Parthenogenesis in Biology Asexual Reproduction Without Fertilization

The process of reproduction is multifaceted and can happen in a lot of different ways. Broadly speaking, there are two kinds of reproduction- sexual and asexual. Sexual reproduction, as we know, involves the sex organs while asexual reproduction can be very different.  Parthenogenesis is a particular variety of asexual reproduction seen in certain beings. 

What is Parthenogenesis?

This process refers to a certain kind of reproduction where the result is the development of a female egg (gamete) that happens without fertilization. This is seen in some species of plants and some small invertebrates. It does not happen in higher classes of vertebrates, since asexual reproduction is not possible for them. The term parthenogenesis is a combination of two Greek words parthos (meaning virgin) and genesis (meaning origin). 

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 the 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 that develops into a haploid adult.

Diploid Parthenogenesis May Occur in Two Ways-

• 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.

• 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 two types as

1. Natural Parthenogenesis

2. 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.

2. 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-

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

• The presence of ultraviolet light induces parthenogenesis.

• Electric shock waves also induce it.

• Pricking an egg with a needle induces parthenogenetic development.

Chemicals that Trigger Parthenogenesis are-

• Chloroform

• Chlorides

• Acids

• Fat solvents

• Urea and Sucrose

• Strychnine

Parthenogenesis in Animals

A few sharks like bonnethead, Zebra sharks 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 reproduction. 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 a nonadaptive combination of genes is limited.

What Variations are Seen in Parthenogenesis?

Due to the fact that this is an asexual form of reproduction, a number of variations are seen in this process. For example, some fees undergo a special kind of parthenogenesis known as cycle parthenogenesis or heterogony. This means that generations of eggs that are produced from fertilization will alternate with those that are formed from unfertilized eggs. This can happen due to a number of reasons, one of them being that there were significant changes in seasons, due to which the temperature may not have been suitable enough for eggs to fertilize. 

Another kind of variation observed is known as pseudogamy, and this is mainly seen in species like mites, salamanders, flatworms, and so on. In this case, the sperm is used only to stimulate the eg, but not for actual fertilization. This means that the genetic material of the sperm is not used at all, it simply stimulates the egg to develop.