The asexual, all-female whiptail species
Cnemidophorus neomexicanus (center), which reproduces via parthenogenesis, is shown flanked by two sexual species having males
C. inornatus(left) and
C. tigris (right), which hybridized naturally to form the
C. neomexicanus species.
Gynogenesis and
pseudogamy are closely related phenomena in which a
sperm or
pollen triggers the development of the egg cell 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 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.
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 mother's
alleles. The offspring having all of the mother's genetic material are called full
clones and those having only half are called half clones. Full clones are usually formed without meiosis. If meiosis occurs, the offspring will get only a fraction of the mother's alleles.
Parthenogenetic offspring in species that use either the
XY or the
X0 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).
Life history types[edit]
Some species reproduce exclusively by parthenogenesis (such as the
Bdelloid rotifers), while others can switch between sexual reproduction and parthenogenesis. This is called facultative parthenogenesis (other terms are cyclical parthenogenesis, 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. In addition, sexual reproduction provides the benefit of meiotic recombination between non-sister chromosomes, a process associated with repair of DNA double-strand breaks and other DNA damages that may be induced by stressful conditions.
[11][12] (See also
Meiosis section: Origin and function of meiosis.)
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). When unfertilized eggs develop into both males and females, the phenomenon is called deuterotoky.
[13]
Types and mechanisms[edit]
Parthenogenesis can occur without meiosis through mitotic oogenesis. This is called
apomictic parthenogenesis. Mature egg cells 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. Examples include aphids.
Parthenogenesis involving
meiosis is more complicated. In some cases, the offspring are haploid (e.g., male
ants). In other cases, collectively called
automictic parthenogenesis, the ploidy is restored to diploidy by various means. This is because haploid individuals are not viable in most species. In automictic parthenogenesis the offspring differ from one another and from their mother. They are called
half clones of their mother.
Automixis[edit]
The effects of central fusion and terminal fusion on heterozygosity
Automixis is a term that covers several reproductive mechanisms, some of which are parthenogenetic.
[14]
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,
[15] to those where the nuclei fuse or to only those where gametes are mature at the time of fusion.
[14] 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
[16][17] or when
central fusion occurs (restitutional meiosis of anaphase I or the fusion of its products), the offspring get all
[16][18] to more than half of the mother's genetic material and heterozygosity is mostly preserved
[19] (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.
[20] In the case of pre-meiotic doubling, recombination -if it happens- occurs between identical sister chromatids.
[16]
If
terminal fusion (restitutional meiosis of anaphase II or the fusion of its products) occurs, a little over half the mother's genetic material is present in the offspring and the offspring are mostly homozygous.
[21] 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 mother's genetic material.
This can result in parthenogenetic offspring being unique from each other and from their mother.
Sex of the offspring[edit]
In apomictic parthenogenesis, the offspring are clones of the mother and hence are usually (except for
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).
[22]
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),
[18][19] ZZ (male), or WW (non-viable in most species
[21] but a fertile,
[dubious – discuss] viable female in a few (e.g., boas)).
[21] ZW offspring are produced by
endoreplication before meiosis or by central fusion.
[18][19] ZZ and WW offspring occur either by terminal fusion
[21] or by endomitosis in the egg cell.
In polyploid obligate parthenogens like the whiptail lizard, all the offspring are female.
[17]
In many hymenopteran insects such as honeybees, female eggs are produced sexually, using sperm from a drone father, while the production of further drones (males) depends on the queen (and occasionally workers) producing unfertilised eggs. This means that females (workers and queens) are always diploid, while males (drones) are always haploid, and produced parthenogenetically.
Facultative parthenogenesis[edit]
Facultative parthenogenesis is the term for what occurs when a species that normally reproduces sexually undergoes asexual reproduction. This is in contrast to obligate parthenogenesis, where the females reproduce exclusively by asexual means. Facultative parthenogenesis is believed to be a response to a lack of a viable male. A female may undergo facultative parthenogenesis if a male is absent from the habitat or if it is unable to produce viable offspring. This behavior has been documented in
sharks, some
snakes,
Komodo dragons and a variety of domesticated birds. In fact, parthenogenesis (facultative and obligate) has been documented in approximately 70 vertebrate species.
[23] The first documented cases of facultative parthenogenesis were observed in females in captivity without viable males. It was thought for some time that facultative parthenogenesis was a phenomenon occurring specifically for those individuals in captivity, but recently researchers found evidence that North American
pit vipers undergo this process in the wild, even if males are in the vicinity.
[24] These pit vipers were the first example of wild vertebrates opting for asexual reproduction. The discovery of facultative parthenogenesis in wild vertebrates has spurred more research into this previously undiscovered aspect of vertebrate reproduction. Facultative parthenogenesis has also been recorded in
crustaceans and
decapods in nature.
