Sexual Reproduction

Involves two individuals (of different sexes) as male and female to produce respective gametes.

Many lower animals have both sexes in the same individual called hermaphrodite (monoeciousor bisexual), e.g., flatworms, annelids.

In higher animals sexes are separate (unisexual) with apparent sexual dimorphism i.e. male and female look different with different structures.

TYPES OF SEXUAL REPRODUCTION

Amphigony– Zygote is formed by the fusion of male and female gametes further of two types :syngamy and conjugation.

1. Syngamy :Complete and permanent fusion of gametes.

2. Conjugation :Involves temporary union of two parents of the same species which exchange their male pronuclei to form synkaryon and then separate to produce daughter individuals. It corresponds to cross-fertilisation of higher animals.e.g. Paramecium, other ciliates

Parthenogenesis (Virginal Reproduction)—Development of an egg (ovum) into an individual without fertilisation by a sperm.

It is of two types : Natural and Artificial.

•  Natural Parthenogenesis—occurs regularly in the life of certain animals. It may be complete, incomplete or paedogenetic.

Complete (Obligatory) Parthenogenesis.

• Parthenogenesis the only form of reproduction in some animals and there is no biparental sexual reproduction. Such organisms are represented by females only and there are no males.

e.g. (i) Lacertasexicolaarmaniaca(Caucasian Rock Lizard)

(ii) Typhlinabrahmina(Smallest snake of India–15.2 cm)

Incomplete (Cyclic) Parthenogenesis

• Found in those animals in which both sexual reproduction and parthenogenesis occur.

e.g.

• Honey bees–fertilized eggs (zygotes) give rise to queens and workers (both females) and unfertilized eggs (ova) develop into drones (males).
• Aphids, wasps–cyclic parthenogenesis is found.
•  Turkeys–40% males are produced by parthenogenesis.
• Paedogenetic Parthenogenesis (Paedogenesis)
• When larva produces a new generation of larvae by parthenogenesis, it is called paedogenesis. Occurs in sporocystsand rediae of liver fluke.
• On the basis of sex of offspring, Natural parthenogenesis is classified into 3 types :

(i) Arrhenotoky–only males are produced, e.g. honeybees, wasps, ticks, mites and certain spiders.

(ii) Thelytoky–only females are produced. e.g. Lacertasexicolaarmaniaca, Typhlinabrahmina.

(iii) Amphitoky–parthenogenetic egg may develop into male or female e.g. Aphis (aphid).

Artificial Parthenogenesis

Egg is induced to develop into a complete individual by artificial stimuli (physical or chemical).

e.g., Eggs of annelids, molluscs, echinoderms, frogs, salamanders, birds (turkey, hen) and even mammals (rabbit).

According to source of gametes

Endogamy – It involves self fertilisation. Thus uniparentale.g.Taenia.

Exogamy – It involves cross fertilisation. Thus biparental e.g. Earthworm, Leech, Rabbit.

According to structure of fusing gametes.

Isogamy – Fusion of two gametes of identical structure e.g, monocystis.

Anisogamy – Fusion of two dissimilar gametes e.g., frog, rabbit, human.

Characteristics of sexual reproduction:

(i) It is generally bi-parental [i.e. it involves two parents]

(ii) It involves formation and fusion of gametes.

(iii) Cell divisions are both meiotic & mitotic during gamete formation and mitotic during development of zygote into an offspring.

(iv) The off springs are not genetically identical to the parents.

(v) Fertilization in case of humans is internal.

(vi) Infants can be fed on mother's milk.

(vii) Parental care is very well developed.

Significance of sexual reproduction:

(i) It results in multiplication and perpetuation of species.

(ii) It contributes to evolution of the species by introducing variation in a population much more rapidly than asexual reproduction.