Two basic patterns of reproduction have been observed among animals and these are asexual and sexual. In asexual reproduction an individual can give rise to daughter individuals by mitotic divisions of a part of its own body; no gametes are required.
In sexual reproduction, genetically distinct two special sex cell called gametes, fuse to form one cell structure the zygote which inturn divides repeatedly to grow into a fully developed new individual. The gametes, male and female differ from each other due to the presence of sex chromosomes of different nature.
The male gamete is usually small in size, motile and contains a very little cytoplasm and stored food, while female gamete is large in size, immobile and contains massive cytoplasm and stored food materials. Each of the two gametes frequently comes from a different parent so that sexual reproduction requires the participation of two parents. Single organism can also form two types of gametes that undergo fusion.
Such an organism that produces both types of gametes is called hermaphrodite. In most hermaphrodites, the two gametes do not mature at the same time, so that self-fertilization does not usually occur, cross-fertilization is common. In brief, sexual reproduction is often biparental but may also be uniparental depending on the species.
The development of an egg cell into a new individual without the participation of sperm cell from the opposite sex is called the parthenogenesis. It is a naturally occurring phenomenon among insects, crustaceans, rotifers and some platyhelminthes. It does not involve the fusion of two gametes. Parthenogenesis is rare in the vertebrates. A breed of white turkeys were found to lay eggs which would hatch without fertilization.
Gynogenesis and Androgenesis:
In gynogenesis, the development of a new individual takes place from the egg which is activated by spermatozoan but spermatozoan does not contribute any genetic material to the egg. The resulting embryo carries only maternal chromosomes. The best examples in which gynogenesis is common are; Poecilia, a fish and Ptinus, a latro beetle.
Androgenesis, is the reverse condition of gynogenesis. When chromosome contribution in the developing egg comes exclusively from the male it is called androgenesis. Androgenesis in animals is known only experimentally, naturally occurring androgenesis has not been reported so- far.
In all multicellular animals it consists of the union of two dissimilar gametes, an egg nucleus with a sperm nucleus-to produce a single called diploid zygote which ultimately develops into a multicellular organism resembling the parents.
The organs which produce the gametes are called the gonads. The gonads which produce sperm cells are called the testes, while which produce egg cells are called the ovaries. Sexual reproduction involves two most fundamental events, meiosis and fertilization.
Meiosis is the means by which gametes from the germinal epithelium of the gonads are formed and reassortment of different genes takes place in the formation of gametes. Fertilization involves the fusion of two dissimilar genes in the production of offsprings.
Almost all species of animals have some methods of sexual reproduction as:
i. External Fertilization:
Most animals produce sperms and eggs but in many aquatic species these gametes unite in water; there is no union between opposite sexes. This is called external fertilization.
ii. Internal Fertilization:
In all land animals the fertilization is internal. The two opposite sexes of the same species undergo copulation, whereby the sperms can be transferred to the body of the female where fertilization takes place. Many aquatic animals also have this process.
iii. Embryonic Development:
Those animals which lay eggs are said to have oviparous reproduction. In such cases the major part of embryonic development takes place outside the female body, even though fertilization has been internal. Those animals which give birth directly to the fully developed young ones are said to have viviparous reproduction.
There is a third type of reproduction known as ovoviviparous reproduction where there is a large egg which furnishes food for the developing embryo but due to internal fertilization egg remains in the females until it hatches. Thus, the young ones are born duly developed and active but they have not derived nourishment from their mother during embryonic development.