Sexual reproduction is characterised by the fusion of two similar or dissimilar cells. This is a sign of relatively advanced stage in evolution. Sex is not an obvious in the lower plants (like algae) as it is in higher plants. But in any way the study of reproductive processes of various classes of Algae throws considerable light on the origin and evolution of sex in the branch of plants like Algae.
Origin of Sex: Most primitive algae the cyanophyceae (Myxophyceae) reproduce only the method of asexual and vegetative reproduction. Sexual reproduction is entirely absent in their case. In forms like Nostoc and oscillatoria, it takes place by means of hormogones in which the plant body breaks up into group of few cells. In class xanthophyceae, chrysophyceae, cryptophyceae and dinopphyceae, sexuality is rare and has not much evolved beyond the stage of isogamy.
In higher classes of algae reproduction takes place by vegetative, asexual and sexual methods. The commonest method of asexual reproduction is by means of Zoospores. The possibility of gametes from zoospores can evolved from the following description.
In genera like ulothrix, ordogonium, cladophora, zoospores were probably produced before the origin of sexual reproduction. In ulothrix three types of swarm spores are formed.
(i) Quadriflagellate macro-zoospores: They are produced either singly or in small numbers from a single cell. These swarmers are capable of developing into normal plants.
(ii) Macro-zoospores: They are either quadriflagellate or biflagellate and are produced in greater number from each cell than macro-zoospores. They give rise to vary weak plants which are smaller in size.
(iii) Gametes: These are always biflagellate, smaller than the micro-zoospores due to more division in the protoplasts of the cell and hence are largest in number. They are unable to develop into any plant individually but if two of these come together they behave as gametes and fuse leading to the formation of a zygote which germinates into a normal plant.
Thus a gradual loss in vitality of swarmers can be traced, the end product (i.e. gametes) being largest in number but smallest in size and physically in capable of developing into usual plants. It is believed that there was a by chance fusion between these swarmers previously accustomed to reduce vegetatively and then the plants regularly took to the method of fusion or sexual reproduction. It may be concluded that the gametes are ordinary zoospores but reduced in size and unable to germinate without the stimules of sexual fusion. These biciliate gametes unite in pair (isogamy) forming a zygospore which forms a new plant. This proves the derivation of gametes from asexual swarmers.
Evolution of sex: In the evolution of sex, the trend is same in various classes of algae and the sex in all orders and genera has evolved from asexual bodies. Isogamy is the simplest mode of sexual reproduction where two morphological similar gametes fuse e.g.: chlamydomonas media and ulothrix. Isogamy gives rise to anisogamy where two morphologically dissimilar gametes fuse. Anisogamy may be advanced and primitive depending upon the difference in size and behaviour of the gametes e.g.: Pandoriva, Eudoriva, species of chlamydomonas.
Oogamy is most advanced type of sexuality where the two gametes are so dissimilar in shape, size, structure and function that one of them is known as oogonuim and other as antheridium e.g.: volvox, chlamydomonas and ordogobium. In chlorophyceae evolution of sex may be seen from isogam to oogamy when we study individual genera like chlamydomonas (C. media iroamous, C. brounii anisogamous, C. Coccifera oogamous) or order volvocales in which pandorina and endorina are anisogamous and volox is oogamous. In order ulotrichales family ulvaceae genus enteromarphs sexually ranges from isogamy to oogamy.
In Rhodophyceae isogamy and anisogamy are found. In Polysiphomia advanced oogamy is present in phacophycal ectocarpus shows isogamy and anisogamy. Anisogamy is also seen in cutariales. Fucus shows primitive type of oogornium. Sargassum shows advanced oogany because only one ovum is produced in oogonium after fertilization takes place.
It may concluded that in the evolution of sex in algae, the differentiation of gametes is associated with the differentiation of sex organ. It also shows that evolution of sex in algae has taken place from simplest type of the highest evolved type.
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