Parthenogenesis

Reproduction Without Fertilization

Water Flea Parthenogenesis
This water flea (Daphnia longispina) can be seen with developing parthenogenetic or unfertilized eggs. Credit: Roland Birke/Photolibrary/Getty Images

What Is Parthenogenesis?

Parthenogenesis is a type of asexual reproduction in which a female gamete or egg cell develops into an individual without fertilization. Animals including most kinds of wasps, bees, and ants that have no sex chromosomes reproduce by this process. Some reptiles and fish are also capable of reproducing in this manner. Many plants are also capable of reproducing by parthenogenesis.

Most organisms that reproduce by parthenogenesis also reproduce sexually. This type of parthenogenesis is known as facultative parthenogenesis and organisms including water fleas, crayfish, snakes, sharks, and Komodo dragons reproduce in this manner. Other parthenogenic species, including some reptiles, amphibians, and fishes, are only capable of reproducing asexually.

Parthenogenesis is an adaptive strategy to ensure the reproduction of organisms when conditions are not favorable for sexual reproduction. Asexual reproduction can be advantageous to organisms that must remain in a particular environment and in places where mates are scarce. Numerous offspring can be produced without "costing" the parent a great amount of energy or time. A disadvantage of this type of reproduction is the lack of genetic variation. There is no movement of genes from one population to another. Due to the fact that environments are unstable, populations that are genetically variable will be able to adapt to changing conditions better than those that lack genetic variation.

How Does Parthenogenesis Happen?

There are two main ways in which parthenogenesis occurs. One method is by apomixis, where egg cells are produced by mitosis. In apomictic parthenogenesis, the female sex cell (oocyte) replicates by mitosis producing two diploid cells. These cells have the full compliment of chromosomes needed to develop into an embryo.

The resulting offspring are clones of the parent cell. Organisms that reproduce in this manner include flowering plants and aphids.

The other main method of parthenogenesis is through automixis. In automictic parthenogenesis, egg cells are produced by meiosis. Normally in oogenesis (egg cell development), the resulting daughter cells are divided unequally during meiosis. This asymmetrical cytokinesis results in one large egg cell (oocyte) and smaller cells called polar bodies. The polar bodies degrade and are not fertilized. The oocyte is haploid and only becomes diploid after it is fertilized by male sperm. Since automictic parthenogenesis does not involve males, the egg cell becomes diploid by fusing with one of the polar bodies or by duplicating its chromosomes and doubling its genetic material. Since the resulting offspring are produced by meiosis, genetic recombination occurs and these individuals are not true clones of the parent cell.

Sexual Activity and Parthenogenesis

In an interesting twist, some organisms that reproduce by parthenogenesis actually need sexual activity for parthenogenesis to occur. Known as pseudogamy or gynogenesis, this type of reproduction requires the presence of sperm cells to stimulate egg cell development.

In the process, no genetic material is exchanged because the sperm cell does not fertilize the egg cell. The egg cell develops into an embryo by parthenogenesis. Organisms that reproduce in this manner include some salamanders, stick insects, ticks, aphids, mites, cicadas, wasps, bees, and ants.

How Is Sex Determined in Parthenogenesis?

In some organisms such as wasps, bees, and ants, sex is determined by fertilization. In arrhenotokous parthenogenesis, an unfertilized egg develops into a male and a fertilized egg develops into a female. The female is diploid and contains two sets of chromosomes, while the male is haploid. In thelytoky parthenogenesis, unfertilized eggs develop into females. Thelytoky parthenogenesis occurs in some ants, bees, wasps, arthropods, salamanders, fish, and reptiles.

In deuterotoky parthenogenesis, both males and females develop from unfertilized eggs.

Other Types of Asexual Reproduction

In addition to parthenogenesis, there are several other modes of asexual reproduction. Some of these methods include:

  • Spores - reproductive cells develop into new organisms without fertilization.
  • Binary fission - an individual replicates and divides by mitosis creating two individuals.
  • Budding - an individual grows out of the body of its parent.
  • Regeneration - a detached part of an individual grows into another individual.

Sources:

  • "Parthenogenesis." New World Encyclopedia, . 26 Mar 2015, 16:00 UTC. 11 Jun 2015, 16:16 (http://www.newworldencyclopedia.org/p/index.php?title=Parthenogenesis&oldid=987045).