Difference between revisions of "Parthenogenesis" - New World Encyclopedia

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[[Image:Cnemidophorus-ThreeSpecies.jpg|thumb|260px|right|The asexual whiptail species ''[[Cnemidophorus neomexicanus]]'' (center) with the sexual species that hybridized to form it, ''[[Cnemidophorus inornatus|C. inornatus]]'' (left) and ''[[Cnemidophorus tigris|C. tigris]]'' (right).]]
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[[Image:Cnemidophorus-ThreeSpecies.jpg|thumb|260px|right|The asexual whiptail species ''[[Cnemidophorus neomexicanus]]'' (center) consists exclusively of females who reproduce via parthenogenesis. ''C. neomexicanus'' is flanked by the sexually reproducing species that hybridized to generate it: ''[[Cnemidophorus inornatus|C. inornatus]]'' (left) and ''[[Cnemidophorus tigris|C. tigris]]'' (right).]]
  
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'''Parthenogenesis''' (from the [[Greek language|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 [[plant]]s, [[invertebrate]]s (e.g. [[water flea]]s, [[aphid]]s, some [[bee]]s and [[parasitic wasp]]s), and [[vertebrate]]s (e.g. some  
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'''Parthenogenesis''' is a form of [[asexual reproduction]] in which offspring develop from unfertilized eggs. A common mode of reproduction in [[anthropod]]s such as [[insect]]s and [[arachnid]]s, parthenogenesis also occurs in some species of fish, amphibians, and reptiles.
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[[reptile]]s, [[fish]],
 
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and, very rarely, [[bird]]s (Halliday, 1986; Savage, 2005).
 
  
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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 dragon]]s 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?)
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Parthenogenesis (which derives from the [[Greek language|Greek]] words for "virgin" and "creation") is more efficient than sexual reproduction because it does not always depend on mating, which requires energy and usually involves risks. Moreover, all members of an asexual population are capable of reproducing. The disadvantage, however, is that asexual reproduction does not generate genotypic diversity, which is an important evolutionary advantage for sexually reproducing species.
  
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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).
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Given the drawbacks of asexual reproduction for the long-term survival of the species, most species that engage in parthenogenesis also participate in sexual reproduction or sexual behaviors. Parthenogenesis thus serves as one available reproductive strategy, often a response to environmental or seasonal conditions such as the amount of available resources. [[Aphid]]s, for example, are parthenogenic in spring and summer, multiplying rapidly while conditions are favorable; during the winter months, they mate and the females hatch fertilized eggs. In rare cases, however, parthenogenesis does not occur in combination with sexual reproduction or behaviors: the bdelloid rotifer ''philodina roseola'', for example, reproduces exclusively by parthenogenesis, and the species has not engaged in sexual reproduction in 85 million years (Judson, 2002).
  
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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]].
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In addition to its reproductive role, parthenogenesis also functions as part of a mechanism for determining sex in some species. In ants and most species of bees and wasps, females develop from unfertilized eggs and are referred to as [[haploid]] (possessing one set of chromosomes), while males develop from fertilized eggs and hence are [[diploid]] (possessing two sets of chromosomes, one from each parent). Thus, in species also capable of sexual reproduction, parthenogenesis can help to regulate the relative number of males and females in a population.
  
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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 '''[[Parthenogenesis#Gynogenesis|gynogenesis]]''' and '''[[Parthenogenesis#Hybridogenesis|hybridogenesis]]'''.
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==Sexual behavior is sometimes required to stimulate development==
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In some species, parthenogenesis requires a sexual act to trigger development of the egg, even though this behavior does not fertilize the egg. In parthenogenic ticks and mites, for example, the eggs develops only after the animals have mated, but the eggs remain unfertilized.
  
