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assifying automixis as a sexual process depends on when the products of anaphase i or of anaphase ii are joined the criterion for sexuality varies from all cases of restitutional meiosis 25 to those where the nuclei fuse or to only those where gametes are mature at the time of fusion 24 the genetic composition of the offspring depends on what type of automixis takes place when endomitosis occurs before meiosis 26 27 or when central fusion occurs restitutional meiosis of anaphase i or the fusion of its products the offspring get all 26 28 to more than half of the mother s genetic material and heterozygosity is mostly preserved 29 if the mother has two alleles for a locus the offspring will likely 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 30 in the case of pre meiotic doubling recombination if it happens occurs between identical sister chromatids 26 if terminal fusion restitutional meiosis of anaphase ii or the fusion of its products occurs a little over half of the mother s genetic material is present in the offspring and the offspring are mostly homozygous 31 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 citation needed this can result in parthenogenetic offspring being unique from each other and from their mother citation needed sex of the offspring edit in apomictic parthenogenesis the offspring are clones of the mother and hence except for aphids are usually 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 32 when meiosis is involved the sex of the offspring depends on the type of sex determination system and the type of apomixis in species that use the xy sex determination system parthenogenetic offspring 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 28 29 zz male or ww non viable in most species 31 but a viable female in a few e g boas 31 zw offspring are produced by endoreplication before meiosis or by central fusion 28 29 zz and ww offspring occur either by terminal fusion 31 or by endomitosis in the egg cell citation needed in polyploid obligate parthenogens like the whiptail lizard all the offspring are female 27 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 unfertilized eggs this means that females workers and queens are always diploid while males drones are always haploid and are produced parthenogenetically citation needed facultative edit facultative parthenogenesis occurs when a female can produce offspring either sexually or via asexual reproduction 33 facultative parthenogenesis is extremely rare in nature with only a few examples of animal taxa capable of facultative parthenogenesis 33 one of the best known examples of taxa exhibiting facultative parthenogenesis are mayflies presumably this is the default reproductive mode of all species in this insect order 34 facultative parthenogenesis has generally been 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 however california condors and the tropical lizard lepidophyma smithii both can produce parthenogenic offspring in the presence of males indicating that facultative parthenogenesis may be more common than previously thought and is not simply a response to a lack of males 16 8 in aphids a generation sexually conceived by a male and a female produces only females the reason for this is the non random segregation of the sex chromosomes x and o during spermatogenesis 35 facultative parthenogenesis is often used to describe cases of spontaneous parthenogenesis in normally sexual animals 36 for example many cases of spontaneous parthenogenesis in sharks some snakes komodo dragons and a variety of domesticated birds were widely attributed to facultative parthenogenesis 37 these cases are examples of spontaneous parthenogenesis 33 36 the occurrence of such asexually produced eggs in sexual animals can be explained by a meiotic error leading to eggs produced via automixis 36 38 obligate edit obligate parthenogenesis is the process in which organisms exclusively reproduce through asexual means 39 many species have transitioned to obligate parthenogenesis over evolutionary time well documented transitions to obligate parthenogenesis have been found in numerous metazoan taxa albeit through highly diverse mechanisms these transitions often occur as a result of inbreeding or mutation within large populations 40 some documented species specifically salamanders and geckos 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 indotyphlops braminus amphibians and fishes in nature for which males are no longer a part of the reproductive process 41 a female produces 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 due to the subsequent lack of gene variation and potentially decreased fitness of the offspring 39 some invertebrate species that feature partial sexual reproduction in their native range are found to reproduce solely by parthenogenesis in areas to which they have been introduced 42 43 relying solely on parthenogenetic reproduction has several advantages for an invasive species it obviates the need for individuals in a very sparse initial population to search for mates and an exclusively female sex distribution allows a population to multiply and invade more rapidly potentially twice as fast examples include several aphid species 42 and the willow sawfly nematus oligospilus which is sexual in its native holarctic habitat but parthenogenetic where it has been introduced into the southern hemisphere 43 natural occurrence edit further information list of taxa that use parthenogenesis parthenogenesis does not apply to isogamous species 44 parthenogenesis occurs 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 45 birds 46 and sharks 47 fish amphibians and reptiles make use of various forms of gynogenesis and hybridogenesis an incomplete form of parthenogenesis 48 the first all female unisexual reproduction in vertebrates was described in the fish poecilia formosa in 1932 49 since then at least 50 species of unisexual vertebrates have been described including at least 20 fish 25 lizards a single snake species frogs and salamanders 48 artificial induction edit use of an electrical or chemical stimulus