Sex chromosomes in snakes

In two recently published articles Rovatsos et al. (2015 a, b) highly differentiated heteromorphic ZZ/ZW sex chromosomes with a heterochromatic W are  basic among the advanced snakes, Colubroidea, while other snake lineages generally lack them. The authors examined the dragonsnake, Xenodermus javanicus (family Xenodermatidae), which is phylogenetically nested between snake lineages with and without differentiated sex chromosomes. Although most snakes have a karyotype with a stable 2n chromosomal number of  36, the dragonsnake has an unusual, derived karyotype  2n = 32 chromosomes. The found that heteromorphic ZZ/ZW sex chromosomes with a heterochromatic W are present in the dragonsnake, which suggests that the emergence of a highly differentiated W sex chromosome within snakes predates the split of Xenodermatidae and the clade including families Pareatidae, Viperidae, Homalopsidae, Lamprophiidae, Elapidae, and Colubridae (the Colubroidae). Although accumulations of interstitial telomeric sequences have not been previously reported in snakes, by using FISH with a telomeric probe they discovered them in six pairs of autosomes as well as in the W sex chromosome of the dragonsnake. Similarly to advanced snakes, the sex chromosomes of the dragonsnake have a significant accumulation of repeats containing a (GATA)n sequence. The results facilitate the dating of the differentiation of sex chromosomes within snakes back to the split between Xenodermatidae and other advanced snakes, about 40-75 mya. In a second  article they document the stability of sex chromosomes in advanced snakes based on the testing of Z-specificity of genes using quantitative PCR (qPCR) across 37 snake species (their qPCR technique is suitable for molecular sexing all advanced snakes). They found that at least part of the sex chromosomes is homologous across all families of caenophidian snakes (Acrochordidae, Xenodermatidae, Pareatidae, Viperidae, Homalopsidae, Colubridae, Elapidae and Lamprophiidae). The emergence of differentiated sex chromosomes can be dated to about 60 Ma, a date that preceded the extensive diversification of advanced snakes, a group with more than 3000 species. The Z-specific genes of caenophidian snakes are (pseudo)autosomal in the snake families Pythonidae, Xenopeltidae, Boidae, Erycidae and Sanziniidae, as well as in outgroups with differentiated sex chromosomes such as monitor lizards, iguanas and chameleons. Along with iguanas, advanced snakes are therefore another example of ectothermic amniotes with a long-term stability of sex chromosomes comparable with endotherms.

Rovatsos, M., Johnson Pokorná, M., & Kratochvíl, L. (2015). Differentiation of Sex Chromosomes and Karyotype Characterisation in the Dragonsnake Xenodermus javanicus (Squamata: Xenodermatidae). Cytogenetic and genome research.

Rovatsos, M., Vukić, J., Lymberakis, P., & Kratochvíl, L. (2015). Evolutionary stability of sex chromosomes in snakes. In Proc. R. Soc. B (Vol. 282, No. 1821, p. 20151992). The Royal Society.

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