Reproduction in Animals – There are two main types of reproduction in animals, asexual and sexual.
Sexual Reproduction
There are several types of asexual reproduction in animals. The most common are:
- Binary Fission
- Multiple Fission
- Budding
- Regeneration
- Parthenogenesis
1. Binary fission
It is the simplest method of asexual reproduction at the level of unicellular organisms. In this process, the unicellular organism divides into two identical daughter cells. During the process, the nucleus becomes elongated followed by the constriction of the cytoplasm. Then the nucleus divides into two and the cell afterwards also divides into two. This is common in Amoeba and Paramecium.
2. Multiple fission
This is a process of repeated divisions of the unicellular organisms into many small cells. In this process nucleus divides into many nuclei divides into many nuclei followed by the division of parent cell into many daughter cells. For example Amoeba becomes enclosed in a covering or cyst under the unfavorable conditions.
Its nucleus divides into many nuclei and each nucleus becomes surrounded by a bit of cytoplasm. When the conditions become favourable, the cyst ruptures and this cause release of a large number of daughter amoebae. Another example is that of malaria parasite, the Plasmodium.
In this case, each plasmodium enters red blood cell and gives rise to 24 daughter cells by multiple fission.
3. Budding
In this process an outgrowth or bud develops by mitosis on the body of parent organism. It grows to a new individual, which either separates immediately or may remain attached to the parent body for sometime. It is commonly appears in Hydra.
Fig 16.14 Budding in Hydra
4. Regeneration
Regeneration means formation of the lost parts of the body through the growth of the remaining parts. For example an arm of starfish if lost in accident can regenerate on the body. Some animals use this power of regeneration for reproduction.
For example, if planarian is divided into two halves, each half can regenerate its lost half. Thus each half can become a complete animal. Regeneration is common in annelids, echinoderms and flatworms.
Planaria reproduces by dividing across the middle. Then each offspring regenerates the missing half of its body.
5. Parthenogenesis
This is a modified form of sexual reproduction in which an unfertilized egg develops into a complete individual. For example in the honey bees, the queen lays eggs which are either fertilized to form diploid workers, or if not fertilized it forms the haploid males called drones. In rotifers, parthenogenesis is a common process of reproduction.