Parthenogenesis (from the Greek παρθένος parthenos, "virgin", + γένεσις genesis, "creation") is an asexual form of reproduction found in females where growth and development of embryos or seeds occurs without fertilization by males. The offspring produced by parthenogenesis almost always are female in species where the XY chromosome system determines gender.
Parthenogenesis occurs naturally in some species, including most lower plants, a Kalanchoe succulent plant genus of South Africa, invertebrates (e.g. water fleas, aphids, some bees, some scorpion species, and parasitic wasps), and vertebrates (e.g. some reptiles, fish, and, very rarely, birds and sharks) and this type of reproduction has been induced artificially in other species.
The term is sometimes used inaccurately to describe reproduction modes in hermaphroditic species which can reproduce by themselves because they contain reproductive organs of both genders.
Parthenogenesis is a form of asexual reproduction in which females produce eggs that develop without fertilization. Parthenogenesis is seen to occur naturally in aphids, daphnia, rotifers, and some other invertebrates, as well as in many plants. Komodo dragons 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).
The offspring of parthenogenesis will be all female if two like chromosomes determine the female gender (such as the XY sex-determination system), but they will be male if two like chromosomes determine the male gender (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 (low genetic diversity and susceptibility to adverse mutations that might occur) and benefits (reproduction without the need for a male) associated with parthenogenesis. Asexual reproduction existed alone from the beginning of life on Earth for many epochs. When sexual reproduction arose (presumably through mutation) it introduced a means to expand genetic diversity through the partial contribution of the male, providing more options for the survival of the species in which it began. Many species followed this reproductive path successfully, some to the exclusion of the asexual pattern from which it arose, some enabling both, some retaining the capacity to revert to asexual reproduction—if necessary, and yet others that abandoned sexual reproduction and reverted to the asexual. There are species, such as some Kalanchoe plants, that once had the capacity to reproduce sexually, but wherein no males ever have been discovered and the species reproduces prolifically via its retained asexual methods alone.
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 that daughter when she becomes a mother and her offspring will be identical to each other, as a parthenogen is homozygous. Each sibling of the first generation of parthenogens will, therefore, create a linage that carries only her unique genes when reproducing via parthenogenesis.
Parthenogenesis may be achieved through an artificial process, however, as described below under the discussion of mammals.
The alternation between parthenogenesis and sexual reproduction is called heterogamy. Forms of reproduction related to parthenogenesis, but that only require the presence of sperm that do not fertilize an egg, are known as gynogenesis and hybridogenesis.
Parthenogenesis in insects can cover a wide range of mechanisms.
The different forms include:
- thelytoky - parthenogenesis in which only female offspring are produced and no mating is observed
- pseudogamy (or gynogenesis or "sperm-dependent parthenogenesis") - here mating occurs and the eggs require activation by entry of sperm but only the maternal chromosomes are expressed
- automixis - parthenogenesis in which the eggs undergo meiosis
- apomixis - parthenogenesis in which the eggs do not undergo meiosis
Polyembryony is another process that produces multiple clonal offspring from a single egg. This is known in some hymenopteran parasitoids and in strepsiptera.
In automictic species the offspring can be haploid or diploid. Diploids are produced by doubling or fusion of gametes after meiosis. Fusion is seen in the Phasmatodea, Hemiptera (Aleurodids and Coccidae), Diptera, and some Hymenoptera.
In addition to these forms is hermaproditism, where both the eggs and sperm are produced by the same individual. This is seen in three species of Icerya scale insects.
Parasitic bacteria like Wolbachia have been noted to induce automictic thelytoky in many insect species with haplodiploid systems. They also cause gamete duplication in unfertilized eggs causing them to develop into female offspring.
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.
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 little fire ants, Wasmannia auropunctata, queens produce more queens through parthenogenesis. Sterile workers usually are produced from eggs fertilized by males. In some of the eggs fertilized by males, however, the fertilization can cause the female genetic material to be ablated from the zygote, in a process called ameiotic parthenogenesis. In this way, males pass on only their genes to become fertile male offspring. This type of reproduction results in a complete separation of the gene pools for females and males. This is the first recognized example of an animal species where both females and males can reproduce separately. 
Crustacean reproduction varies both across and within species. The water flea Daphnia pulex alternates between sexual and parthenogenetic reproduction . Among the better-known large decapod crustaceans, only one species is known to reproduce by parthenogensis. "Marmorkrebs" are parthenogenetic crayfish that were discovered in the pet trade in the 1990s . This species currently has no formal scientific name, and its origin is unknown. Offspring are genetically identical to the parent, indicating it reproduces by apomixis .
Parthenogenesis has been studied extensively 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 behaviour is due to the hormonal cycles of the females, which cause them to behave like males shortly after laying eggs, when levels of progesterone are high, and to take the female role in mating before laying eggs, when estrogen dominates. Lizards who 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 behavioral stimuli for maximum reproductive success.
Recently, the Komodo dragon, which normally reproduces sexually, was found also to 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, and 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 known 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 with them to maintain higher level of genetic diversity than asexual reproduction alone can generate.
Parthenogenesis may also occur naturally when males and females are both present, explaining why the wild Komodo dragon population is approximately 75 percent male.
