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Thelytoky (from the Ancient Greek θῆλυς thēlys "female" and τόκος tókos "birth") is a type of parthenogenesis and is the absence of mating and subsequent production of all female diploid offspring as for example in . Thelytokous parthenogenesis is rare among animals and reported in about 1,500 species, about 1 in 1000 of described animal species, according to a 1984 study. It is more common in invertebrates, like arthropods, but it can occur in vertebrates, including , fish, and such as some Cnemidophorus.
Thelytoky can occur by different mechanisms, each of which has a different impact on the level of zygosity. It is found in several groups of Hymenoptera, including Apidae, Aphelinidae, Cynipidae, Formicidae, Ichneumonidae, and Tenthredinidae. It can be induced in Hymenoptera by the bacteria Wolbachia and Cardinium.
Thelytoky can occur naturally, or it can be induced by scientists in a laboratory setting. In some species, thelytoky can also occur through the fusion of two female gametes.
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.
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 cynipid , 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.
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 of cerapachys ants and some species of Teiidae.
that undergo automixis with central fusion often display a reduced rate of crossover recombination. A low rate of recombination in automictic oocytes favors maintenance of Zygosity, and only a slow transition from heterozygosity to homozygosity over successive generations. This allows avoidance of immediate inbreeding depression. Species that display central fusion with reduced recombination include the ants P. punctata and W. auropunctata, the brine shrimp A. parthenogenetica, and the honey bee A. m. capensis. In A. m. capensis, the recombination rate during the meiosis associated with thelytokous parthenogenesis is reduced by more than 10-fold. In W. auropunctata the reduction is 45-fold.
Single queen colonies of the narrow headed ant Formica 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. Reduced colony survival appears to be due to decreased queen lifespan resulting from queen homozygosity and expression of deleterious recessive mutations (inbreeding depression).
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