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==Parthenogenesis often occurs in combination with sexual reproduction or behavior==
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Some species of beetles that have no males require sperm to trigger development; these beetles mate with males of closely related species. However, the sperm does not contribute genetic material.
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examples; related topics – gynogenesis, etc
 
  
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Most reptiles reproduce sexually, but parthenogenesis has been observed in certain species of [[Cnemidophorus|whiptail]]s, [[gecko]]s, [[Darevskia|rock lizard]]s<ref name="reptiles"/>, and [[Komodo dragon]]s.  
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In other parthenogenic species lacking males, females stimulate each other to activate the neuroendocrine mechanisms necessary for egg maturation. This phenomenon has been extensively studied in the [[Cnemidophorus neomexicanus|New Mexico whiptail]] (genus ''[[Cnemidophorus]]''), of which 15 species reproduce exclusively by parthenogenesis. 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.
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Parthenogenesis has been extensively studied in the [[Cnemidophorus neomexicanus|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.
 
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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.
 
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Recently, the [[Komodo dragon]] which normally reproduces sexually was found to also be able to reproduce asexually by parthenogenesis.<ref>[http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2006/12/21/ndragon21.xml "No sex please, we're lizards"], Roger Highfield, Daily Telegraph, 21 December 2006</ref><ref>[http://www.nature.com/nature/journal/v444/n7122/full/4441021a.html "Parthenogenesis in Komodo dragons"]Watts PC, et al. . ''Nature'' 444, p1021, 2006.</ref> 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. <ref name="The Hindu">[http://www.hindu.com/2007/01/25/stories/2007012506101400.htm Virgin birth of dragons], The Hindu, 25 January 2007, Retrieved 3 February 2007 </ref> 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.<ref name="The Hindu"/> Parthenogenesis may also occur when males and females are both present, as the wild Komodo dragon population is approximately 75 percent male.
 
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==Sexual behavior sometimes needed==
 
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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 [[salamander]]s 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.
 
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Or: can’t females of certain species simulate that role?
 
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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.
 
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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]]''.
 
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A graphical representation of this can be seen [http://www.tolweb.org/notes/?note_id=579 here].
 
  
 
==Parthenogenesis can be a means of determining sex==
 
==Parthenogenesis can be a means of determining sex==
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examples like the honeybee – the unfertilized egg can be male or female depending on the chromosomal scheme
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Parthenogenesis involves the inheritance and subsequent duplication of only a single sex chromosome. The unfertilized egg can thus be male or female depending on the chromosomal scheme of the species:
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*If two like chromosomes determine the female sex (such as the [[XY sex-determination system]]), the offspring will be female.  
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An example of non-viable parthenogenesis is common among domesticated [[Western honey bee|honey bee]]s.  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 [[bee]]s may be truly parthenogenetic, for example, at least one species of small carpenter bee, in the genus ''[[Ceratina]]''.  Many [[parasitic wasp]]s are known to be parthenogenetic, sometimes due to infections by ''[[Wolbachia]]''.
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*If two like chromosomes determine the male sex (such as the [[ZW sex-determination system]]), the offspring will be male.
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In ''[[Cataglyphis cursor]]'', a European [[Formicinae|formicine ant]], the queen can reproduce by parthenogenesis. The workers are fertile and can mate with the males.<ref>[http://www.sciencemag.org/cgi/content/full/306/5702/1780 "Conditional Use of Sex and Parthenogenesis for Worker and Queen Production in Ants"] Pearcy M, et al. ''Science'' 306:1780, 2004.</ref>
 
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In [[Wasmannia|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.<ref>[http://www.nature.com/nature/journal/v435/n7046/full/nature03705.html "Clonal reproduction by males and females in the little fire ant"] Fournier D, et al. ''Nature'' 435:1230, 2005.</ref> (huh?)
 
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==section on controversial examples of parthenogenesis==
 
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(do we want to call this parthenogenesis?)
 
