PHYLUM: PROTOZOA Sexual Reproduction By:AAKRITI
SEXUAL REPRODUCTIO PROTOZOANS: Reproduction is an important life process to produce young ones of the same kind As single celled organisms, protozoans lack special reproductive structures like gonads. Protozoans reproduce both by asexual and sexual reproduction. Out of these two, asexual reproduction is common. . In some of the protozoans asexual reproduction is the only mode of reproduction.
. Sexual reproduction takes place by fusion of pronuclei with the formation of gametes or without the formation of the gametes. There are numerous chances of genetic recombination in sexual reproduction. . This genetic recombination occurs during the formation of pronuclei and during the fusion. Unlike asexual reproduction, sexual reproduction is induced by the unfavorable conditions. . Sexual reproduction in protozoans occur by the following methods, Syngamy: It is the fusion of the pronuclei of two gametes. This is a complete fusion of the two sex cells resulting in the formation of the zygote. The fusion nucleus of the zygote is called as synkaryon.
Depending on the degree of differentiation displayed by the fusing gametes syngamy is of following types: Autogamy: This is the fusion of the gametes derived from the same parent cell. For example, in Actinophrys during sexual reproduction, the pseudopodia are withdrawn and a cyst is formed.Now a meiotic division takes place and two daughter nuclei with half the number of chromosomes are formed. No cell division takes place but after sometime, gametic nuclei fuse to form a zygote nucleus.
Hologamy: In this type of reproduction, the two mature protozoan individuals themselves behave as gametes and fuse together to form zygote. Hologamy occurs in few species of Sarcodina and Mastigophora. (Ex:Copromonas) .Isogamy: When the two fusing gametes are similar in size and shape but different in behaviour they are called as isogametes. The fusion of the isogametes is called as isogamy. Isogametes are generally produced by multiple fissions. Isogamy is common in few species of Foraminifera (Ex:Elphidium), Gergarinia (Ex:Monocystis) and Phytomonatida (Ex: Chlamydomonas)
Anisogamy: When the two fusing gametes differ morphologically as well as in terms of behaviour they are called as anisogametes. Generally small and motile gametes are male or microgametes whereas the large and non-motile gametes are female or macrogametes. The fusion of two such anisogametes is called as anisogamy. This mode of reproduction is seen in Sporozoa (Ex Plasmodium), Phytomonatida (Ex:Volvox)
.Conjugation: It involves temporary fusion of two individuals called as conjugants at the oral or buccal regions. This type of reproduction is the characteristic of the suctorians and holotrich ciliates Fusion of the protoplasm takes place at the point of contact. Macronuclei break up and disappear but the micronuclei undergo meiotic division. After the meiotic division of the micronuclei, all the micronuclei get degenerated and only one remains. This remaining micronucleus divides forming two gametic micronuclei. Out of these two one is considered male pronucleus and the other female pronucleus.The male pronucleus of one of the conjugants moves through fused protoplasm into the other conjugant.
In each conjugant, these male and female pronuclei fuse together forming zygote nucleus. Now two individuals separate and are called as ex- conjugants. Each ex-conjugant undergoes further nuclear and cytoplasmic divisions forming four daughter individuals. The asociation during the conjugation processis highly specialized The unique feature of conjugation is the exchange of hereditary material so that each of the conjugant benefits from a renewed hereditary constitution.
A B Micronucleus undergoes 2 successive divisions Unite to form Conjugants Macronucleus disintegrates (1 of it is reduction division) Two Paramecia of opposite mating types come in contact by ventral surface 3 nuclei disintegrate Conjugants separate to form exconjugants Male pronucleus of one conjugant fuses with female pronucleus of other conjugant Formation of Zygote nucleus 3 successive divisions produce 8 nuclei
3 successive divisions produce 8 nuclei Formation of 2 daughter Paramecia Micronucleus divides with the division of each daughter paramecium 3 nuclei disintegrate Mitotitic division of micronucleus 3 nuclei Mitotitic d disintegrate vision of micronucleus Formation of 2 daughter Paramecia divides with the division of each daughter paramecium STEP WISE REPRESENTATION OF CONJUGATION PROCESS IN PARAMECIUM
Graduate in Biology Pursuing M.Sc. in Zoology Power is gained by sharing knowledge not hoarding it.