. . Sexual reproduction 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. POWER OF REGENERATION Sponge when macerated and squeezed through fine silk cloth, the cluster of cells pass through, and these can regenerate new sponges. This technique is used for cultivation of sponges. Different kinds of syngamy (fertilization) are prevalent among living organisms.
a) 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 . b) 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 beings – dioecious or unisexual animal. c) Hologamy - 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. XII Std Biology-Zoology Chapter- XII Std Biology-Zoology Chapter- Reproduction in Organisms d) Paedogamy- It is the sexual union of young individuals produced immediately after the division of the adult parent cell by mitosis. e.g. Actinophrys.
e) Merogamy- The fusion of small sized and morphologically different gametes (merogametes) takes place. e.g. Protozoa. f) Isogamy- The fusion of morphological and physiological identical gametes (isogametes) is called isogamy .
e.g. Monocystis. g) Anisogamy- 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.