Asexual reproduction occurs in several ways which include binary fission, multiple fission, sporulation, budding and fragmentation.
It involves the division of the nucleus followed by that of the cytoplasm, breaking the body into two young ones. It occurs in unicellular organisms, E.g., Amoeba, Planaria and Paramecium. Binary fission may be Irregular – Amoeba, Longitudinal – Euglena, Transverse – Paramecium and Planaria, Oblique – Dinoflagellates.
In this the nucleus divides into several daughter nuclei, followed by the simultaneous division of cytoplasm. The nuclei move towards the periphery and each one is surrounded by small amount of cytoplasm which gives rise to new daughter cells. It occurs in protozoa, E.g., Plasmodium.
In many protozoa and bacteria the asexual reproduction occurs by the division of nucleus into several daughter nuclei, and then each daughter nucleus gets enclosed by small amount of cytoplasm to form a spore.
Budding is an unequal division of the parent where the identity of the parent body is still maintained. It is in contrast to binary fission where parent body divides equally into two and no identity of the parent is left. Bud is an outgrowth formed on the surface of the body which finally gets detached and develops into a young one.
If the bud is formed on the external surface, it is called exogenous bud but in some cases it could be an internal bud or a specialized form like gemmules as in sponges. Exogenous budding is common in Hydra.
The body may break into two or more fragments, and each fragment develops into a complete individual. E.g. Filamentous algae, Spirogyra.
II. Sexual Reproduction:
The process of sexual reproduction involves the formation and fusion of gametes (Syngamy) and results in the formation of new organisms genetically different from parent. Sexual reproduction is usually biparental, i.e., involves a male and female individual producing male and female gametes respectively.
Such animals are called unisexual or dioecious. E.g., frog, cockroach, human beings. In certain animals, the male and female sex organs are present in the same individual. Such species are called monoecious or bisexual. E.g., earthworm, leech.
If the fusing gametes are similar, they are called isogametes. If dissimilar, the gametes are termed as anisogametes.