Parthenogenesis is a form of asexual reproduction where growth and development of embryos occur without fertilization. In plants, parthenogenesis means development of an embryo from an unfertilized egg cell, and is a component process of apomixis.
Gynogenesis and Pseudogamy are closely related phenomena where a sperm or pollen triggers the development of the egg into an embryo but makes no genetic contribution to the embryo. The rest of the cytology and genetics of these phenomena are mostly identical to that of parthenogenesis.
The word "parthenogenesis" comes from the Greek παρθένος, parthenos, meaning "virgin", and γένεσις, genesis, meaning "birth".[1] The term is sometimes used inaccurately to describe reproduction modes in hermaphroditic species that can reproduce by themselves because they contain reproductive organs of both sexes in a single individual's body.
Parthenogenesis occurs naturally in many plants, some invertebrate animal species (e.g., nematodes, water fleas, some scorpions, aphids, some bees, some Phasmida, and parasitic wasps) and a few vertebrates (e.g., some fish,[2] amphibians, reptiles,[3][4]and very rarely birds[5] ). This type of reproduction has been induced artificially in a few species including fish and amphibians.[6]
Normal egg cells form after meiosis and are haploid, with half as many chromosomes as their mother's body cells. Haploid individuals, however, are usually non-viable, and parthenogenetic offspring usually have the diploid chromosome number. Depending on the mechanism involved in restoring the diploid number of chromosomes, parthenogenetic offspring may have anywhere between all and half of the mothers alleles. The offspring having all of the mothers genetic material is called a full clone and those having only half are called "half clone". Full clones are usually formed without meiosis. If meiosis occurs, the offspring will get only a fraction of the mothers alleles.
Parthenogenetic offspring in species that use the XY sex-determination system have two X chromosomes and are female. In species that use the ZW sex-determination system, they have either two Z chromosomes (male) or two W chromosomes (mostly non-viable but rarely a female), or they could have one Z and one W chromosome (female).
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Some species reproduce exclusively by parthenogenesis (e.g., the bdelloid rotifers), while others can switch between sexual reproduction and parthenogenesis. This is called facultative parthnogenesis or cyclical parthenogenesis. This is also referred to by the terms heterogamy[7][8] or heterogony.[9][10] The switch between sexuality and parthenogenesis in such species may be triggered by the season (aphid, some gall wasps), or by a lack of males or by conditions that favour rapid population growth (rotifers and cladocerans like daphnia). In these species asexual reproduction occurs either in summer (aphids) or as long as conditions are favourable. This is because in asexual reproduction a successful genotype can spread quickly without being modified by sex or wasting resources on male offspring who won't give birth. In times of stress, offspring produced by sexual reproduction may be fitter as they have new, possibly beneficial gene combinations.
Many taxa with heterogony have within them species that have lost the sexual phase and are now completely asexual. Many other cases of obligate parthenogenesis (or gynogenesis) are found among polyploids and hybrids where the chromosomes cannot pair for meiosis.
The production of female offspring by parthenogenesis is referred to as thelytoky (e.g., aphids) while the production of males by parthenogenesis is referred to as arrhenotoky (e.g., bees).
Parthenogenesis can occur without meiosis through mitotic oogenesis. This is called apomictic parthenogenesis. Mature eggs are produced by mitotic divisions, and these cells directly develop into embryos. In flowering plants, cells of the gametophyte can undergo this process . The offspring produced by apomictic parthenogenesis are full clones of their mother.(e.g., aphids)
Parthenogenesis involving meiosis is more complicated. In some cases, the offspring are haploid (e.g., male ants) while in most others, the ploidy is restored to diploidy by various means. This is because haploid individuals are not viable in most species. These types of parthenogenesis are collectively called automictic parthenogenesis. In automictic parthenogenesis the offspring differ from one another and from their mother. They are called half clones of their mother.
Automixis is a term that covers several reproductive mechanisms, some of which are parthenogenetic.[11]
Diploidy might be restored by the doubling of the chromosomes without cell division before meiosis begins or after meiosis is completed. This is referred to as an endomitotic cycle. This may also happen by the fusion of the first two blastomeres. Other species restore their ploidy by the fusion of the meiotic products. The chromosomes may not separate at one of the two anaphases (called restitutional meiosis), or the nuclei produced may fuse or one of the polar bodies may fuse with the egg cell at some stage during its' maturation.
