Asexual Reproduction:
It is a reproduction without the union of gametes. It the only means by which primitive organisms could increase their number. But these species that reproducing asexually tends to evolve very slowly, due to less genetic diversity for evolutionary selection. It is common among protozoa as well among lower invertebrates such as sponges, jelly fishes, flatworms and many segmented worms. It is rare among the higher invertebrates. The ability to reproduce asexually often correlates with marked capacity for regeneration.
In the lower invertebrates its most common forms are fission, budding (internal and external) and fragmentation. Parthenogenesis which is comparatively uncommon also occurs in few invertebrates.
Fission:
Protists and some multi-cellular animals (cnidarians, annelids) may reproduce by fission. Fission is the division of one cell, body or body part into two. In this process, the cell pinches in two by an inward furrowing of the plasma membrane. Binary fission occurs when the division is equal; each offspring contains approximately equal amount of protoplasm and associated structures. Binary fission is common in protozoa for some; it is their only means of reproduction.
In fission the plane of division may be asymmetrical, transverse or longitudinal, depending on the species. For example Planarian reproduces by longitudinal fission. Some flatworms and annelids reproduce by forming numerous constrictions along the length of the body; a chain of daughter individuals results. This type of asexual reproduction is called multiple fission.
Budding:
In cnidarian Hydra and many species of sponges, certain cells divide rapidly and develop on the body surface to form an external bud. The bud cells proliferate to form a cylindrical structure which develops into new animals, usually breaking away from the parent. If the buds remain attached to the parent, they form a colony. Internal budding (as in freshwater sponges) produces gemmules, which are collections of many cells surrounded by body wall, when the body of parent dies and degenerates each gammule gives rise to new individual.
Fragmentation (regeneration):
It is a type of asexual reproduction where by a body part is lost and then regenerates into new organisms. Fragmentation occurs in some cnidarians, Platyhelminthes, Rhychocoels and Echinoderms. For example in sea anemones, as the organisms moves, small pieces breakdown from the adult and develop into new individuals.
Parthenogenesis:
It is spontaneous activation of mature eggs followed by normal egg divisions and subsequent embryonic development without fertilization by pricking them with a needle by exposing them to high concentration of calcium or by altering their temperatures. E.g. Flatworms, Rotifers, Round worms, Insects, Lobsters, some lizards and some fishes. Because parthenogenetic eggs are not fertilized, they do not receive male chromosomes. The offspring would thus be expected to have only a haploid set of chromosomes. Parthenogenesis plays an important role in social organisation in colonies of certain bees, wasps and ants. In these insects large number of male (drone) are produced parthenogenetically where as sterile female workers and reproductive females (queens) are produced asexually.
Advantages of Asexual Reproduction:
(1) The predominance of asexual reproduction is protists and some invertebrates can be partially explained by the environment in which they live.
(2) Marine environment is usually very stable. Stable environments may favour this form of reproduction because a combination of genes that matches the unchanging environment is an advantage over greater number of gene combinations, many of which do not match the environment.
(3) Asexual reproduction is seasonal. The season during which asexual reproduction occurs coincides with the period when the environment is predicately hospitable. Under such conditions it is advantageous for the animal to produce asexually large number of progeny with identical characters.
(4) Large number of animals, well adapted to a given environment can be produced even if only one parent is present.
Disadvantages:
(1) Due to this reproduction a population of genetically identical animals can be devastated by single disease or environment insult such as long drought.
(2) Asexually reproducing animals can cope with changing environment only through rare spontaneous mutations that prove to be beneficial. But most mutations are lethal and here in lies one of the greatest disadvantages of asexual reproduction.
(3) All such mutations are passed on to every offspring with the normal, unmutated genes. Consequently, typical asexual animal may have only one ‘good’ copy of each hereditary unit (gene); the one on the homologous chromosomes may be a mutated form that is lethal.
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