Thelytoky (from theGreek θῆλυςthēlys "female" and τόκοςtókos "birth") is a type ofparthenogenesis and is the absence of mating and subsequent production of all female diploid offspring as for example inaphids. Thelytokous parthenogenesis is rare among animals and reported in about 1,500species, about 1 in 1000 of described animal species, according to a 1984 study.[1] It is more common ininvertebrates, likearthropods, but it can occur invertebrates, includingsalamanders,fish, andreptiles such as somewhiptail lizards.
Thelytoky can occur by different mechanisms, each of which has a different impact on the level ofhomozygosity. It is found in several groups ofHymenoptera, includingApidae,Aphelinidae,Cynipidae,Formicidae,Ichneumonidae, andTenthredinidae.[2] It can be induced in Hymenoptera by thebacteriaWolbachia andCardinium.[3]
Species may encounter a few advantages employing this form of mating system. Thelytoky allows females to pass along genotypes that ensure success in that particular environment, having only daughters increases the species output, and energy that would otherwise be exerted into finding or attracting a mate can directly be invested in reproduction.[4]
Thelytoky can occur naturally, or it can be induced by scientists in a laboratory setting.[5] In some species, thelytoky can also occur through the fusion of two female gametes.[6]
Facultative thelytoky refers to an individual being capable of reproducing sexually or asexually depending on environmental conditions. For example,smalltooth sawfish in Florida populations can be facultatively thelytokous, meaning that they will reproduce sexually when conditions are favorable, but switch to thelytoky when resources and mates become scarce.[7]
Accidental thelytoky occurs when a female organism produces offspring asexually due to the absence or failure of fertilization by a male. This can occur in species that normally reproduce sexually but are unable to find a mate, or in species in which mating is unsuccessful due to physical or behavioral barriers. While accidental thelytoky can provide a short-term reproductive solution in the absence of a mate, it is typically not sustainable over the long-term due to the loss of genetic diversity.[8]
Cyclical thelytoky is a form of thelytoky in which organisms alternate between sexual and asexual reproduction in a regular cycle. This type of reproduction is seen in cynipidgall wasps, in which sexual reproduction occurs in alternate generations. The asexual reproduction that occurs in between these sexual generations is typically facilitated by the presence of specific environmental cues, such as temperature or photoperiod. The genetic diversity generated by sexual reproduction in these organisms is thought to play an important role in their ability to adapt to changing environmental conditions.[9]
Obligate thelytoky refers to a form of asexual reproduction in which an individual is unable to reproduce sexually and must rely on asexual reproduction for reproduction. Species that are obligately thelytokous do not have the genetic or physiological mechanisms necessary to produce males, and thus rely solely on female offspring to perpetuate their lineage. Examples of obligately thelytokous species include some members ofcerapachys ants and some species ofwhiptail lizards.[10]
Hymenoptera (ants,bees,wasps, andsawflies) have ahaplodiploid sex-determination system. They produce haploid males from unfertilized eggs (arrhenotoky), a form ofparthenogenesis. However, in a few social hymenopterans,queens or workers are capable of producing diploid female offspring by thelytoky.[11] The daughters produced may or may not be complete clones of their mother depending on the type of parthenogenesis that takes place.[12][13] The offspring can develop into either queens or workers. Examples of such species include the Cape bee,Apis mellifera capensis,Mycocepurus smithii and clonal raider ant,Ooceraea biroi.
Automixis is a form of thelytoky. In automicticparthenogenesis,meiosis takes place and diploidy is restored by fusion of first division non-sister nuclei (central fusion) or the second division sister nuclei (terminal fusion).[14] (see diagram).
Automixis with central fusion tends to maintain heterozygosity in the passage of the genome from mother to daughter. This form of automixis has been observed in several ant species including the desert antCataglyphis cursor,[11] the clonal raider antCerapachys biroi,[15] the predaceous antPlatythyrea punctata,[14] and the electric ant (little fire ant)Wasmannia auropunctata.[16] Automixis with central fusion also occurs in the Cape honey beeApis mellifera capensis,[13] the brine shrimpArtemia parthenogenetica,[17] and the termiteEmbiratermes neotenicus.[18]
Oocytes that undergo automixis with central fusion often display a reduced rate ofcrossover recombination. A low rate of recombination in automictic oocytes favors maintenance ofheterozygosity, and only a slow transition from heterozygosity to homozygosity over successive generations. This allows avoidance of immediateinbreeding depression. Species that display central fusion with reduced recombination include the antsP. punctata[14] andW. auropunctata,[16] the brine shrimpA. parthenogenetica,[17] and the honey beeA. m. capensis.[13] InA. m. capensis, the recombination rate during the meiosis associated with thelytokous parthenogenesis is reduced by more than 10-fold.[13] InW. auropunctata the reduction is 45-fold.[16]
Single queen colonies of the narrow headed antFormica exsecta provide an illustrative example of the possible deleterious effects of increased homozygosity. In this ant the level of queen homozygosity is negatively associated with colony age.[19] Reduced colony survival appears to be due to decreased queen lifespan resulting from queen homozygosity and expression of deleterious recessive mutations (inbreeding depression).
Automixis with terminal fusion tends to promote homozygosity in the passage of the genome from mother to daughter. This form of automixis has been observed in the water fleaDaphnia magna[20] and the Colombian rainbow boa constrictorEpicrates maurus.[21] Parthenogenesis inE. maurus is only the third genetically confirmed case of consecutive virgin births of viable offspring from a single female within any vertebrate lineage.[21] However, survival of offspring over two successive litters was poor, suggesting that automixis with terminal fusion leads to homozygosity and expression of deleteriousrecessive alleles (inbreeding depression).
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