Some authors consider all forms of automixis sexual as they involve recombination. Many others classify the endomitotic variants as asexual, and consider the resulting embryos parthenogenetic. Among these authors the threshold for classifying automixis as a sexual process depends on when the products of anaphase I or of anaphase II are joined together. The criterion for "sexuality" varies from all cases of restitutional meiosis,[12] to those where the nuclei fuse or to only those where gametes are mature at the time of fusion.[11] Those cases of automixis that are classified as sexual reproduction are compared to self-fertilization in their mechanism and consequences.
The genetic composition of the offspring depends on what type of apomixis takes place. When endomitosis occurs before meiosis[13][14] or when central fusion occurs (restitutional meiosis of anaphase I or the fusion of it's products), the offspring get all[15][13] to more than half of the mothers genetic material and heterozygosity is mostly preserved[16] (if the mother has two alleles for a locus, it is likely that the offspring will get both). This is because in anaphase I the homologous chromosomes are separated. Heterozygosity is not completely preserved when crossing over occurs in central fusion[17]. In the case of pre-meiotic doubling, recombination -if it happens- occurs between identical sister chromatids.[13]
If terminal fusion (restitutional meiosis of anaphase II or the fusion of it's products) occurs, a little over half the mothers genetic material is present in the offspring and the offspring are mostly homozygous.[18] This is because at anaphase II the sister chromatids are separated and whatever heterozygosity is present is due to crossing over. In the case of endomitosis after meiosis the offspring is completely homozygous and has only half the mothers genetic material.
This can result in parthenogenetic offspring being unique from each other and from their mother.
In apomictic parthenogenesis, the offspring are clones of the mother and hence are usually (except for e.g., aphids) female. In the case of aphids, parthenogenetically produced males and females are clones of their mother except that the males lack one of the X chromosomes (XO).[19]
When meiosis is involved, the sex of the offspring will depend on the type of sex determination system and the type of apomixis. In species that use the XY sex-determination system, parthenogenetic offspring will have two X chromosomes and are female. In species that use the ZW sex-determination system the offspring genotype may be one of ZW (female),[15][16] ZZ (male), or WW (non-viable in most species[18] but a fertile, viable female in a few (e.g., boas)).[18] ZW offspring are produced by endoreplication before meiosis or by central fusion.[15][16] ZZ and WW offspring occur either by terminal fusion[18] or by endomitosis in the egg.
In polyploid obligate parthenogens like the whiptail lizard, all the offspring are female.[14]
Parthenogenesis is seen to occur naturally in aphids, Daphnia, rotifers, nematodes and some other invertebrates, as well as in many plants and certain lizards. Komodo dragons and the hammerhead- and blacktip 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). As with all types of asexual reproduction, there are both costs (low genetic diversity and therefore susceptibility to adverse mutations that might occur) and benefits (reproduction without the need for a male) associated with parthenogenesis.
Parthenogenesis is distinct from artificial animal cloning, a process where the new organism is necessarily genetically identical to the cell donor. In cloning, the nucleus of a diploid cell from a donor organism is inserted into an enucleated egg cell and the cell is then stimulated to undergo continued mitosis, resulting in an organism that is genetically identical to the donor. Parthenogenesis is different, in that it originates from the genetic material contained within an egg cell.
Parthenogenesis may be achieved through an artificial process as described below under the discussion of mammals.
Parthenogenesis in insects can cover a wide range of mechanisms.[20] The offspring produced by parthenogenesis may be of both sexes, only female (thelytoky, e.g. aphids) or only male (arrhenotoky, e.g. most hymenopterans). Both true parthenogenesis and pseudogamy (gynogenesis or sperm-dependent parthenogenesis) are known to occur. The eggs, depending on the species may be produced without meiosis (apomictically) or by one of the several automictic mechanisms.
