Parthenogenesis

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 παρθένος parthenos, "virgin", + γένεσις genesis, "creation") is 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, fish, and, very rarely, birds (Halliday, 1986; Savage, 2005).

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. Komodo dragons and sharks have recently been added to the list of vertebrates—along with 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). (what are the differences between these forms?)

The offspring of parthenogenesis will be all female if two like chromosomes determine the female sex (such as the XY sex-determination system), but male if two like chromosomes determine the male sex (such as the ZW sex-determination system), 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 mate) associated with parthenogenesis (expand).

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. In animals with an XY chromosome system where parthenogenic offspring are female, 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.

Parthenogenesis often occurs in combination with sexual reproduction or behavior

examples; related topics – gynogenesis, etc

Most reptiles reproduce sexually, but parthenogenesis has been observed in certain species of whiptails, geckos, rock lizards, 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 behaviors are still seen, although the populations are all female. One female plays the role played by the male in closely related species, and mounts the female that is about to lay eggs. This behavior 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. 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. It has been postulated that this gives an advantage to colonization 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. Parthenogenesis may also occur when males and females are both present, as the wild Komodo dragon population is approximately 75 percent male.

Sexual behavior sometimes needed

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.

Or: can’t females of certain species simulate that role?

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.

Parthenogenesis can be a means of determining sex

examples like the honeybee – the unfertilized egg can be male or female depending on the chromosomal scheme

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. 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.

In little fire ants, Wasmannia auropunctata, queens produce more queens through parthenogenesis. 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. (huh?)

section on controversial examples of parthenogenesis

(do we want to call this parthenogenesis?)

In 2001 a bonnethead, a type of small hammerhead shark was thought to have produced a pup, born live on the 14th December 2001, in captivity in a tank containing three female hammerheads but no males; thought to be through parthenogenic means at Henry Doorly Zoo in Nebraska. The shark pup was apparently killed by a stingray within three days of birth. The investigation of the birth was conducted by the research team from Queen's University Belfast, the Southeastern University in Florida, and Henry Doorly Zoo itself and concluded after DNA testing that the reproduction took place under parthenogenic circumstances. The testing showed the pup's DNA matched only one female that lived in the tank, and that no male DNA was present in the pup. The pup was not a twin or clone of the mother, but rather contained only half her DNA ("automictic parthenogenesis"). The type of reproduction exhibited had been seen before in bony fish but never in cartilaginous fish such as sharks.

In 2002, two white-spotted bamboo sharks were born at the Belle Isle Aquarium in Detroit after hatching 15 weeks after laying. The birth baffled experts as the mother shared an aquarium with only one other female shark. The female bamboo sharks had laid eggs in the past. This is not unexpected, as many animals will lay infertile eggs even if there is not a male to mate with. Normally, the eggs are assumed to be infertile and are discarded. This batch was left alone by the curator as he had heard about the previous birth in 2001 in Nebraska.

Other possibilities had been considered for the birth of the Detroit bamboo sharks included thoughts that the sharks had been fertilized by a male and stored the sperm for a period of time and also the possibility that the Belle Isle bamboo shark is a hermaphrodite, harboring both male and female sex organs, and capable of fertilizing its own eggs.

The repercussions of self fertilization in sharks, which reduces the genetic diversity of the offspring, is a matter of concern for shark experts, taking into consideration conservation management strategies for this species, particularly in areas where there may be a shortage of males due to fishing or environmental pressures.

Unlike Komodo dragons, which have a WZ chromosome system and produce male (ZZ) offspring by parthenogenesis, sharks have an XY chromosome system, so they produce only female (XX) offspring by parthenogenesis. As a result, sharks cannot restore a depleted male population through parthenogenesis, so an all-female population must come in contact with an outside male before normal sexual reproduction can resume.

(is this section legit? Condense dramatically)

Differences between parthenogenesis and cloning

There are no known cases of mammalian parthenogenesis in the wild.

In April 2004, scientists at Tokyo University of Agriculture used parthenogenesis to successfully create fatherless mice: see Kaguya.

The beginnings of artificial human parthenogenesis has been performed in the lab. In theory, the process could be used to reproduce humans, but this is unlikely due to ethical concerns. Parthenogenesis in mice and monkeys often results in abnormal development. This high level of birth defects, plus the fact that parthenotes have only half the genetic diversity of their parent, means that research on human parthenogenesis is focused on the production of embryonic stem cells for use in medical treatment, not as a reproductive strategy.

Apomixis is a similar process of asexual reproduction in plants

In botany, apomixis is asexual reproduction, without fertilization. Apomixis mainly occurs in two forms: In agamogenesis, the embryo arises from an unfertilized egg via a modified meiosis. In agamospermy (also called apogamy), a nucellar embryo is formed from the surrounding embryo sac tissue. Apomictically produced seeds are genetically identical to the parent plant. Although the evolutionary advantages of sexual reproduction are lost, apomixis does pass along traits fortuitous for individual evolutionary fitness.

As apomictic plants are genetically identical from one generation to the next, each has the characters of a true species, maintaining distinctions from other congeneric apomicts, while having much smaller differences than is normal between species of most genera. They are therefore often called microspecies. In some genera, it is possible to identify and name hundreds or even thousands of microspecies, which may be grouped together as aggregate species, typically listed in Floras with the convention "Genus species agg." (e.g., the bramble, Rubus fruticosus agg.). Good examples of apomixis can be found in the genera Crataegus (hawthorns), Sorbus (rowans and whitebeams), Rubus (brambles or blackberries), Hieracium (hawkweeds) and Taraxacum (dandelions).

A unique example of male apomixis has recently been discovered in the Saharan Cypress, Cupressus dupreziana, where the seeds are derived entirely from the pollen with no genetic contribution from the female "parent" (Pichot, et al., 2000, 2001).

In zoology parthenogenesis is the animal equivalent of apomixis. Recently, Matthew Meselson won the Lasker Award 2004. He and his students are probing why sex is necessary for evolution. Some small aquatic animals, bdelloid rotifers, are apomictic and have survived for millions of years without sex. They serve as an experimental model system. Meselson assumes that the advantage of sex may lie in its ability to reduce what he calls "genetic parasites" (i.e.transposable elements). These are pieces of DNA that multiply on their own and can cause genetic damage. Bdelloid rotifers don't appear to have such parasites.

References

ISBN links support NWE through referral fees

• add purves and Dr. Tatiana

>Halliday, Tim R. and Kraig Adler (eds.) (1986). Reptiles & Amphibians. Torstar Books, p. 101. ISBN 0-920269-81-8. </ref> and sharks). It

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

External links

• Parthenogenesis of Mice at Nature.com

• Reproductive behavior in whiptails at Crews Laboratory

• Types of asexual reproduction

• Parthenogenesis in Incubated Turkey Eggs from Oregon State University

• National Geographic NEWS: Virgin Birth Expected at Christmas — By Komodo Dragon

• BBC NEWS: 'Virgin births' for giant lizards (Komodo dragon)

• REUTERS: Komodo dragon proud mum (and dad) of five

• Female sharks capable of virgin birth

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