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<ref>http://www.msnbc.msn.com/id/18809674/</ref>
 
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<ref>http://news.bbc.co.uk/2/hi/science/nature/6681793.stm</ref>
 
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<ref>http://news.nationalgeographic.com/news/2002/09/0925_020925_virginshark.html</ref>
 
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<ref>http://www.livescience.com/animals/070522_asexual_sharks.html</ref>
 
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<ref>http://www.peteducation.com/article.cfm?cls=0&cat=1967&articleid=2710</ref>
 
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<ref>http://www.nytimes.com/2007/05/23/science/23shark.html Female Shark Reproduced Without Male DNA, Scientists Say</ref>
 
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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.<ref>http://edition.cnn.com/2007/TECH/science/05/23/virgin.sharks.ap/index.html</ref> 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|automictic parthenogenesis]]"). The type of reproduction exhibited had been seen before in bony fish but never in cartilaginous fish such as sharks.
 
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In 2002, two [[white-spotted bamboo shark]]s 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.  
 
  
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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.  
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[[Western honey bee|Honey bee]]s example (something on social altruism). Most females in the colony are sterile workers, but a few become fertile queens. After the queen mates, she has a supply of sperm that she controls, enabling her to produce either fertilized or unfertilized eggs. Thus, the queen determines when and how much of the colony’s resources are expended on the production of males (called drones).  
  
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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.
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==Recent discoveries of parthenogenic behavior in sexually reproducing species==
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The [[Komodo dragon]], which normally engages in sexual reproduction, was recently found to be able to reproduce asexually via parthenogenesis (Highfield, 2006; Watts, 2006). 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 parthenogenesis will be male (ZZ) or inviable (WW), with no WZ females being born. It has been postulated that this strategy might give the Komodo dragon an advantage in the colonization of islands, where a single female could theoretically have male offspring asexually, then switch to sexual reproduction to maintain a higher level of genetic diversity than asexual reproduction alone could produce.  
  
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Unlike [[Komodo dragon]]s, 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.
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In 2001, a bonnethead (a type of small [[hammerhead shark]]) was thought to have produced a pup in captivity at a zoo in Nebraska. The tank contained three female hammerheads and no males. DNA testing showed that the pup's DNA matched only one female living in the tank, and that no male DNA was present in the pup.  The pup was not a twin or clone of the mother; rather, it contained only half her DNA (a process called automictic parthenogenesis"). The type of reproduction exhibited had been seen before in bony fish but never in cartilaginous fish such as sharks (Sample, 2007). Another apparent parthenogenic shark birth occurred in 2002, when two [[white-spotted bamboo shark]]s were born at the [[Belle Isle Aquarium]] in Detroit. The birth baffled experts as the mother shared an aquarium with only one other female shark.
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(is this section legit? Condense dramatically)
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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 dragon]]s, 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.
  
 
==Differences between parthenogenesis and cloning==
 
==Differences between parthenogenesis and cloning==
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There are no known cases of mammalian parthenogenesis in the wild.
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Parthenogenesis is distinct from artificial [[animal cloning]], a process in which the new organism is identical to the cell donor.  Parthenogenesis is truly a reproductive process that creates a new individual or individuals from the naturally varied genetic material contained in the eggs of the mother. However, in animals with an XY chromosome system where parthenogenic offspring (called parthenogens) are female, the offspring of a parthenogen are all genetically identical to each other and to the mother, as a parthenogen is [[homozygous]] (possessing two identical sets of genes).
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In April [[2004]], scientists at [[Tokyo|Tokyo University of Agriculture]] used parthenogenesis to successfully create fatherless mice: see [[Kaguya]].
 
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The beginnings of artificial human parthenogenesis has been performed in the lab.<ref>http://news.bbc.co.uk/1/hi/sci/tech/4228992.stm</ref>  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.
 
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==Apomixis is a similar process of asexual reproduction in plants==
 
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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 [[seed]]s are genetically identical to the parent plant. Although the [[Evolution|evolutionary]] advantages of [[sexual reproduction]] are lost, apomixis does pass along traits fortuitous for individual evolutionary fitness.
 
