Asexual reproduction

Production of offspring by a single parent without the formation and fusion of gametes is called asexual reproduction. It is more primitive and hence simpler than sexual reproduction. It is involves only mitotic cell division, and is also termed somatogenic reproduction. It ensures rapid increase in numbers. Asexual reproduction produces identical offspring commonly referred to as a clone. The only genetic variation arises as a result of random mutation among the members of a clone.

Asexual reproduction occurs in protozoans and some animals such sponges, coelenterates, certain worms.

Types. Asexual reproduction take place in four ways, binary fission, multiple fission, budding, fragmentation.

1. Binary fission is the division of the parents into two small, nearly equal-sized daughter individuals. B.F. is common in protozoans, but also occurs in certain flatworms such as planarian.

2. Multiple fission is the division of the parents into many small daughter individuals simultaneously. M.F. occurs in many protozoans such as Plasmodium and Ameoba.

3. Budding formation of a daughter individual from a small projection, the bud, arising on the parent body is called budding. Budding occurs in some protozoans and certain lower animals such as coelenterates (Hydra), annelids (Syllis).

4. Fragmentation. It is the breaking up of an animals body into two or more species, each of which growth into new individual. It occurs in the flatworm microstomum.

Characteristics of asexual reproduction

1. A single parent produces offspring, that is, asexual reproduction is uniparental.

2. Gametes are not formed.

3. Cell division are only mitotic.

4. The new individuals formed are usually genetically identical to the parent. Variability, if it occurs, is restricted to mutation only.

4. Multiplication occurs rapidly.

5. The offspring are often formed in large numbers.

Significance of asexual reproduction: A.R. brings about multiplication of the species only. It does not play role in evolution as no variation is introduced into the new individuals formed by it.

Sexual reproduction

It is the production of offspring usually by two parents, male and female, involving formation of special haploid cells, the gametes, by meiosis (gametogenesis); fusion of the gametes in pairs, forming diploid cells, the zygotes (fertilization); and repeated mitotic divisions of the zygotes to form embryos and than new individual (development).

Sexual reproducing also called syngenesis (G. Syn – with, together; genesis – origin).

Types. 2 main types: Amphigony and parthenogenesis.

I. Amphigony (G. amphi – two sides; gonos – offspring). It involves the complete and permanent fusion of two gametes or gametic nuclei from different parents or from the same parent to form a composite cell, the zygote, or a composite nucleus, the zygotic nucleus.

It is further of two kinds: syngamy and conjugation.

Syngamy (G. syn - together, gamos – marriage). It involves the fusion of two entire gametes to form a zygote. It is further of two types with regard to the source of the fusing gametes; Endogamy and exogamy.

Endogamy. It is involves self-fertilization, the fusion of two gametes of the same parent (uniparental) it is not common. It is found in Taenia tapeworm.

Exogamy. It involves cross-fertilization, the fusion of two gametes formed by different parents. It is thus, biparental. It is very common, found in frog, rabbit, man.

Syngamy is also of two kinds with regard two the structure of the fusing gametes; isogamy and anisogamy or heterogamy.

Isogamy (G.isos – equal, gamos – marriage) the fusing gametes are similar morphologically as in monocyst, a protozoan. Such gametes are known as isogametes and their production is termed isogamy.

Anisogamy (G. an – not, without; isos – equal, gamos – marriage) or heterogamy (heteros – others, gamos – marriage). The fussing gametes are different in form, size, structure and behavior as in frog and humans. Such gametes known are known as anisogametes or heterogametes.

There are two special forms of syngamy: Neoteny and polyembryony.

Neoteny. Development of gonads and sexual reproduction in the larval stage of an animal is called neoteny. It is found in the axolotl larva of the Salamander Ambystoma.

Polyembryony. The blastomeres formed by division of the zygote separate in early stages of development, each producing a complete individual. Identical twins in human beings is example.

Conjugation. It involves temporary pairing of two parents which exchange their male pronuclei and then separate. It is found in Paramecium and other ciliates.

Parthenogenesis ( G. Parthenos – virgin; genesis – product) It is a modification of sexual reproduction in which an egg develops into a complete offspring without fertilization. It is monoparental. It is found in many non-vertebrates such as rotifers, bees, crustaceans (and also in a few vertebrates)

Parthenogenesis may be complete or incomplete.