Parthenogenesis
From Wikipedia, the free encyclopedia
- For the Christian belief, see Virgin Birth.
Parthenogenesis or virgin birth (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 (called agamospermy), invertebrates (e.g. water fleas, aphids, some bees and parasitic wasps), and vertebrates (e.g. some reptiles,[1] fish, and, very rarely, birds[2]). Parthenogenetic populations must be all-female because there is no contribution from a male. The offspring may be capable of sexual reproduction, however, if that exists in the species. As with all types of asexual reproduction, there are both costs and benefits associated with parthenogenesis.
Parthenogenesis has nothing to do with 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 unique siblings without any twins or multiple numbers from the same genetic material, but they would all be female.
In April 2004, scientists at Tokyo University of Agriculture used parthenogenesis to successfully create fatherless mice. The process could be used to reproduce humans after extensive testing and perfection. 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.
Contents |
[edit] Asexual reproduction versus sexual reproduction
Asexual reproduction is relatively rare among multicellular organisms, for reasons that are not completely understood. Current hypotheses suggest that, while asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, over the long term sexual reproduction offers a net advantage by allowing more rapid adaptation to changing environments.
Asexual lineages can increase their numbers rapidly because all members are female and can therefore produce viable eggs. In sexual populations, some of the individuals are male and cannot themselves produce offspring. This means that an asexual lineage will have roughly double the rate of population growth under ideal conditions when compared with a sexual population half composed of males. This is known as the two-fold cost of sex.
Also, organisms that can reproduce through parthenogenesis are more able to settle isolated habitats like oceanic islands, as a single (female) member of the species is enough to start a population.
Another consequence of asexual reproduction, which may have both benefits and costs, is that offspring are typically genetically similar to their parent, with as broad a range as that animal receives from one parent. Fewer genetic alternatives result than with sexual reproduction. This genetic similarity may be beneficial if the genotype is well-suited to a stable environment, but disadvantageous if the environment is changing. For example, if a new predator or pathogen appears and a genotype is particularly defenseless against it, an asexual lineage is more likely to be completely wiped out by it. In contrast, a lineage that reproduces sexually has a higher probability of having more members survive due to the genetic recombination that produces a novel genotype in each individual. Similar arguments apply to changes in the physical environment.
Some species alternate between the sexual and asexual strategies, an ability known as heterogamy, depending on conditions. For example, the freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases.
[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. 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).
Most reptiles reproduce sexually, but parthenogenesis has been observed in certain species of rock lizards, geckos, and whiptails[1]. 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 behaviors are still seen even though the populations are entirely female. One female plays the role formerly played by the male and mounts the female that is about to produce eggs. This behavior is due to the hormonal cycles of the animals, which cause them to mate 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.
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.
[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 replaced. Females mate with males and both contribute genetic material to the offspring. But when the female offspring produce their own eggs, the eggs contain no genetic material from their father, only the chromosomes from the offspring's own mother. 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.
Recently it was seen that little fire ants (Wasmannia auropunctata) can fertilize but males can cause the female genetic material to be ablated from the zygote, in a process called ameiotic parthenogenesis. [Fournier et al. 2005 Nature] In this way, males can reproduce themselves. This is the first example of an animal species where both females and males can reproduce themselves. Females can reproduce themselves by conventional parthenogenesis.
[edit] See also
- Apomixis for a similar process in plants
- Parthenocarpy
- Jacques Loeb
- Gregory Goodwin Pincus
[edit] Notes
- ^ a b Halliday, Tim R., Kraig Adler (eds.) (1986). Reptiles & Amphibians. Torstar Books, p. 101. ISBN 0-920269-81-8.
- ^ 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.
[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.
[edit] External links
- Parthenogenesis in Incubated Turkey Eggs from Oregon State University