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As apomictic plants are genetically identical from one generation to the next, each has the characters of a true [[species]], maintaining distinctions from other [[Congener#Biology|congeneric]] apomicts, while having much smaller differences than is normal between species of most [[Genus|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 [[Flora (book)|Flora]]s 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]]'' ([[rowan]]s and [[whitebeam]]s), ''[[Rubus]]'' (brambles or blackberries), ''Hieracium'' ([[hawkweed]]s) and ''Taraxacum'' ([[dandelion]]s).
 
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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).
 
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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==
 
==References==
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<references/>
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*Highfield, Roger. 2006. [http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2006/12/21/ndragon21.xml No sex please, we're lizards.] ''Daily Telegraph''. Retrieved July 28, 2007.
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*add purves and Dr. Tatiana
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*Judson, O. 2002. ''Dr. Tatiana’s Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex''. New York, NY: Metropolitan Books. ISBN 0-805-06331-5
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>{{cite book
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*Purves, W., D. Sadava, G. Orians, and C. Heller. 2004. ''Life: The Science of Biology,'' 7th edition. Sunderland, MA: Sinauer.
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  | last = Halliday
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*Sample, Ian. 2007. [http://www.guardian.co.uk/science/2007/may/23/uknews.sciencenews Study confirms virgin birth of zoo shark pup.] ''The Guardian''. Retrieved August 6, 2007.
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  | first = Tim R.
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*Watts, P.C., et al. 2006. [http://www.nature.com/nature/journal/v444/n7122/full/4441021a.html Parthenogenesis in Komodo dragons.] ''Nature'' 444:1021.
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  | coauthors = Kraig Adler (eds.)
 
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  | title = Reptiles & Amphibians
 
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  | publisher = Torstar Books
 
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  |date= 1986
 
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  | pages = p. 101
 
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  | id = ISBN 0-920269-81-8 }}</ref>
 
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and [[shark]]s<ref>[http://www.washingtonpost.com/wp-dyn/content/article/2007/05/22/AR2007052201405.html "Female Sharks Can Reproduce Alone, Researchers Find"], Washington Post, Wednesday, May 23, 2007; Page A02</ref>). It
 
  
 
==Further reading==
 
==Further reading==
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* 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.
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* Dawley, R.M. and J.P. Bogart, James P. 1989. ''Evolution and Ecology of Unisexual Vertebrates''. Albany, NY: New York State Museum. ISBN 1-55557-179-4.
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* Futuyma, Douglas J. & Slatkin, Montgomery. (1983). ''Coevolution''. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-228-3.
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* Futuyma, D.J. and M. Slatkin. 1983. ''Coevolution''. Sunderland, MA: Sinauer Associates. ISBN 0-87893-228-3.
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* Maynard Smith, John. (1978). ''The Evolution of Sex''. Cambridge: Cambridge University Press. ISBN 0-521-29302-2.
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* Maynard Smith, J. 1978. ''The Evolution of Sex''. Cambridge: Cambridge University Press. ISBN 0-521-29302-2.
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* Michod, Richard E. & Levin, Bruce R. (1988). ''The Evolution of Sex''. Sunderland, Mass: Sinauer Associates.  ISBN 0-87893-459-6.
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* Michod, R.E. and Levin, B.R. 1988. ''The Evolution of Sex''. Sunderland, MA: Sinauer Associates.  ISBN 0-87893-459-6.
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* Schlupp, I. (2005) The evolutionary ecology of gynogenesis. Annu. Rev. Ecol. Evol. Syst. 36: 399-417.
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* Schlupp, I. 2005. The evolutionary ecology of gynogenesis. ''Annu Rev Ecol Evol Syst'' 36: 399-417.
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* Simon, Jean-Christophe, Rispe, Claude & Sunnucks, Paul. (2002). Ecology and evolution of sex in aphids. ''Trends in Ecology & Evolution, 17'', 34-39.
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* Simon, J, Rispe, C. and P. Sunnucks. 2002. Ecology and evolution of sex in aphids. ''Trends in Ecology & Evolution'' 17: 34-9.
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* Stearns, Stephan C. (1988). ''The Evolution of Sex and Its Consequences'' (Experientia Supplementum, Vol. 55). Boston: Birkhauser. ISBN 0-8176-1807-4.
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* Stearns, S.C. 1988. ''The Evolution of Sex and Its Consequences'' Experientia Supplementum, Vol. 55. Boston: Birkhauser. ISBN 0-8176-1807-4.
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* Phillip C. Watts, Kevin R. Buley, Stephanie Sanderson, Wayne Boardman, Claudio Ciofi and Richard Gibson. (2006). Parthenogenesis in Komodo dragons. ''Nature'' 444, 1021-1022
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* Watts, P.C., Buley, K.R., Sanderson, S., Boardman, W., Claudio, C. and R. Gibson. 2006. Parthenogenesis in Komodo dragons. ''Nature'' 444:1021-22.
  
