Parthenogenesis

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For the Christian belief, see Virgin Birth.
The asexual whiptail species Cnemidophorus neomexicanus (center) with the sexual species that hybridized to form it, C. inornatus (left) and C. tigris (right).
The asexual whiptail species Cnemidophorus neomexicanus (center) with the sexual species that hybridized to form it, C. inornatus (left) and C. tigris (right).

Parthenogenesis (from the Greek παρθενος parthenos, "virgin", + γενεσις genesis, "birth") describes the growth and development of an embryo or seed without fertilization by a male. Parthenogenesis occurs naturally in some species, including lower plants, invertebrates (e.g. water fleas, aphids, some bees and parasitic wasps), and vertebrates (e.g. some reptiles,[1] fish, and, very rarely, birds[2]). It is sometimes also used to describe reproduction modes in hermaphroditic species which can self-fertilize.

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[edit] Parthenogenesis

Parthenogenesis is a form of asexual reproduction in which females produce eggs that develop without fertilization. Parthenogenesis is seen in aphids, daphnia, rotifers, and some other invertebrates, as well as in some plants. Most recently Komodo Dragons have been added to this list. Among vertebrates, there are several genera of fish, amphibians, and reptiles that exhibit differing forms of asexual reproduction, including true parthenogenesis, gynogenesis, and hybridogenesis (an incomplete form of parthenogenesis).

The offspring of parthenogenesis will be all female if two like chromosomes determine the female sex (such as systems where XX is female and XY is male), but male if the female sex is determined by unlike chromosomes (such as systems where WZ is female and ZZ is male), because the process involves the inheritance and subsequent duplication of only a single sex chromosome. The offspring may be capable of sexual reproduction, if this mode exists in the species. A parthenogenetic offspring is sometimes called a parthenogen. As with all types of asexual reproduction, there are both costs (reduced genetic diversity generated and susceptibility to adverse mutation) and benefits (reproduction without the need for a male) associated with parthenogenesis.

Parthenogenesis is distinct from artificial animal cloning, a process where the new organism is identical to the cell donor. Parthenogenesis is truly a reproductive process which creates a new individual or individuals from the naturally varied genetic material contained in the eggs of the mother. A litter of animals resulting from parthenogenesis may contain all genetically unique siblings without any twins or multiple numbers from the same genetic material. Parthenogenic offspring of a parthenogen are, however, all genetically identical to each other and to the mother, as a parthenogen is homozygous.

The alternation between parthenogenesis and sexual reproduction is called heterogamy. Forms of reproduction related to parthenogenesis but that require the presence of sperm are known as gynogenesis and hybridogenesis.

[edit] Examples

[edit] Reptiles

Most reptiles reproduce sexually, but parthenogenesis has been observed in certain species of whiptails, geckos, rock lizards[1], and Komodo Dragons.

Parthenogenesis has been extensively studied in the New Mexico whiptail (genus Cnemidophorus), of which 15 species reproduce exclusively by parthenogenesis. These lizards live in the dry and sometimes harsh climate of the southwestern United States and northern Mexico. All these asexual species appear to have arisen through the hybridization of two or three of the sexual species in the genus leading to polyploid individuals. The mechanism by which the mixing of chromosomes from two or three species can lead to parthenogenetic reproduction is unknown. Because multiple hybridization events can occur, individual parthenogenetic whiptail species can consist of multiple, independent asexual lineages. Within lineages, there is very little genetic diversity, but different lineages may have quite different genotypes.

An interesting aspect to reproduction in these asexual lizards is that mating behaviours are still seen even though the populations are entirely female. One female plays the role played by the male in closely related species, and mounts the female that is about to produce eggs. This behaviour is due to the hormonal cycles of the females, which cause them to behave like males shortly after laying eggs, when levels of progesterone are high, and to take the female role in mating before laying eggs, when estrogen dominates. Lizards that act out the courtship ritual have greater fecundity than those kept in isolation, due to the increase in hormones that accompanies the mounting. So, although the populations lack males, they still require sexual stimuli for maximum reproductive success.

Recently, the Komodo dragon which normally reproduces sexually was found to also be able to reproduce asexually by parthenogenesis.[3][4] Because the genetics of sex determination in Komodo Dragons uses the WZ system (where WZ is female, ZZ is male, WW is inviable) the offspring of this process will be ZZ (male) or WW (inviable), with no WZ females being born. A case has been documented of a Komodo Dragon switching back to sexual reproduction after a parthenogenetic event. [5] It has been postulated that this gives an advantage to colonisation of islands, where a single female could theoretically have male offspring asexually, then switch to sexual reproduction to maintain higher level of genetic diversity than asexual reproduction alone can generate.[5] Parthenogenesis may also occur when males and females are both present, as the wild Komodo dragon population is approximately 75 percent male.

