Useful notes on the origin and evolution of sex in algae!
Primarily there are two methods of reproduction in plant, i.e., asexual and sexual. In asexual type of reproduction, no sex is involved and there is no fusion of any kind of gametes or cells.
This type of reproduction is being effected by special cells known as spores. Each such spore is capable to develop into a new plant.
On the other hand, in sexual reproduction the two cells or the gametes unite together, and the zygote is resulted, which develops into a new plant. Normally, the individual gametes are incapable to produce new plants without fusion.
Origin of sex:
The most primitive algae, i.e., the members of Cyanophyceae (Myxophyceae), e.g., Gloeocapsa, Chroococcus, etc., reproduce by means of fusion, whereas Oscillatoria, Nostoc, Anabaena, etc., reproduce vegetatively, by means of a group of few cells called hormogonia, which later on give rise to new plants by further division.
Some members of Cyanophyceae reproduce by means of arthrospores. This shows that all the blue-green algae reproduce asexually and the sexual reproduction is altogether absent. In rest of algae, whether unicellular or filamentous, both asexual and sexual methods of reproduction prevail.
It is thought that probably the gametes have been originated from the motile asexual spores or zoospores. Except few cases, the sexual reproduction has not replaced the asexual method but has been added as a supplementary method. The zoospores usually resemble the gametes except for their size.
For example, in algae such as Chlamydomonas, Ulothrix, Cladophora, etc., it is thought that the zoospores were probably produced before the gametes originated. 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 produced, in favourable conditions. In adverse conditions, the gametes are formed in the vegetative cells which earlier produced zoospores in favourable conditions. The gametes are quite similar to zoospores but for their size and behaviour. In forms like Chlamydomonas debaryanum the zoospores and gametes are quite similar.
In Ulothrix, two types of zoospores, i.e., the micro- and the macrozoospores, are produced and the gametes are produced in the same manner as the zoospores are formed. The microzoospores are quite similar to the gametes. At the end of the season, when the plants run in the shortage of food and other favourable conditions, the cells begin to produce gametes. Cholnosky, however, advocates that the gametes in Ulothrix variabilis develop from potential zoospores which fail to escape and undergo more divisions to give rise to the gametes. According to him, the gametes are quite similar except their size and number of flagella. In Oedogonium, the gametes and the zoospores are quite similar except their size.
One can conclude that the gametes are formed as a result of aging of cells and unfavourable conditions. It may be said that the gametes are the reduced type of zoospores. Such reduced zoospores cannot reproduce asexually. They can give rise to a new offspring only with the result of fusion and formation of the zygote. All these examples support that the gametes have been originated from zoospores by means of reduction.
Evolution of sex:
The members of Myxophyceae do not show any trace of sexuality. In other classes of algae, the sexuality has been established. Now to trace out the evolution of sex in algae, it becomes necessary to know about the nature of the gametes in beginning. It is thought, that in the beginning the gametes were of similar shape and size.
These gametes were morphologically identical, but physiologically different, e.g., in Ulothrix and certain species of Chlamydomonas, the gametes are of plus and minus strains. This is known as isogamy. Thereafter anisogamy developed. In this process, the two uniting gametes are of different sizes, e.g., in Chlamydomonas braunii and Pandorina. In this condition, the number of divisions in one cell is more or less than the other cell, and therefore, the uniting gametes are unequal in size. Thereafter developed oogamy.
The oogamy is the highly evolved condition of heterogamy. In oogamy, one cell does not divide at all it simply increases in size, accumulates sufficient food material and acts as female cell or egg. The other cell divides repeatedly producing small motile cells, acting as male gametes. However, in Polysiphonia the resultant male gametes are non-motile. Less evolved oogamy is found in Chlamydomonas coccifera, Volvox and Oedogonium.
The evolution of sex in algae has not taken place in any one phylogenetic line. This has taken place along several independent lines. The example of this statement is found in Volvocales, where Gonium is isogamous, Pandorina slightly anisogalnous, Eudorina and Pleodorina marked by anisogamy and Volvox oogamous. This Gonium-Pandorina-Eudorina-Pleodorina-Volvox series also shows progressive somatic differentiation. It has been assumed that it is correlated with the evolution from isogamy to oogamy through anisogamy. Whereas on the other hand, in another genus of Volvocales, i.e., Chlamydomonas, where species within the genus show all gradations from isogamy to oogamy, e.g., Chlamydomonas snowiae is isogamous, C. braunii is anisogamous and C. coccifera is oogamous.
In a few green algae, such as Spirogyra, Vaucheria, and Chara and in nearly all the brown and red algae a far more advanced condition has been reached. In Spirogyra, the two uniting gametes are produced inside the ordinary vegetative cells known as gametangia. The gametangia are not differentiated into male and female structures.
Those which receive the contents are supposed to be female ones while those from which the contents pass out are supposed to be male ones. In Vaucheria, the gametes are borne in well-differentiated gametangia, known as antheridia and oogonia. The antheridia produce antherozoids and the oogonium contains an egg.
In Chara, there is further advancement. Here the sex organs are not developed all over the plant body, but are confined to nodes on the branches of limited growth. The male and female sex organs are highly developed and specialized organs known as globule and nucule respectively. The spermatogenous filaments develop within globule while the nucule possesses an egg.
In brown algae, Fucus and Sargassum possess fertile conceptacles on the receptacles which contain male and female sex organs. The oogonia may be liberated even before fertilization, and the fertilization takes place in the water. Fucus contains eight eggs in its oogonium whereas in Sargassum seven eggs degenerate but one. The highest evolution of sex in algae has been seen in the red algae.
In Polysiphonia, the male spermatia are nonmotile. After their liberation, the spermatia are carried away to the female by means of water currents. From these examples, it becomes quite clear, that there is no evolutionary sequence in the evolution of sex in algae. It is thought that it developed and evolved independently in green, brown and red algae.
In conclusion, we can say that the sex has been developed in response to circumstances and gametes have been developed from zoospores by means of reduction. The isogamy gave rise to oogamy through anisogamy. During oogamy, the female sex organ increases in size and stores food material to face adverse weather conditions, whereas, the male sex organ gives rise to numerous male gametes which ensure fertilization.
The most important feature of sexual reproduction is the union of two gametic nuclei, which results in the formation of a diploid zygote nucleus. Sooner or later reduction division takes place which provides a means of reshuffling of paternal and maternal chromosomes brought together during the act of fertilization.
Asexual reproduction results in organic similarity while sexual reproduction in diversity. The sexual reproduction is not primarily meant for the multiplication of individuals. It is actually meant for reproduction of heritable variations that accelerate the process of evolution.