[25]
Obligate parthenogenesis[edit]
Obligate parthenogenesis is the process in which organisms exclusively reproduce through asexual means.
[26] Many species have been shown to transition to obligate parthenogenesis over evolutionary time. Among these species, one of the most well documented transitions to obligate parthenogenesis was found in almost all metazoan taxa, albeit through highly diverse mechanisms. These transitions often occur as a result of inbreeding or mutation within large populations.
[27] There are a number of documented species, specifically salamanders and geckos, that rely on obligate parthenogenesis as their major method of reproduction. As such, there are over 80 species of unisex reptiles, mostly lizards but including a single snake species, amphibians and fishes in nature for which males are no longer a part of the reproductive process.
[24] A female will produce an ovum with a full set (two sets of genes) provided solely by the mother. Thus, a male is not needed to provide sperm to fertilize the egg. This form of asexual reproduction is thought in some cases to be a serious threat to biodiversity for the subsequent lack of gene variation and potentially decreased fitness of the offspring.
[26]
Natural occurrence[edit]
Parthenogenesis is seen to occur naturally in
aphids,
Daphnia,
rotifers,
nematodes and some other invertebrates, as well as in many plants. Among
vertebrates, strict parthenogenesis is only known to occur in lizards, snakes,
[28] birds
[29] and sharks,
[30] with fish, amphibians and reptiles exhibiting various forms of gynogenesis and hybridogenesis (an incomplete form of parthenogenesis).
[31] The first all-female (unisexual) reproduction in
vertebrateswas described in the fish
Poecilia formosa in 1932.
[32] Since then at least 50 species of unisexual vertebrate have been described, including at least 20 fish, 25 lizards, a single snake species, frogs, and salamanders.
[31] Other, usually sexual, species may occasionally reproduce parthenogenetically and
Komodo dragons and the
hammerhead and
blacktip sharks are recent additions to the known list of facultative parthenogenetic vertebrates. 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 and the new organism is not necessarily genetically identical to the parent.
Parthenogenesis may be achieved through an artificial process as described below under the discussion of mammals.
Insects[edit]
Parthenogenesis in insects can cover a wide range of mechanisms.
[33] 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.
[34] The egg cells, 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 cell. This is known in some hymenopteran parasitoids and in
Strepsiptera.
[33]
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.
[33]
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.
[33]
Honey Bee on a plum blossom
Among the
hymenopterans (ants, bees and wasps),
haploid males are produced from unfertilized eggs. Usually eggs are laid only by the queen, but the unmated workers may also lay haploid, male eggs either regularly (e.g. stingless bees) or under special circumstances. 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. This is a result of the lack of the queen's pheromones and the pheromones secreted by uncapped brood, which normally suppress ovarian development in workers. 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 if they have not been able to mate before the collapse of the colony. This behaviour is believed to have evolved to allow a doomed colony to produce drones which may mate with a virgin queen and thus preserve the colony's genetic progeny.
A few ants and bees are capable of producing diploid female offspring parthenogenetically. These include a honey bee subspecies from South Africa,
Apis mellifera capensis, where workers are capable of producing
diploid eggs parthenogenetically, and replacing the queen if she dies; other examples include some species of small carpenter bee, (genus
Ceratina). Many
parasitic wasps are known to be parthenogenetic, sometimes due to infections by
Wolbachia.
The workers in five
[20] ant species and the queens in some ants are known to reproduce by parthenogenesis. In
Cataglyphis cursor, a European
formicine ant, the queens and workers can produce new queens by parthenogenesis. The workers are produced sexually.
[20]
In Central and South American
electric ants,
Wasmannia auropunctata, queens produce more queens through automictic parthenogenesis with central fusion. 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 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.
[35] As a consequence, the males will only have fathers and the queens only mothers, while the sterile workers are the only ones with both parents of both genders.
These ants get both the benefits of both asexual and sexual reproduction
[20][35] — the daughters who can reproduce (the queens) have all of the mother's genes, while the sterile workers whose physical strength and disease resistance are important are produced sexually.
Another example of insect parthenogenesis can be found in gall-forming aphids (e.g.
Pemphigus betae), where females reproduce parthenogenetically during the gall-forming phase of their life cycle.
Crustaceans[edit]
Crustacean reproduction varies both across and within species. The
water flea Daphnia pulex alternates between sexual and parthenogenetic reproduction.
[36] 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.
[37] Offspring are genetically identical to the parent, indicating it reproduces by apomixis, i.e. parthenogenesis in which the eggs did not undergo meiosis.
[38] 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,
[39] although no individuals of this species have been reared this way in the lab.
