[[Image:Mouse 180.jpg|thumb|Kaguya is one success from 460 attempts at growing embryos. (c) T. Kono]] Parthenogenesis (Partheno-genesis from the Greek language παρθενος, "virgin", + γενεσις, "birth") means the growth and development of an embryo or seed without fertilization by a male. Parthenogenesis occurs naturally in some lower plants (called ''agamospermy''), invertebrates (e.g. water fleas, aphids) and some vertebrates (e.g. lizards, salamanders, some fish, and even Turkey_(bird)). Parthenogenetic populations are typically all-female. As with all types of asexual reproduction, there are both costs and benefits associated with parthenogenesis. Parthenogenesis is seen as a possible way to cloning primates, with the emphasis on human cloning. In April 2004, scientists at Tokyo University of Agriculture used parthenogenesis to successfully create fatherless mice. The alteration 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''. ==Asexual versus sexual reproduction== Asexuality 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 (since individuals are always female) everyone can produce viable eggs. In sexual populations half 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. Organisms that can reproduce through parthenogenesis are also more able to settle isolated habitats like oceanic islands, as only a single (female) member of the species has to reach the habitat to start the population. Another consequence of asexual reproduction, which may have both benefits and costs, is that offspring are typically genetically identical or nearly identical to their parent. This genetic similarity can 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 at least some members surivive 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 assexual 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. ==Parthenogenesis== ''Parthenogenesis'' is a particular form of asexual reproduction in which females produce eggs that develop without fertilization. Parthenogenesis is seen in aphids, 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 form of asexual reproduction, including true parthenogenesis, gynogenesis, and hybridogenesis, an incomplete form of parthenogenesis. Among the reptiles, about fifteen species of whiptail lizard (genus Cnemidophorus) 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 whiptail lizards is that mating behaviors are still seen even though the populations are entirely female. One female plays the role of a male lizard and mounts the female that is about to produce eggs. The reason the animals act this way is due to their hormonal cycles, which cause some to act as males when levels of estrogen are low, and others to take the role of female when estrogen levels are high. Lizards that act out the courtship ritual have greater fecundity than those kept in isolation due to the increase in hormones that accompanies the fake sex. So, even though asexual whiptail lizards populations lack males, they still require sexual stimuli for maximum reproductive success. ==Gynogenesis== A form of asexual reproduction related to parthenogenesis is ''gynogenesis''. In gynogenesis, 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. ==Hybridogenesis== In ''hybridogenesis'' reproduction is not completely asexual but instead ''hemiclonal'', with half the genome passing intact to the next generation while the other half is replaced. In hybridogenetic species, females mate with males and both individuals contribute genetic material to the offspring. But when the female offspring produce their own eggs, the eggs contain no genetic material from their father; instead the eggs contains an exact copy of the chromosomes those offspring got from their 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 of the guppy species in the genus Poeciliopsis . ==See also== * Apomixis for a similar process in plants. * Parthenocarpy * Virgin Birth * Jacques Loeb * Gregory Goodwin Pincus ==References== * Dawley, Robert M. & Bogart, James P. (1989). Evolution and Ecology of Unisexual Vertebrates. Albany, New York: New York State Museum. ISBN 1555571794. * Futuyma, Douglas J. & Slatkin, Montgomery. (1983). Coevolution. Sunderland, Mass: Sinauer Associates. ISBN 0878932283. * Maynard Smith, John. (1978). The Evolution of Sex. Cambridge: Cambridge University Press. ISBN 0521293022. * Michod, Richard E. & Levin, Bruce R. (1988). The Evolution of Sex. Sunderland, Mass: Sinauer Associates. ISBN 0878934596. * 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 0817618074. ==External links== *[http://www.nature.com/news/2004/040419/pf/040419-8_pf.html Parthenogenesis of Mice at Nature.com] *[http://www.utexas.edu/research/crewslab/index.html Reproductive behavior in whiptails at Crews Laboratory] *[http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AsexualReproduction.html Types of asexual reproduction] Biological reproduction