[edit] Insects

An example of non-viable parthenogenesis is common among domesticated honey bees. The queen bee is the only fertile female in the hive; if she dies without the possibility for a viable replacement queen, it is not uncommon for the worker bees to lay eggs. Worker bees are unable to mate, and the unfertilized eggs produce only drones (males), which can only mate with a queen. Thus, in a relatively short period, all the worker bees die off, and the new drones follow. In one subspecies from South Africa, Apis mellifera capensis, workers are capable of producing diploid eggs parthenogenetically, and thus the queen can be replaced if she dies. It is believed that a few other bees may be truly parthenogenetic (for example, at least one species of small carpenter bee, in the genus Ceratina), and many parasitic wasps are known to be parthenogenetic, sometimes due to infections by Wolbachia.

In Cataglyphis cursor, a European formicine ant, the queen can reproduce by parthenogenesis. The workers are fertile and can mate with the males.[6]

In little fire ants, Wasmannia auropunctata, queens produce more queens through parthenogensis. Usually, eggs fertilized by the males will develop into sterile workers. In some eggs, males cause the female genetic material to be ablated from the zygote, in a process called ameiotic parthenogenesis. In this way, males can pass on their genes by cloning themselves. This is the first example of an animal species where both females and males can clone themselves.[7]

[edit] Mammals

In April 2004, scientists at Tokyo University of Agriculture used parthenogenesis to successfully create fatherless mice: see Kaguya. In theory, the process could be used to reproduce humans after extensive testing and perfection.

[edit] Gynogenesis

A form of asexual reproduction related to parthenogenesis is gynogenesis. Here offspring are produced by the same mechanism as in parthenogenesis, but with the requirement that the egg be stimulated by the presence of sperm in order to develop. However, the sperm cell does not contribute any genetic material to the offspring. Since gynogenetic species lack males, activation of the egg requires mating with males of a closely related species. Some salamanders of the genus Ambystoma are gynogenetic and appear to have been so for over a million years. It is believed that the success of those salamanders may be due to the rare (perhaps only one mating out of a million) actual fertilization of eggs by a male, introducing new material to the gene pool. .

[edit] Hybridogenesis

In hybridogenesis reproduction is not completely asexual but instead hemiclonal: half the genome passes intact to the next generation while the other half is discarded.

Hybridogenetic females can mate with males of a "donor" species and both will contribute genetic material to the offspring. When the female offspring produce their own eggs, however, the eggs will contain no genetic material from their father, only the chromosomes from the offspring's own mother; the set of genes from the father is invariably discarded. This process continues, so that each generation is half (or hemi-) clonal on the mother's side and half new genetic material from the father's side. This form of reproduction is seen in some livebearing fish of the genus Poeciliopsis and in the waterfrog Rana esculenta and the donor waterfrog species Rana lessonae.

A graphical representation of this can be seen here.

[edit] See also

[edit] Notes

  1. ^ a b Halliday, Tim R.; Kraig Adler (eds.) (1986). Reptiles & Amphibians. Torstar Books, p. 101. ISBN 0-920269-81-8. 
  2. ^ Savage, Thomas F. (September 12, 2005). A Guide to the Recognition of Parthenogenesis in Incubated Turkey Eggs. Oregon State University. Retrieved on 2006-10-11.
  3. ^ "No sex please, we're lizards", Roger Highfield, Daily Telegraph, 21 December 2006
  4. ^ "Parthenogenesis in Komodo dragons"Watts PC, et al. . Nature 444, p1021, 2006.
  5. ^ a b Virgin birth of dragons, The Hindu, 25 January 2007, Retrieved 3 February 2007
  6. ^ "Conditional Use of Sex and Parthenogenesis for Worker and Queen Production in Ants"Pearcy M, et. al. . Science 306, p1780, 2004.
  7. ^ "Clonal reproduction by males and females in the little fire ant"Fournier D, et. al. . Nature 435, p1230, 2005.

[edit] Further reading

  • Dawley, Robert M. & Bogart, James P. (1989). Evolution and Ecology of Unisexual Vertebrates. Albany, New York: New York State Museum. ISBN 1-55557-179-4.
  • Futuyma, Douglas J. & Slatkin, Montgomery. (1983). Coevolution. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-228-3.
  • Maynard Smith, John. (1978). The Evolution of Sex. Cambridge: Cambridge University Press. ISBN 0-521-29302-2.
  • Michod, Richard E. & Levin, Bruce R. (1988). The Evolution of Sex. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-459-6.
  • Schlupp, I. (2005) The evolutionary ecology of gynogenesis. Annu. Rev. Ecol. Evol. Syst. 36: 399-417.
  • Simon, Jean-Christophe, Rispe, Claude & Sunnucks, Paul. (2002). Ecology and evolution of sex in aphids. Trends in Ecology & Evolution, 17, 34-39.
  • Stearns, Stephan C. (1988). The Evolution of Sex and Its Consequences (Experientia Supplementum, Vol. 55). Boston: Birkhauser. ISBN 0-8176-1807-4.
  • Phillip C. Watts, Kevin R. Buley, Stephanie Sanderson, Wayne Boardman, Claudio Ciofi and Richard Gibson. (2006). Parthenogenesis in Komodo dragons. Nature 444, 1021-1022

[edit] External links

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