Rotifers[edit]
In
bdelloid rotifers, females reproduce exclusively by parthenogenesis (obligate parthenogenesis),
[40] 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.
[41]
Flatworms[edit]
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.
[42] 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.
Reptiles[edit]
Komodo dragon,
Varanus komodoensis, is confirmed to be able to reproduce naturally by parthenogenesis.
Most reptiles reproduce sexually, but parthenogenesis has been observed to occur naturally in certain species of
whiptails, some
geckos,
rock lizards,
[3][46][47]Komodo dragons and a
blindsnake. Some of these like the
mourning gecko Lepidodactylus lugubris,
Indo-Pacific house gecko Hemidactylus garnotii, the hybrid whiptails
Cnemidophorus, Caucasian rock lizards
Darevskia, and the
brahminy blindsnake,
Indotyphlops braminus are unisexual and obligately parthenogenetic. Others reptiles, such as the Komodo dragon, other monitor lizards,
[48]and some species of
boas,
[6][21][49] pythons,
[19] filesnakes,
[50][51] gartersnakes[52] and
rattlesnakes[53][54] may be facultatively parthenogenic. There is a concern that facultative parthenogenesis in endangered reptiles, like Komodo dragons, being maintained in single-sex groups or individually, could have a damaging effect on the genetic diversity of the species
[23] and it may be necessary to maintain males and females together.
In 2012, facultative parthenogenesis was reported in wild vertebrates for the first time by US researchers amongst captured pregnant copperhead and cottonmouth female pit-vipers.
[55]
Some reptile species use a ZW chromosome system, which produces either males (ZZ) or females (ZW). 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.
[56]
Parthenogenesis has been studied extensively in the
New Mexico whiptail in the genus
Cnemidophorus (also known as
Aspidoscelis) 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
polyploidindividuals. 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 lizard was bred in the laboratory from a cross between an asexual and a sexual whiptail
[57] 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.
[58]
Some lizard parthenogens show a pattern of geographic parthenogenesis, occupying high mountain areas where their ancestral forms have an inferior competition ability.
[59] In Caucasian rock lizards of genus
Darevskia, which have six parthenogenetic forms of hybrid origin
[46][47][60] hybrid parthenogenetic form
D. "dahli" has a broader niche then either of its bisexual ancestors and its expansion throughout the Central
Lesser Caucasuscaused decline of the ranges of both its maternal and paternal species
[61]
The
Komodo dragon, which normally reproduces sexually, has also been found able to reproduce asexually by parthenogenesis.
[23][62] A case has been documented of a Komodo dragon switching back to sexual reproduction after a known parthenogenetic event.
[63] 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.
[63]
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.
Amphibians[edit]
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.
[66] 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 the same year, a female
Atlantic blacktip shark in Virginia reproduced via parthenogenesis.
[67] 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.
[64]
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.
[68]
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.
In 2011, recurring shark parthenogenesis over several years was demonstrated in a captive
zebra shark, a type of carpet shark.
[65][69]
Parthenogenesis in turkeys appears to result from a conversion of
haploid cells to
diploid;
[70] most embryos produced in this way die early in development. Rarely, viable birds result from this process, and the rate at which this occurs in turkeys can be increased by
selective breeding,
[71] however male turkeys produced from parthenogenesis exhibit smaller
testes and reduced fertility.
[72]
Mammals[edit]
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.
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
[76] 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.
[77]
Induction of parthenogenesis in swine. Parthenogenetic development of swine oocytes.
[76] 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.
[78] 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. 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 and is approximately 50% smaller than biparental offspring placentae: see free image (Figure 1) in linked reference.
[76]
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.
[78] 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 to developmental failure after ~30 days of gestation. The swine parthenote placentae often appears hypo-vascular: see free image (Figure 1) in linked reference.
[76]
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
[79] illustrating a breakthrough in the use of parthenogenesis to produce human stem cells that are
homozygous in the
HLAregion 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.
[80] 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.
[81] The truth was discovered in 2007, long after the embryos were created by him and his team in February 2004. This made Hwang the first, unknowingly, to successfully perform the process of parthenogenesis to create a human embryon and, ultimately, a human parthenogenetic stem cell line.
Oomycetes[edit]
Apomixis can apparently occur in
Phytophthora,
[82] 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.
Similar phenomena[edit]
Gynogenesis[edit]
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
[who?] 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.
Hybridogenesis[edit]
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[83] as well as in some of the
Pelophylax spp. ("green frogs" or "waterfrogs"):
In popular culture[edit]
- In the television series "House" in episode "Joy to the World" Gregory House diagnosed the first human case of parthenogenesis to virgin mother in the clinic, which turns out to be a lie.
- The group Shriekback make reference to parthenogenesis in their song Nemesis.
See also[edit]
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