Temperature-Induced Sex Reversal in Reptiles: Prevalence, Discovery, and Evolutionary Implications

Sex reversal is the process by which an individual develops a phenotypic sex that is discordant with its chromosomal or genotypic sex. It occurs in many lineages of ectothermic vertebrates, such as fish, amphibians, and at least one agamid and one scincid reptile species. Sex reversal is usually triggered by an environmental cue that alters the genetically determined process of sexual differentiation, but it can also be caused by exposure to exogenous chemicals, hormones, or pollutants. Despite the occurrence of both temperature-dependent sex determination (TSD) and genetic sex determination (GSD) broadly among reptiles, only 2 species of squamates have thus far been demonstrated to possess sex reversal in nature (GSD with overriding thermal influence). The lack of species with unambiguously identified sex reversal is not necessarily a reflection of a low incidence of this trait among reptiles. Indeed, sex reversal may be relatively common in reptiles, but little is known of its prevalence, the mechanisms by which it occurs, or the consequences of sex reversal for species in the wild under a changing climate. In this review, we present a roadmap to the discovery of sex reversal in reptiles, outlining the various techniques that allow new occurrences of sex reversal to be identified, the molecular mechanisms that may be involved in sex reversal and how to identify them, and approaches for assessing the impacts of sex reversal in wild populations. We discuss the evolutionary implications of sex reversal and use the central bearded dragon (Pogona vitticeps) and the eastern three-lined skink (Bassiana duperreyi) as examples of how species with opposing patterns of sex reversal may be impacted differently by our rapidly changing climate. Ultimately, this review serves to highlight the importance of understanding sex reversal both in the laboratory and in wild populations and proposes practical solutions to foster future research.