Sexual reproduction involves the fusion of male and female gametes to form a diploid zygote, which develops into a new organism. It leads to genetic variation. The types of sexual reproduction seen in animals are syngamy ( fertilization) and conjugation. In syngamy, the fusion of two haploid gametes takes place to produce a diploid zygote. Depending upon the place where the fertilization takes place, it is of two types. In external fertilization, the fusion of male and female gametes takes place outside the body of female organisms in the water medium. e.g. sponges, fishes and amphibians. In internal fertilization, the fusion of male and female gametes takes place within the body of female organisms. e.g. reptiles, aves and mammals.
Different kinds of syngamy (fertilization) are prevalent among living organisms. In autogamy, the male and female gametes are produced by the same cell or same organism and both the gametes fuse together to form a zygote e.g. Actinosphaerium and Paramecium. In exogamy, the male and female gametes are produced by different parents and they fuse to form a zygote. So it is biparental. e.g. Human – dioecious or unisexual animal.
In lower organisms, sometimes the entire mature organisms do not form gametes but they themselves behave as gametes and the fusion of such mature individuals is known as hologamy e.g. Trichonympha. Paedogamy is the sexual union of young individuals produced immediately after the division of the adult parent cell by mitosis. In merogamy, the fusion of small sized and morphologically different gametes (merogametes) takes place. The fusion of morphological and physiological identical gametes (isogametes) is called isogamy. e.g. Monocystis, whereas the fusion of dissimilar gametes is called anisogamy (Gr. An-without; iso-equal; gam-marriage). Anisogamy occurs in higher animals but it is customary to use the term fertilization instead of anisogamy or syngamy. e.g. higher invertebrates and all vertebrates.
Conjugation is the temporary union of the two individuals of the same species. During their union both individuals, called the conjugants exchange certain amount of nuclear material (DNA) and then get separated. Conjugation is common among ciliates, e.g. Paramecium, Vorticella and bacteria (Prokaryotes).
Phases of life cycle: Organisms have three phases – Juvenile phase, reproductive phase and senescent phase. Juvenile phase/ vegetative phase is the period of growth between the birth of the individual upto reproductive maturity. During reproductive phase/ maturity phase the organisms reproduce and their offsprings reach maturity period. On the basis of time, breeding animals are of two types: seasonal breeders and continuous breeders. Seasonal breeders reproduce at particular period of the year such as frogs, lizards, most birds, deers etc., Continuous breeders continue to breed throughout their sexual maturity e.g. honey bees, poultry, rabbit etc., Senescent phase begins at the end of reproductive phase when degeneration sets in the structure and functioning of the body.
(Gr. Parthenos – virgin, Genesis-produce)
Development of an egg into a complete individual without fertilization is known as parthenogenesis. It was first discovered by Charles Bonnet in 1745. Parthenogenesis is of two main types namely, Natural Parthenogenesis and Artificial Parthenogenesis. In certain animals, parthenogenesis occurs regularly, constantly and naturally in their life cycle and is known as natural parthenogenesis.
Natural parthenogenesis may be of two types, viz., complete and incomplete. Complete parthenogenesis is the only form of reproduction in certain animals and there is no biparental sexual reproduction. These are no male organisms and so, such individuals are represented by females only. Incomplete parthenogenesis is found in some animals in which both sexual reproduction and parthenogenesis occurs. e.g. In honeybees; fertilized eggs (zygotes) develop into queen and workers, whereas unfertilized eggs develop into drones (male). In paedogenetic parthenogenesis (paedogenesis) the larvae produce a new generation of larvae by parthenogenesis. It occurs in the sporocysts and Redia larvae of liver fluke. It is also seen in the larvae of some insects. e.g. Gall fly. In artificial parthenogenesis, the unfertilized egg (ovum) is induced to develop into a complete individual by physical or chemical stimuli. e.g., Annelid and seaurchin eggs.
Animals are classified mainly into three groups namely – Oviparous, Viviparous and Ovoviviparous depends on the site of development of embryo and whether they lay eggs (unfertilized or fertilized) or give birth to young ones.
In Oviparous (Fig 1.14) (L., Ovum -egg-, Parere- to produce) animals (egg laying animals), the young hatch from eggs laid outside the mother’s body. e.g. reptiles and birds (their eggs are covered by hard calcareous shells), invertebrates, fishes and amphibians (eggs are not covered by hard calcareous shells but covered by a membrane). Viviparous (Fig 1.15) (L., Vivus - alive, Parere - to produce) animals give rise to young ones.
Viviparity is a type of development in which the young ones are born alive after being nourished in the uterus through the placenta. Majority of mammals including human beings are viviparous. In Ovoviviparous animals, the embryo develops inside the egg and remains in the mother’s body until they are ready to hatch (Fig 1.16). This method of reproduction is similar to viviparity but the embryos have no placental connection with the mother and receive their nourishment from the egg yolk. Ovoviviparity is seen in fishes like shark.