A related phenomenon, polyembryony is a process that produces multiple clonal offspring from a single egg. This is known in some hymenopteran parasitoids and in Strepsiptera.[20]
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.[20]
In addition to these forms is hermaphroditism, where both the eggs and sperm are produced by the same individual, but is not a type of parthenogenesis. This is seen in three species of Icerya scale insects.[20]
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.[20]
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 mate only 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 the colony.[17]
In Central and South American electric 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 is the first recognized example of an animal species where both females and males can reproduce clonally resulting in a complete separation of male and female gene pools.[21]
Crustacean reproduction varies both across and within species. The water flea Daphnia pulex alternates between sexual and parthenogenetic reproduction.[22] Among the better-known large decapod crustaceans, some crayfish reproduce by parthenogensis. "Marmorkrebs" are parthenogenetic crayfish that were discovered in the pet trade in the 1990s.[23] Offspring are genetically identical to the parent, indicating it reproduces by apomixis, i.e. parthenogenesis in which the eggs did not undergo meiosis.[24] Spinycheek crayfish (Orconectes limosus) can reproduce both sexually and by parthenogenesis.[25] The Louisiana red swamp crayfish (Procambarus clarkii), which normally reproduces sexually, has also been suggested to reproduce by parthenogenesis,[26] although no individuals of this species have been reared this way in the lab.
In bdelloid rotifers, females reproduce exclusively by parthenogenesis,[27] while in monogonont rotifers, females can alternate between sexual and asexual reproduction (cyclical parthenogenesis). At least in one normally cyclical parthenogenetic species obligate parthenogenesis can be inherited: a recessive allele leads to loss of sexual reproduction in homozygous offspring.[28]
At least two species in the genus Dugesia, flatworms in the Turbellaria sub-division of the phylum Platyhelminthes, include polyploid individuals that reproduce by parthenogenesis.[29] This type of parthenogenesis requires mating, but the sperm does not contribute to the genetics of the offspring (the parthenogenesis is pseudogamous, alternatively referred to as gynogenetic). A complex cycle of matings between diploid sexual and polyploid parthenogenetic individuals produces new parthenogenetic lines.
Parthenogenetic Thiarid snails have slender cone shaped shells. They live in muddy stream bottoms and feed on detritus and algae.
Most reptiles reproduce sexually, but parthenogenesis has been observed to occur naturally in certain species of whiptails, geckos, rock lizards,[3] blindsnakes, Komodo dragons and boa constrictors.[6] Some of these like the gecko Lepidodactylus lugubris, the hybrid whiptails and the brahmini blind snake are unisexual and obligately parthenogenetic. Others like the Komodo dragon and some boas and pythons are facultatively parthenogenic.
Reptiles use the ZW chromosome system, which produces either males (ZZ) or females (ZW or WW). Until 2010, it was thought that the ZW chromosome system used by reptiles was incapable of producing viable WW offspring, but a (ZW) female boa constrictor was discovered to have produced viable female offspring with WW chromosomes.[30]
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. Recently, a hybrid parthenogenetic whiptail (genus Aspidoscelis) was bred in the laboratory from an cross between a asexual and a sexual whiptail[31]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.[32]
The Komodo dragon, which normally reproduces sexually, has also been found able to reproduce asexually by parthenogenesis.[33][34] A case has been documented of a Komodo Dragon switching back to sexual reproduction after a known parthenogenetic event.[35] 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 a higher level of genetic diversity than asexual reproduction alone can generate.[35]
Parthenogenesis may also occur naturally when males and females are both present, which might explain why the wild Komodo dragon population is approximately 75 percent male.
A bonnethead, a type of small hammerhead shark, was found to have produced a pup, born live on 14 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 days of birth.[36] 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, which was female. The female bamboo sharks had laid eggs in the past. This is not unexpected, as many animals will lay eggs even if there is not a male to fertilize them. Normally, the eggs are assumed to be inviable 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. In the same year, a female Atlantic blacktip shark in Virginia reproduced via parthenogenesis.[37]
On 10 October 2008 scientists confirmed the second case of a virgin birth in a shark. The Journal of Fish Biology reported a study in which scientists said DNA testing proved that a pup carried by a female Atlantic blacktip shark in the Virginia Aquarium & Marine Science Center contained no genetic material from a male.
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.
Sharks have an XY sex-determination 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.[38][39][40][41][42][43][44]
There were early claims of parthenogenesis in birds but this can be attributed to non-scientific reasoning and motivation. According to Giraldus Cambrensis and Conor O'Cathfai who wrote an account of the history and topography of Ireland in the latter part of the 12th century, the Barnacle Goose reproduces without mating. It was claimed that since this makes it not the progeny of flesh, it could not be considered flesh itself, and this was used as a justification for eating it during periods of fasting. This appears to be the only basis for the claim that the goose is parthenogenetic, and no scientific evidence has been discovered in the modern era.