 
==External links==
 
==External links==

Revision as of 14:58, 6 August 2007

The asexual whiptail species Cnemidophorus neomexicanus (center) consists exclusively of females who reproduce via parthenogenesis. C. neomexicanus is flanked by the sexually reproducing species that hybridized to generate it: C. inornatus (left) and C. tigris (right).

Parthenogenesis is a form of asexual reproduction in which offspring develop from unfertilized eggs. A common mode of reproduction in anthropods such as insects and arachnids, parthenogenesis also occurs in some species of fish, amphibians, and reptiles.

Parthenogenesis (which derives from the Greek words for "virgin" and "creation") is more efficient than sexual reproduction because it does not always depend on mating, which requires energy and usually involves risks. Moreover, all members of an asexual population are capable of reproducing. The disadvantage, however, is that asexual reproduction does not generate genotypic diversity, which is an important evolutionary advantage for sexually reproducing species.

Given the drawbacks of asexual reproduction for the long-term survival of the species, most species that engage in parthenogenesis also participate in sexual reproduction or sexual behaviors. Parthenogenesis thus serves as one available reproductive strategy, often a response to environmental or seasonal conditions such as the amount of available resources. Aphids, for example, are parthenogenic in spring and summer, multiplying rapidly while conditions are favorable; during the winter months, they mate and the females hatch fertilized eggs. In rare cases, however, parthenogenesis does not occur in combination with sexual reproduction or behaviors: the bdelloid rotifer philodina roseola, for example, reproduces exclusively by parthenogenesis, and the species has not engaged in sexual reproduction in 85 million years (Judson, 2002).

In addition to its reproductive role, parthenogenesis also functions as part of a mechanism for determining sex in some species. In ants and most species of bees and wasps, females develop from unfertilized eggs and are referred to as haploid (possessing one set of chromosomes), while males develop from fertilized eggs and hence are diploid (possessing two sets of chromosomes, one from each parent). Thus, in species also capable of sexual reproduction, parthenogenesis can help to regulate the relative number of males and females in a population.

Sexual behavior is sometimes required to stimulate development

In some species, parthenogenesis requires a sexual act to trigger development of the egg, even though this behavior does not fertilize the egg. In parthenogenic ticks and mites, for example, the eggs develops only after the animals have mated, but the eggs remain unfertilized.

Some species of beetles that have no males require sperm to trigger development; these beetles mate with males of closely related species. However, the sperm does not contribute genetic material.

In other parthenogenic species lacking males, females stimulate each other to activate the neuroendocrine mechanisms necessary for egg maturation. This phenomenon has been extensively studied in the New Mexico whiptail (genus Cnemidophorus), of which 15 species reproduce exclusively by parthenogenesis. 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.

