Complete (Obligatory) Parthenogenesis.

It occurs in somerotifers. There are no male sat all. Hence, there is no biparental sexual reproduction. Females develop exclusively by parthenogenesis. The Caucasian rock lizard, lacerta sexicola armaniaca, shows complete parthenogenesis. It has no males the females develop by parthenogenesis. In the small Indian snake Typhlina brahmina also females develop by parthenogenesis and there are no males.

Incomplete (Cyclic) Parthenogenesis

Some animals have both sexual and parthenogenetic individuals, which may alternate. In honeybee unfertilized eggs develop into male bees (drones) with haploid cells and fertilized eggs give rise to females (queen bees and worker bees) with diploid cells.

In certain insects larval is called paedogenesis (G. pais – child; genesis – origin).

Significance. Parthenogenesis provides an easy mode of reproduction and eliminates variability and need for mating.

Reproductive system

Mammals are unisexual. The reproductive system of each sex consist of many organs. There are primary and secondary sex organs. The primary sex organs are called gonads. These give rise to sex cells or gametes and secrete sex hormones.

The growth, maintains and functions of the gonads are regulated by specific hormones, the follicle-stimulating hormone (FSM) and luteinising hormone (LH) from the anterior lobe of the pituitary gland.

The gonads of the male are called testes or spermaries, they produce male gametes, the spermatozoa (sperm).

The gonads of the female are termed ovaries. They form in them the female gametes or ova.

The secondary sex organs include the reproductive (genital) ducts that transport gametes and reproductive (genital) glands that help in reproductive process. They do not produce gametes or secrete hormones. Man is unisexual. There is a distinct sexual dimorphism males visibly differ from female in physical standards, external genital organs and accessory sex characters.

Characteristics of sexual reproduction

1. It is generally biparental

2. It involves formation and fusion of gametes

3. Cell division are meiotic during gametes formation and mitotic during development of zygote into an offspring.

4. The offspring are not genetically identical to the parents. They show variation as they receive characters (chromosomes) from two different parents. Sexual reproduction is a source of variety in population.

5. Sexual reproduction may not result in an increase in the number of individuals.

Significance of sexual reproduction

1. It results in multiplication and perpetuation of the species.

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

6. Gametogenesis.

The process of the formation of haploid gametes from the undifferentiated, diploid germ cells in the gonads for sexual reproduction called gametogenesis.

Spermatogenesis is formation of sperm.

Oogenesis is the formation of ova.

Both the process are basically similar, both involve three important phases:

Multiplication phase, in which the germ cells of the gonads multiply by mitosis.

Growth phase, in which meiosis take place to produce the gametes.

Maturation phase, in which, meiosis takes place to produce the gametes.