For over 50 years, biologists have accepted that all extant snakes share the same ZW sex chromosomes derived from a common ancestor, with different species exhibiting sex chromosomes at varying stages of differentiation. Accordingly, snakes have been a well-studied model for sex chromosome evolution in animals. A review of the literature, however, reveals no compelling support that boas and pythons possess ZW sex chromosomes. Furthermore, phylogenetic patterns of facultative parthenogenesis in snakes and a sex-linked color mutation in the ball python (Python regius) are best explained by boas and pythons possessing an XY sex chromosome system. In a recent paper Gamble et al. (2017) demonstrate that the boa, Boa imperator, and a python, Python bivittatus, indeed possess XY sex chromosomes, based on the discovery of male-specific genetic markers in both species. The authors used these markers, along with transcriptomic and genomic data, to identify distinct sex chromosomes in boas and pythons, demonstrating that XY systems evolved independently in each lineage. This discovery highlights the dynamic evolution of vertebrate sex chromosomes and further enhances the value of snakes as a model for studying sex chromosome evolution.
Gamble T, Castoe TA, Nielsen SV, Banks JL, Card DC, Schield DR, Schuett GW, Booth W. 2017. The discovery of XY sex chromosomes in a boa and python. Current Biology. 27(14):2148-53.
Facultative parthenogenesis in the Green Anaconda
In reptiles, the mode of reproduction is typically sexual. However, facultative parthenogenesis occurs in some Squamata, such as Komodo dragon (Varanus komodoensis) and Burmese python (Python bivittatus). Shibata et al. (2017) report facultative parthenogenesis in the green anaconda (Eunectes murinus). They found two fully developed female neonates and 17 undeveloped eggs in the oviduct of a female anaconda isolated from other individuals for eight years and two months at Ueno Zoo, Japan. To clarify the zygosity of the neonates, we analyzed 18 microsatellite markers of which 16 were informative. The authors observed only maternal alleles and no paternal alleles for all 16 markers. To examine the possibility of the long-term sperm storage, we estimated allele frequencies in a putative parental stock by genotyping five unrelated founders. If all founders, including the mother, are originated from a single Mendelian population, then the probability that the neonates were produced by sexual reproduction with an unrelated male via long-term sperm storage was infinitesimally small (2.31E-32 per clutch). They examined samples from two additional offspring that the mother delivered eight years before her death. The authors consistently observed paternal alleles in these elder offspring, indicating that the mother had switched from sexual reproduction to asexual reproduction during the eight years of isolation. This is the first case of parthenogenesis in Eunectes to be validated by DNA analysis and suggests that facultative parthenogenesis is widespread in the Boidae.
Shibata H, Sakata S, Hirano Y, Nitasaka E, Sakabe A (2017) Facultative parthenogenesis validated by DNA analyses in the green anaconda (Eunectes murinus). PLoS ONE 12(12): e0189654. https://doi.org/10.1371/journal.pone.0189654
The discovery of facultative parthenogenesis in elapid snakes
Parthenogenesis is a form of asexual reproduction by which embryos develop from unfertilized eggs. Parthenogenesis occurs in reptiles; however, it is not yet known to occur in the widespread elapid snakes (Elapidae), which include well-known taxa such as cobras, mambas, taipans and sea snakes. Allen et al. (2018) describe the production of viable parthenogens in two species of Australo-Papuan elapids with divergent reproductive modes: the oviparous coastal/Papuan taipan (Oxyuranus scutellatus) and the viviparous southern death adder (Acanthophis antarcticus). Analyses of nuclear SNP data excluded paternity for putative fathers and convincingly demonstrated asexual reproduction, thus representing the first evidence of facultative parthenogenesis in Elapidae. Our finding has broad implications for understanding the evolution of reproductive diversity in snakes, as well as managing the conservation of genetic diversity in wild and captive populations.
Allen L, Sanders KL, Thomson VA. 2018. Molecular evidence for the first records of facultative parthenogenesis in elapid snakes. Royal Society Open Science. 5(2):171901.