My genetics course said that parthenogenesis in mammals is impossible, or at least ''extremely'' difficult. Obviously, there are examples of experiments on this main page, but I suspect they had problems with this: parthenogenesis is difficult for mammals because mammals posses a unique genetic characteristic: X-linked innactivation. The general idea is that mammals, more so than other vertebrates, are likely to have a female inseminated by more than one male at a time, and to actually have embryos within the same womb from different fathers. This led to the natural selection directed process whereby certain genes from the male chromosomes are methylated, turning them off, etc. and this tends to give that male's embryo's an advantage over the others. Meanwhile, the other embryos are doing the same thing; this leads to a tug of war, essentially, while at the same time the mother's genes in the embryo are methylated in a way that will favor the survival off ''all'' of the embryos. --->At any rate, the practical result is that if two DNA strands from only the mother were to combine (the parthenogenesis process as it normally occurs in certain lizards), the resulting mammal embryos would lack essential X-linked inactivations. X-linked innactivation mistakes, when they occur naturally in humans because of non-disjunction, etc., cause crippling genetic disorders in humans, i.e. "Laughing Puppet Syndrome" and others. I would imagine that any live parthenogenesis mammal births would suffer from devastating genetic syndromes. More research for the wiki article needs to go into that---Ricimer, April 12, 2005 Unless someone has a source to back up the claim that snakes can reproduce by parthenogenesis, it should be removed (as I have done) and stay removed. Parthenogenesis is well noted in some species of lizards and salamanders (such as the Jefferson's Salamander and various Blue Salamander hybrids) for which there are no males. I'm unaware of anything remotely like that in snakes, unless you want to count false pregnancies (where a female snake which has not been impregnated lay unfertilized eggs, or in the case of ovoviviporous [spelled wrong, trust me] snakes, weird jelly-bean-like nuggets). I know enough about reptiles and amphibians that I'm comfortable making those edits, but I'm not so sure about Turkeys... Anyone have some evidence they wanna post? -- I've just done some research and parthenogenesis has occured in some snakes, but it is by no means a common occurence (as it is with whiptails and some salamanders) and is not very well understood. As far as I could find are four known occurences (one timber rattlesnake which produced one litter, one wandering garter snake which consistently produces litters for ten years, a brahminy blind snake, and one other snake). ==Sharks== It would be nice to have some information about Shark on this page. Here's a general [http://news.nationalgeographic.com/news/2002/09/0925_020925_virginshark.html article] on putative parthenogenesis in Sharks. --User:Viriditas 10:13, 7 Sep 2004 (UTC) ---- The exact process, difference in parts involved and biochemistry, etc, still needs expansion and explanation. --User:Tchalvak
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Parthenogenesis
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[[Image:Mouse 180.jpg|thumb|Kaguya is one success from 460 attempts at growing embryos. (c) T. Kono]] Parthenogenesis (Partheno-genesis from the Greek language παρθενος, "virgin", + γενεσις, "birth") means the growth and development of an embryo or seed without fertilization by a male. Parthenogenesis occurs naturally in some lower plants (called ''agamospermy''), invertebrates (e.g. water fleas, aphids) and some vertebrates (e.g. lizards, salamanders, some fish, and even Turkey_(bird)). Parthenogenetic populations are typically all-female. As with all types of asexual reproduction, there are both costs and benefits associated with parthenogenesis. Parthenogenesis is seen as a possible way to cloning primates, with the emphasis on human cloning. In April 2004, scientists at Tokyo University of Agriculture used parthenogenesis to successfully create fatherless mice. The alteration 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''. ==Asexual versus sexual reproduction== Asexuality 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 (since individuals are always female) everyone can produce viable eggs. In sexual populations half 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. Organisms that can reproduce through parthenogenesis are also more able to settle isolated habitats like oceanic islands, as only a single (female) member of the species has to reach the habitat to start the population. Another consequence of asexual reproduction, which may have both benefits and costs, is that offspring are typically genetically identical or nearly identical to their parent. This genetic similarity can 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 at least some members surivive 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 assexual 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. ==Parthenogenesis== ''Parthenogenesis'' is a particular form of asexual reproduction in which females produce eggs that develop without fertilization. Parthenogenesis is seen in aphids, 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 form of asexual reproduction, including true parthenogenesis, gynogenesis, and hybridogenesis, an incomplete form of parthenogenesis. Among the reptiles, about fifteen species of whiptail lizard (genus Cnemidophorus) 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 whiptail lizards is that mating behaviors are still seen even though the populations are entirely female. One female plays the role of a male lizard and mounts the female that is about to produce eggs. The reason the animals act this way is due to their hormonal cycles, which cause some to act as males when levels of estrogen are low, and others to take the role of female when estrogen levels are high. Lizards that act