can produce the beginning of the process of parthenogenesis in the asexual development of viable offspring 50 induction of parthenogenesis in pigs 51 52 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 51 52 to initiate unfertilised development 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 the development of unfertilised eggs in swine presumably by continual inhibition of mpf cyclin b 52 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 51 the resulting embryos can be surgically transferred to a recipient oviduct for further development but will succumb to developmental failure after 30 days of gestation the swine placenta in these cases often appears hypo vascular see free image figure 1 in linked reference 51 induced parthenogenesis of this type in mice and monkeys 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 for development to proceed normally a mammal developing from parthenogenesis would have double doses of maternally imprinted genes and lack paternally imprinted genes leading to developmental abnormalities it has been suggested that defects in placental folding or interdigitation are one cause of swine parthenote abortive development 51 as a consequence research on the induced development of unfertilised eggs in humans is focused on the production of embryonic stem cells for use in medical treatment not as a reproductive strategy citation needed in 2022 researchers reported that they had produced viable offspring born from unfertilized eggs in mice addressing the problems of genomic imprinting by targeted dna methylation rewriting of seven imprinting control regions 11 in humans edit in 1955 the geneticist helen spurway who had studied the reproductive biology of the guppy poecilia reticulata claimed that parthenogenesis may rarely occur in humans leading to so called virgin births this created some sensation among her colleagues and the lay public alike 53 sometimes an embryo may begin to divide without fertilization but it cannot fully develop on its own so while it may create some skin and nerve cells it cannot create others such as skeletal muscle and becomes a type of benign tumor called an ovarian teratoma 54 in 1995 there was a reported case of partial human parthenogenesis a boy was found to have some of his cells such as white blood cells lacking any genetic content from his father scientists believe that an unfertilized egg began to self divide but then had some but not all of its cells fertilized by a sperm cell this must have happened early in development as self activated eggs quickly lose their ability to be fertilized the unfertilized cells eventually duplicated their dna boosting their chromosomes to 46 when the unfertilized cells hit a developmental block the fertilized cells took over and developed that tissue the boy had asymmetrical facial features and learning difficulties but was otherwise healthy this would make him a parthenogenetic chimera a child with two cell lineages in his body 55 while over a dozen similar cases have since been reported there have been no confirmed reports of a non chimeric clinically healthy human parthenote i e produced from a single parthenogenetic activated oocyte 54 in 2007 the international stem cell corporation of california announced that elena revazova had intentionally created human stem cells from unfertilized human eggs using parthenogenesis the process may offer a way to create stem cells genetically matched to a particular female to treat degenerative diseases the same year revazova and iscc published an article describing how to produce human stem cells that are homozygous in the hla region of dna 56 these stem cells are called hla homozygous parthenogenetic human stem cells hpsc hhom and would allow derivatives of these cells to be implanted without immune rejection with the selection of oocyte donors according to hla haplotype it would be 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 57 an independent investigation found that the discredited south korean scientist hwang woo suk had unknowingly produced the first human embryos by parthenogenesis hwang had claimed falsely that his team had extracted stem cells from cloned human embryos the chromosomes of these cells showed indications of parthenogenesis in extracted stem cells similar to those of the mice created by tokyo scientists in 2004 58 similar phenomena edit androgenesis edit androgenesis is a system of asexual reproduction and occurs when a zygote is produced with only paternal nuclear genes 59 requiring the presence of eggs one of two things can occur to produce offspring with exclusively paternal genetic material the maternal nuclear genome can be eliminated from the zygote or the female can produce an egg with no nucleus resulting in an embryo developing with only the genome of the male gamete citation needed although androgenesis requires both male and females gametes it is not strictly considered a form of sexual reproduction because the offspring have uniparental nuclear dna that has not undergone recombination and the proliferation of androgenesis can lead to exclusively asexually reproducing species citation needed gynogenesis edit further information gynogenesis and parthenogenesis in amphibians gynogenesis 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 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 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 60 hybridogenesis edit further information hybridogenesis in water frogs example crosses between pool frog pelophylax lessonae marsh frog p ridibundus and their hybrid edible frog p kl esculentus on the left primary hybridisation generating a hybrid while the middle is the most widespread type of hybridogenesis 61 62 large circles adult frogs small circles gametes lack of gametes containing genome of one of the parental species hybridogenesis is a mode of reproduction of hybrids hybridogenetic hybrids for example ab genome usually females during gametogenesis exclude one of parental genomes a and produce gametes with unrecombined 61 genome of second parental species b instea...
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