Parthenogenesis: Parthenogenesis is a form of asexual reproduction in which an unfertilized egg develops into a new individual. It is a method in which a new individual developed without fertilization. Here, males do not have any role to play and only female gametes develop into new offspring. Examples of plants showing parthenogenesis include honey bees, ants, birds. 

Parthenogenesis is sometimes considered to be an asexual form of reproduction; however, it may be more accurately described as an “incomplete form of sexual reproduction,” since offspring of parthenogenic species develop from gametes.

Parthenogenesis can operate on either a haploid or a diploid cell. In haploid parthenogenesis, a rare form of parthenogenesis that occurs in a few species of bees, nematodes, and plants, offspring develop from haploid eggs to produce haploid adults. On the other hand, the process of diploid parthenogenesis, a more common and varied form of the phenomenon, may proceed along two pathways. Automixis (automictic parthenogenesis) is a postmeiotic process in which a haploid cell may either duplicate its chromosomes or join with another haploid cell. In both cases, diploid zygotes develop and grow into diploid adults. Such organisms are not true clones of the mother, however, because the meiotic process separates and recombines the genetic material.

Types of Parthenogenesis

There are two types of parthenogenesis:

• Natural Parthenogenesis: In certain animals, parthenogenesis occurs naturally in their life cycles. This is known as natural parthenogenesis. Natural parthenogenesis can be further divided into two types:
  • Complete Parthenogenesis
  • Incomplete Parthenogenesis
• Artificial Parthenogenesis: Various chemical or physical methods may trigger the development of fertilized eggs. This is called artificial parthenogenesis.
  • Physical Factors that Trigger Parthenogenesis are-
    • Temperature induces parthenogenesis in eggs.
    • Parthenogenesis is caused by ultraviolet light.
    • Electrical shocks cause parthenogenesis.
    • When an egg is pricked by a needle, the development occurs parthenogenetically.
  • Chemical factors that Trigger Parthenogenesis are-
    • Chloroform
    • Urea and Sucrose
    • Strychnine
    • Fat solvents
    • Acids
    • Chlorides

How Parthenogenesis happens?

The egg cells are haploid cells produced from the ovaries by meiosis. A precursor cell after meiosis produces an ovum and three polar bodies (small byproducts of meiotic division). In sexual reproduction, the polar bodies eventually degenerate. That’s what happens in human egg cells following meiosis. But for other organisms, the ovum may fuse with the polar body such as in the case of automixis parthenogenesis (also called meiotic parthenogenesis) described in the previous section. The egg fuses with the second polar body. Thus, although there is no sperm cell involved in increasing gene diversity, this form of parthenogenesis offers a degree of genetic variation among offspring.

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By Team Learning Mantras