PHYLUM: PROTOZOA Evolution of Sex By:AAKRITI
EVOLUTION OF SEX In sexually reproducing population only 50% individuals produce offspring, while the rest half just contribute their genes, thus reducing the reproductive capacity. Whereas in asexuals all individuals produce offsprings with no apparent help from others. Asexual reproduction saves energy and time.
Basic steps in the origin of sexual selection in Protozoa: First there was asexual reproduction in the beginning which was fast and simple but had its disadvantages as it produced clones and could not get rid of harmful effects of mutation. . Meiosis evolved to reduce the no. of chromosomes to half Isogamy producedsimilar and equal sized gametes. . Different mating types originated. . Anisogamy to some extent tried to solve the problem of cell fusion.
Large number of mating types was reduced to two Larger gametes specialized in the storage of nutrients and development of embryo Larger gametes could now be produced in small numbers. Larger gametes being in small numbers became limiting resource, triggering competition. Individuals producing larger gametes were called females.
Evolution of Sex in Protozoa Early protozoan probably reproduced by asexual means such as binary fission, multiple fission, budding etc. as these methods were quick, had lesser energy demands and all individuals participated in producing offspring Asexual reproduction was best suited to animals which had delicate bodies and no apparent defense system and, therefore, must have needed a speedy way of multiplication for the recovery of population.
Gametogenesis in protozoan is variable and can be classified in the following three forms: The protozoa in which adult (trophozoite stage) is haploid. Those protozoa in which adult stage is diploid. . In ciliates gametic nuclei rather than gametes are formed. Haploid protozoan such as sporozoans, Elphidium etc., perhaps evolved when diploid individuals produced gametes. cium a produce gametes by meiosis and then by mitosis.
Conjugation is a perfect sexual process in which exchange of genetic material takes place. In sporozoans andsome other protozoans, haploid adults are formed and whole individual behaveslike a gamete. In Elphidium, Volvox and mycetozoa, biflagellate isogametes are formed. Sexual reproduction rejuvenates protozoan populations instantly.
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