Sex reversal can be induced under controlled conditions in the laboratory, but importantly it also occurs in the wild across the species range.

In the case of the Australian central bearded dragon Pogona vitticeps, the most well studied reptile with sex reversal, genetic males (ZZ sex chromosomes) incubated at high temperatures (> 32 ☌) undergo sex reversal so the animal develops as a female despite being genetically male. Some species possess sex chromosomes with a thermal override that can cause sex reversal. Sex determination in vertebrates exists on a continuum spanning from genetic sex determination (GSD) to temperature dependent sex determination (TSD). These findings bring new insights to the mechanisms underlying sex reversal, improving our understanding of thermosensitive sex systems in vertebrates. We found inter-individual variation in gene expression changes following temperature switches, suggesting both genetic sensitivity to, and the timing and duration of the temperature cue influences sex reversal. We revealed ovotestes exist along a spectrum of femaleness to male-ness at the transcriptional level. Any exposure to low temperatures (28 ☌) caused testes differentiation, whereas sex reversal required longer exposure to high temperatures. We found sexual fate is more strongly influenced by the ZZ genotype than temperature. To test this, we conducted temperature switching experiments at key developmental stages, and analysed the effect on gonadal phenotypes using histology and transcriptomics.

Previous research showed ovotestes, a rare gonadal phenotype with traits of both sexes, develop during sex reversal, leading to the hypothesis that sex reversal relies on high temperature feminisation to outcompete the male genetic cue. In the central bearded dragon ( Pogona vitticeps), the genetic influence of sex chromosomes (ZZ/ZW) can be overridden by high incubation temperatures, causing ZZ male to female sex reversal. In some vertebrate species, gene-environment interactions can determine sex, driving bipotential gonads to differentiate into either ovaries or testes.

Bearded dragon egg incubator