sex organ grows larger and saves food to withstand bad weather.

The joining of two gametic nuclei to form a diploid zygote nucleus is the most significant aspect of sexual reproduction. Reduction division, which occurs eventually, allows the paternal and maternal chromosomes that are brought together during fertilization to be rearranged.

Whereas sexual reproduction produces variation, asexual reproduction produces organic similarity. The primary purpose of sexual reproduction is not individual multiplication.

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