Parthenogenesis occurs in turkeys through doubling of haploid cells to diploid, and the rate at which this occurs could be increased by selective breeding.[45] The offspring produced by parthenogenesis were healthy, and as doubled haploids they were homogametic, and consequently all were male (in turkeys, like in many birds and in contradiction to mammals, the males are homogametic).
There are no known cases of naturally occurring mammalian parthenogenesis in the wild. Parthenogenetic progeny of mammals would have two X chromosomes, and would therefore be female.
In 1936, Gregory Goodwin Pincus reported successfully inducing parthenogenesis in a rabbit.[46] In April 2004, scientists at Tokyo University of Agriculture used parthenogenesis successfully to create a fatherless mouse. Using gene targeting, they were able to manipulate two imprinted loci H19/IGF2 and DLK1/MEG3 to produce bi-maternal mice at high frequency[47] and subsequently show that fatherless mice have enhanced longevity.[48]
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 have double doses of maternally imprinted genes and lack paternally imprinted genes, leading to developmental abnormalities. It has been suggested[49] that defects in placental folding or interdigitation are one cause of swine parthenote abortive development. 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.
Use of an electrical or chemical stimulus can produce the beginning of the process of parthenogenesis in the asexual development of viable offspring.
During oocyte development, high metaphase promoting factor (MPF) activity causes mammalian oocytes to arrest at the metaphase II stage until fertilization by a sperm. The fertilization event causes intracellular calcium oscillations, and targeted degradation of cyclin B, a regulatory subunit of MPF, thus permitting the MII-arrested oocyte to proceed through meiosis.
To initiate parthenogenesis of swine oocytes, various methods exist to induce an artificial activation that mimics sperm entry, such as calcium ionophore treatment, microinjection of calcium ions, or electrical stimulation. Treatment with cycloheximide, a non-specific protein synthesis inhibitor, enhances parthenote development in swine presumably by continual inhibition of MPF/cyclin B.[50] As meiosis proceeds, extrusion of the second polar is blocked by exposure to cytochalasin B. This treatment results in a diploid (2 maternal genomes) parthenote [(Bischoff et al., 2009), PMID 19571260] Parthenotes can be surgically transferred to a recipient oviduct for further development, but will succumb by developmental failure after ~30 days of gestation. The swine parthenote placentae often appears hypo-vascular: see free image (Figure 1) in linked reference.[49]
On June 26, 2007, International Stem Cell Corporation (ISCC), 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. In December 2007, Dr. Revazova and ISCC published an article[51] illustrating a breakthrough in the use of parthenogenesis to produce human stem cells that are homozygous in the HLA region of DNA. These stem cells are called HLA homozygous parthenogenetic human stem cells (hpSC-Hhom) and have unique characteristics that would allow derivatives of these cells to be implanted into millions of people without immune rejection.[52] 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.
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 later found to be fabricated. Further examination of the chromosomes of these cells show indicators of parthenogenesis in those extracted stem cells, similar 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.[53] Although the truth about the results of Hwang's work was just discovered, those embryos were created by him and his team before February 2004, making Hwang the first, unknowingly, to perform the process of parthenogenesis to create a human embryo and ultimately a human parthenogenetic stem cell line successfully.
Apomixis can apparently occur in Phytophthora,[54] an Oomycete. Oospores derived after an experimental cross were germinated, and some of the progeny were genetically identical to one or other parent, which would imply that meiosis did not occur and the oospores developed by parthenogenesis.
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 rare fertilization of eggs by males, introducing new material to the gene pool, which may result from perhaps only one mating out of a million. In addition, the amazon molly is known to reproduce by gynogenesis.
In hybridogenesis, reproduction is not completely asexual, but instead hemiclonal: Half the genome is passed intact to the next generation, while the other half is discarded. It occurs in some animals that are themselves hybrids between two different species.
Hybridogenetic females can mate with males of a "donor" species and both will contribute genetic material to the offspring. When each female offspring produces her own eggs, however, the eggs will contain no genetic material from her father (the donor), only the chromosomes from her 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 has half new genetic material from the father's side. This form of reproduction is seen in some live-bearing fish of the genus Poeciliopsis[55] as well as in the waterfrog Rana esculenta.[56]
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