Parthenogenesis in squamates
Among all the sexual vertebrates, the only examples of true parthenogenesis, in which all-female populations reproduce without the involvement of males, are found in squamate reptiles (snakes and lizards).
The best-known and perhaps most evolutionarily derived example of parthenogenesis in reptiles occurs within the Teiid genus of whiptail lizards known as Aspidoscelis.
The often quoted parthenogeneetic species N. arnouxi is nomen rejectum (ICZN 1991) and therefore a synonym of N. pelagicus, while Gehyra ogasawarisimae is a misidentified L.
Surprisingly, parthenogenetic females of this species occasionally produce male offspring, which are thought to be the result of non-genetic hormonal inversions.
Unlike most parthenogenetic reptiles, Lepidophyma lizards show very low genetic heterozygosity, suggesting a non-hybrid origin.
[13][14] In addition, asexually produced offspring in vertebrates exhibit extremely high levels of sterility, highlighting that this mode of reproduction is not adaptive.
[17] Three species of the Neotropical pit viper Bothrops atrox group have been show to be capable of facultative parthenogenesis based on information concerning their captivity and also by testing using molecular markers (heterologous microsatellites).
[20] The offspring were found to be clones of their mother and reproduction was apparently by a parthenogenetic mechanism involving a variation of the meiotic process.
[21] In species that are able to undergo facultative parthenogenesis, the transition to the completely homozygous condition can lead to exposure of their genetic load resulting in an elevated rate of congenital malformations and embryonic mortality.
Thus, A. arizonae is an example of facultative parthenogenesis that can potentially allow purifying selection to occur with the consequence that all lethal recessive alleles are purged in only one generation.
Hybridogenesis is a variation of parthenogenesis in which males mate with females, but only the mother's genetic material is propagated by these offspring to their own young.
In all parthenogenetic reptile species studied to date, chromosomal evidence supports the theory that parthenogenesis arose through a hybridization event, although members of the genus Lepidophyma may be exceptions to this rule.
As no crosses of two sexual species in captivity have ever produced parthenogenetic offspring, it is unclear how a hybridization event would actually lead to asexual reproduction.
Triploid unisexual geckos of the species Heteronotia binoei have greater endurance and aerobic capacity than their diploid ancestors, and this advantage may be the result of polyploidy and a form of hybrid vigor.
[24] It has also been observed that obligate parthenotes are often found at high altitudes and in sparse or marginal habitats, a pattern known as "geographical parthenogenesis," and their distribution in suboptimal territories may be a result of their increased colonization ability.