out the courtship ritual have greater fecundity than those kept in isolation due to the increase in hormones that accompanies the fake sex. So, even though asexual whiptail lizards populations lack males, they still require sexual stimuli for maximum reproductive success. ==Gynogenesis== A form of asexual reproduction related to parthenogenesis is ''gynogenesis''. In gynogenesis, 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. ==Hybridogenesis== In ''hybridogenesis'' reproduction is not completely asexual but instead ''hemiclonal'', with half the genome passing intact to the next generation while the other half is replaced. In hybridogenetic species, females mate with males and both individuals contribute genetic material to the offspring. But when the female offspring produce their own eggs, the eggs contain no genetic material from their father; instead the eggs contains an exact copy of the chromosomes those offspring got from their 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 of the guppy species in the genus Poeciliopsis . ==See also== * Apomixis for a similar process in plants. * Parthenocarpy * Virgin Birth * Jacques Loeb * Gregory Goodwin Pincus ==References== * Dawley, Robert M. & Bogart, James P. (1989). Evolution and Ecology of Unisexual Vertebrates. Albany, New York: New York State Museum. ISBN 1555571794. * Futuyma, Douglas J. & Slatkin, Montgomery. (1983). Coevolution. Sunderland, Mass: Sinauer Associates. ISBN 0878932283. * Maynard Smith, John. (1978). The Evolution of Sex. Cambridge: Cambridge University Press. ISBN 0521293022. * Michod, Richard E. & Levin, Bruce R. (1988). The Evolution of Sex. Sunderland, Mass: Sinauer Associates. ISBN 0878934596. * 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 0817618074. ==External links== *[http://www.nature.com/news/2004/040419/pf/040419-8_pf.html Parthenogenesis of Mice at Nature.com] *[http://www.utexas.edu/research/crewslab/index.html Reproductive behavior in whiptails at Crews Laboratory] *[http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AsexualReproduction.html Types of asexual reproduction] Biological reproduction
Parthenogenesis
My genetics course said that parthenogenesis in mammals is impossible, or at least ''extremely'' difficult. Obviously, there are examples of experiments on this main page, but I suspect they had problems with this: parthenogenesis is difficult for mammals because mammals posses a unique genetic characteristic: X-linked innactivation. The general idea is that mammals, more so than other vertebrates, are likely to have a female inseminated by more than one male at a time, and to actually have embryos within the same womb from different fathers. This led to the natural selection directed process whereby certain genes from the male chromosomes are methylated, turning them off, etc. and this tends to give that male's embryo's an advantage over the others. Meanwhile, the other embryos are doing the same thing; this leads to a tug of war, essentially, while at the same time the mother's genes in the embryo are methylated in a way that will favor the survival off ''all'' of the embryos. --->At any rate, the practical result is that if two DNA strands from only the mother were to combine (the parthenogenesis process as it normally occurs in certain lizards), the resulting mammal embryos would lack essential X-linked inactivations. X-linked innactivation mistakes, when they occur naturally in humans because of non-disjunction, etc., cause crippling genetic disorders in humans, i.e. "Laughing Puppet Syndrome" and others. I would imagine that any live parthenogenesis mammal births would suffer from devastating genetic syndromes. More research for the wiki article needs to go into that---Ricimer, April 12, 2005 Unless someone has a source to back up the claim that snakes can reproduce by parthenogenesis, it should be removed (as I have done) and stay removed. Parthenogenesis is well noted in some species of lizards and salamanders (such as the Jefferson's Salamander and various Blue Salamander hybrids) for which there are no males. I'm unaware of anything remotely like that in snakes, unless you want to count false pregnancies (where a female snake which has not been impregnated lay unfertilized eggs, or in the case of ovoviviporous [spelled wrong, trust me] snakes, weird jelly-bean-like nuggets). I know enough about reptiles and amphibians that I'm comfortable making those edits, but I'm not so sure about Turkeys... Anyone have some evidence they wanna post? -- I've just done some research and parthenogenesis has occured in some snakes, but it is by no means a common occurence (as it is with whiptails and some salamanders) and is not very well understood. As far as I could find are four known occurences (one timber rattlesnake which produced one litter, one wandering garter snake which consistently produces litters for ten years, a brahminy blind snake, and one other snake). ==Sharks== It would be nice to have some information about Shark on this page. Here's a general [http://news.nationalgeographic.com/news/2002/09/0925_020925_virginshark.html article] on putative parthenogenesis in Sharks. --User:Viriditas 10:13, 7 Sep 2004 (UTC) ---- The exact process, difference in parts involved and biochemistry, etc, still needs expansion and explanation. --User:Tchalvak
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Parthenogenesis
Parthenogenesis
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