Parthenogenesis

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Introduction

Parthenogenesis is a type of asexual reproduction in which the development of mostly a female gamete takes place without fertilization. This type of reproduction is mostly shown by lower plants, some reptiles, and fishes, who do not possess sex chromosomes. Parthenogenesis is a Greek word that literally translates to “Virgin Origin”. Parthenogenetic animals may be incapable of sexual reproduction or capable of switching between the modes of reproduction under certain conditions. It can be naturally occurring between some plants or can be artificially induced in some amphibians or fishes.

A normal egg cell forms from the mother containing half the number of chromosomes. The offspring may be a full clone of the mother containing the mother’s full genetic material or maybe a half clone with half of the genetic material from the mother. Organisms that can switch between sexual reproduction and parthenogenesis, mostly get triggered for such a form of asexual reproduction under bad weather conditions or under conditions of stress. This is because parthenogenesis does not involve modification of genes or the formation of male gametes who do not give birth further. The formation of female gametes by parthenogenesis is called thelytoky. And the formation of male gametes, which is very rare, is called arrhenotoky.


Mechanism of Parthenogenesis

Parthenogenesis can be considered as an incomplete sexual reproduction since the resultant offspring develops from the gametes. Gametes are reproductive or sex cells formed by meiosis. There are two types of cells in individuals- Haploid and diploid cells. Haploid cells contain a single set of chromosomes and diploid cells consist of two complete sets of chromosomes which are double in number than in haploid cells. Similarly, in haploid parthenogenesis, a haploid egg gives rise to an offspring which develops into a haploid adult.


Diploid Parthenogenesis may occur in two ways-

  1. Automixis- It is haploid parthenogenesis in which a haploid cell may either duplicate its chromosome or pair with another haploid cell. In either of the cases, a diploid zygote is formed which develops into a diploid adult.

  2. Apomixis- In apomictic parthenogenesis, a parent cell produces two genetically identical diploid egg cells by mitosis. These develop into diploid offspring.

Examples- Mostly wasps, bees, crayfish, snakes, sharks, Komodo dragons reproduce by parthenogenesis.


Types of Parthenogenesis

Depending upon the way Parthenogenesis occurs, it is classified as-

  • Natural Parthenogenesis

  • Artificial Parthenogenesis


Natural Parthenogenesis

In some organisms parthenogenesis is a regular and constant process occurring naturally. This is natural parthenogenesis. Natural Parthenogenesis can be of two types-

  1. Complete Parthenogenesis

Some insects completely rely on self-reproduction either due to the absence of the sexual phase or males. Parthenogenesis is the only means of reproduction in such organisms. And it is called complete or obligatory parthenogenesis.

  1. Incomplete Parthenogenesis

The life cycle of certain insects has two generations. One is the sexual generation and the other is parthenogenetic generation. In such insects, diploid eggs produce females and the unfertilized eggs produce males.

Examples- Bees, wasps, ticks, mites, spiders, etc.


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-

  1. A rise in temperature induces parthenogenesis in some animal eggs.

  2. The presence of ultraviolet light induces parthenogenesis.

  3. Electric shock waves also induce it.

  4. Pricking an egg by a needle induces parthenogenetic development.

Chemicals that Trigger Parthenogenesis are-

  1. Chloroform

  2. Chlorides

  3. Acids

  4. Fat solvents

  5. Urea and Sucrose

  6. Strychnine


Parthenogenesis in Animals

A few sharks like bonnethead, Zebra shark reproduce by parthenogenesis. In an experiment, a bonnethead pup was born in a tank containing only three female sharks and no male shark. The DNA analysis showed the pup was neither a clone or a twin but was found to have half of the mother’s DNA. Which is predominant in this type of reproductions. Apart from this, crustaceans like crayfish reproduce by apomixis. Goblin spiders are also believed to be parthenogenetic as they do not have a male population. Some lizards also can reproduce parthenogenetically along with reproducing sexually. Reptiles like pythons, rattlesnakes, may also reproduce by this method given the circumstances of their physical habitat.

 

Parthenogenesis in Insects

A few ants and bees show the capability to produce diploid female offspring. Worker bees of a few species produce diploid eggs even when the queen dies thus showing parthenogenetic reproduction. Some carpenter bees and parasitic wasps reproduce by this method naturally or by some obligation.


Significance of Parthenogenesis

  • Parthenogenesis is a type of adaptive strategy to reproduce when environmental conditions are not proper and sexual reproduction is not possible.

  • It allows the species to continue thriving and multiplying in some environments where the male population is scarce or none.

  • Many offspring are produced by this method without costing much parental energy or time.

  • It enables sex determination is some organisms like wasps, bees.

  • It is a simple, easy, and stable process of reproduction.

  • It supports the chromosomal theory of inheritance which proves that chromosomes are the vehicles of genetic heredity.

  • Parthenogenesis allows the cells of organisms to have more than two sets of chromosomes, called polyploidy

  • Advantageous mutant characters may develop through this method of reproduction.

  • No chances of sterile races and nonadaptive combination of genes is limited.

FAQ (Frequently Asked Questions)

1. What is meant by Parthenogenesis?

Ans. Parthenogenesis is a type of asexual reproduction in which the development of mostly a female gamete takes place without fertilization. This type of reproduction is mostly shown by lower plants, some reptiles, and fishes, who do not possess sex chromosomes. It can be considered as an incomplete sexual reproduction since the resultant offspring develops from the gametes.

2. What is the significance of Parthenogenesis?

Ans. Significance of Parthenogenesis is as follows-

  • Parthenogenesis is a type of adaptive strategy to reproduce when environmental conditions are not proper and sexual reproduction is not possible.

  • It allows the species to continue thriving and multiplying in some environments where the male population is scarce or none.

  • Many offspring are produced by this method without costing much of parental energy or time.

  • It enables sex determination is some organisms like wasps, bees.

  • It is a simple, easy, and stable process of reproduction.

  • It supports the chromosomal theory of inheritance which proves that chromosomes are the vehicles of genetic heredity.

  • Parthenogenesis allows the cells of organisms to have more than two sets of chromosomes, called polyploidy.

  • Advantageous mutant characters may develop by this method of reproduction.

  • No chances of sterile races and non-adaptive combination of genes is limited.