In 2001 a bonnethead, a type of small hammerhead shark, was found to have produced a pup, born live on the 14th December, 2001 at Henry Doorly Zoo in Nebraska, in a tank containing three female hammerheads, but no males. The pup was thought to have been conceived through parthenogenic means. 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, Southeastern University in Florida, and Henry Doorly Zoo itself, and it was concluded after DNA testing that the reproduction was parthenogenic. The testing showed the female pup's DNA matched only one female who lived in the tank, and that no male DNA was present in the pup. The pup was not a twin or clone of her mother, but rather, contained only half of her mother's DNA ("automictic parthenogenesis"). This type of reproduction had been seen before in bony fish, but never in cartilaginous fish such as sharks, until this documentation.
In 2002, two white-spotted bamboo sharks were born at the Belle Isle Aquarium in Detroit. They hatched 15 weeks after being laid. The births baffled experts as the mother shared an aquarium with only one other shark, who was female. 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 fertilize them. Normally, the eggs are assumed to be infertile and are discarded. This batch of eggs was left undisturbed by the curator as he had heard about the previous birth in 2001 in Nebraska and wanted to observe whether they would hatch.
Other possibilities had been considered for the birth of the Detroit bamboo sharks including thoughts that the sharks had been fertilized by a male and stored the sperm for a period of time, as well as 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, but that is not confirmed.
In 2008, a Hungarian aquarium had another case of parthenogenesis after its lone female shark produced a pup without ever having come into contact with a male shark.
The repercussions of parthenogenesis in sharks, which fails to increase 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. Although parthenogenesis may help females who cannot find mates, it does reduce genetic diversity.
Unlike Komodo dragons, which have a WZ chromosome system and produce even 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 sexual reproduction resulting in males can occur.
There are no known cases of naturally-occurring mammalian parthenogenesis in the wild.
It is highly doubtful that artificial human parthenogenesis would be used to reproduce humans, due to technical (see imprinting below) concerns. Use of an electrical or chemical stimulus can produce the beginning of the process of parthenogenesis in the asexual development of viable offspring.
Induced parthenogenesis in mice and monkeys often results in abnormal development. This is because mammals have imprinted genetic regions, where either the maternal or the paternal chromosome is inactivated in the offspring in order for development to proceed normally. A mammal created by parthenogenesis would thus have double doses of maternally imprinted genes and lack paternally imprinted genes, leading to developmental abnormalities if any were present in the genes of the mother. As a consequence, research on human parthenogenesis is focused on the production of embryonic stem cells for use in medical treatment, not as a reproductive strategy.
On June 26, 2007 International Stem Cell Corporation (ISC), a California based stem cell research company, announced that their lead scientist, Dr. Elena Revazova, and her research team were the first to intentionally create human stem cells from unfertilized human eggs using parthenogenesis.  The process may offer a way for creating stem cells that are genetically matched to a particular woman for the treatment of degenerative diseases that might affect her.
On August 2, 2007, after much independent investigation, it was revealed that discredited South Korean scientist, Hwang Woo-Suk, unknowingly produced the first human embryos resulting from parthenogenesis. Initially, Hwang claimed he and his team had extracted stem cells from cloned human embryos, a result which was later found to be fabricated. Further examination of the chromosomes of these cells show indicators of parthenogenesis in those extracted stem cells, similarly to those found in the mice created by Tokyo scientists in 2004. Although Hwang deceived the world about being the first to create artificially cloned human embryos, he did contribute a major breakthrough to stem cell research by creating human embryos using parthenogenesis. 
On December 18, 2007 Dr. Revazova and ISC published an on-line article in the journal Cloning and Stem Cells illustrating a breakthrough in the use of parthenogenesis to produce human stem cells that are homozygous in the "HLA" region of the DNA. These stem cells are called HLA homozygous parthenogenetic human stem cells (hpSC-Hhom) and have unique characteristics that will allow derivatives of these cells to be implanted into millions of people without immune rejection.  With proper selection of oocyte donors according to HLA haplotype, it is possible to generate a bank of cell lines, whose tissue derivatives collectively, could be MHC-matched with a significant number of individuals within the human population.
Although the truth about the results of Hwang's work were just discovered, those embryos were created by him and his team before February 2004, making Hwang the first, although unknowingly, to perform the process of parthenogenesis to create a human embryo and ultimately a human parthenogenetic stem cell line successfully. In 2006, a group of Italian researchers announced they had achieved the same feat, but have yet to publish their results. Therefore, ISC is the first organization to achieve artificial parthenogenesis that intentionally led to the creation of human parthenogenetic stem cell lines from unfertilized eggs. 
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 merely 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 are all female, activation of their eggs requires mating with males of a closely related species for the needed stimulus. 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 a rare fertilization of eggs by a male, introducing new material to the gene pool, which may result from perhaps, only one mating out of a million.
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 live-bearing fish of the genus Poeciliopsis as well as in the waterfrog Rana esculenta and the donor waterfrog species Rana lessonae.
A graphical representation of this can be seen through this link.
This is defined as a reproduction resulting when the set of chromosomes acquired from the mother, pairs with an exact copy of itself, which can be described as "half a clone". The animal still is unique and not a clone of her mother. In typical parthenogenesis the individual offspring differ from one another and their mother.
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