A study of thermally-induced sex reversal in casque-headed lizards

ABSTRACT

Non-avian reptiles, unlike mammals and birds, have undergone numerous sex determination turnovers. For example, casque-headed lizards replaced the ancestral XY system shared across pleurodonts with a new pair of XY chromosomes. However, the evolutionary forces that triggered this transition have remained unclear. An interesting evolutionary hypothesis suggests that species with intermediate states, with sex chromosomes but also thermal-induced sex reversal at specific incubation temperatures, could be more susceptible to sex determination turnovers. We contrasted genotypic data (presence/absence of the Y chromosome) against the histology of gonads of embryos from stages 35-37 incubated at various temperatures, including typical male-producing (26°C) and female-producing (32°C) temperatures. We observed perfect concordance between genotype and phenotype at all temperatures. However, analysis of transcriptomic data from embryos incubated at 26°C and 32°C identified transcript variants of the chromatin modifiers JARID2 and KDM6B that have been linked to temperature-dependent sex determination in other reptiles. Besides, our work reports for the first time to our knowledge the histology of gonads, including morphological changes, from stages 35-37 of development in the Corytophanidae family. We also observed that all embryos developed hemipenes, suggesting sex-linked developmental heterochrony.Our work tested the validity of a mixed sex determination system in the Corytophanidae family. We showed that XY chromosomes are dominant, however, our work supports the hypothesis of a conserved transcriptional response to incubation temperatures across non-avian reptiles that could be the reminiscence of an ancestral sex determination system.