Parthenogenesis can be a means of determining sex

Parthenogenesis involves the inheritance and subsequent duplication of only a single sex chromosome. The unfertilized egg can thus be male or female depending on the chromosomal scheme of the species:

  • If two like chromosomes determine the female sex (such as the XY sex-determination system), the offspring will be female.
  • If two like chromosomes determine the male sex (such as the ZW sex-determination system), the offspring will be male.

Honey bees example (something on social altruism). Most females in the colony are sterile workers, but a few become fertile queens. After the queen mates, she has a supply of sperm that she controls, enabling her to produce either fertilized or unfertilized eggs. Thus, the queen determines when and how much of the colony’s resources are expended on the production of males (called drones).

Recent discoveries of parthenogenic behavior in sexually reproducing species

The Komodo dragon, which normally engages in sexual reproduction, was recently found to be able to reproduce asexually via parthenogenesis (Highfield, 2006; Watts, 2006). 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 parthenogenesis will be male (ZZ) or inviable (WW), with no WZ females being born. It has been postulated that this strategy might give the Komodo dragon an advantage in the colonization of islands, where a single female could theoretically have male offspring asexually, then switch to sexual reproduction to maintain a higher level of genetic diversity than asexual reproduction alone could produce.

In 2001, a bonnethead (a type of small hammerhead shark) was thought to have produced a pup in captivity at a zoo in Nebraska. The tank contained three female hammerheads and no males. DNA testing showed that the pup's DNA matched only one female living in the tank, and that no male DNA was present in the pup. The pup was not a twin or clone of the mother; rather, it contained only half her DNA (a process called automictic parthenogenesis"). The type of reproduction exhibited had been seen before in bony fish but never in cartilaginous fish such as sharks (Sample, 2007). Another apparent parthenogenic shark birth occurred in 2002, when two white-spotted bamboo sharks were born at the Belle Isle Aquarium in Detroit. The birth baffled experts as the mother shared an aquarium with only one other female shark.

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.

Differences between parthenogenesis and cloning

Parthenogenesis is distinct from artificial animal cloning, a process in which the new organism is identical to the cell donor. Parthenogenesis is truly a reproductive process that creates a new individual or individuals from the naturally varied genetic material contained in the eggs of the mother. However, in animals with an XY chromosome system where parthenogenic offspring (called parthenogens) are female, the offspring of a parthenogen are all genetically identical to each other and to the mother, as a parthenogen is homozygous (possessing two identical sets of genes).

References
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Further reading

  • Dawley, R.M. and J.P. Bogart, James P. 1989. Evolution and Ecology of Unisexual Vertebrates. Albany, NY: New York State Museum. ISBN 1-55557-179-4.
  • Futuyma, D.J. and M. Slatkin. 1983. Coevolution. Sunderland, MA: Sinauer Associates. ISBN 0-87893-228-3.
  • Maynard Smith, J. 1978. The Evolution of Sex. Cambridge: Cambridge University Press. ISBN 0-521-29302-2.
  • Michod, R.E. and Levin, B.R. 1988. The Evolution of Sex. Sunderland, MA: Sinauer Associates. ISBN 0-87893-459-6.
  • Schlupp, I. 2005. The evolutionary ecology of gynogenesis. Annu Rev Ecol Evol Syst 36: 399-417.
  • Simon, J, Rispe, C. and P. Sunnucks. 2002. Ecology and evolution of sex in aphids. Trends in Ecology & Evolution 17: 34-9.
  • Stearns, S.C. 1988. The Evolution of Sex and Its Consequences Experientia Supplementum, Vol. 55. Boston: Birkhauser. ISBN 0-8176-1807-4.
  • Watts, P.C., Buley, K.R., Sanderson, S., Boardman, W., Claudio, C. and R. Gibson. 2006. Parthenogenesis in Komodo dragons. Nature 444:1021-22.

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