Sex reversal in vertebrates: drivers and consequences in a changing world

Naomi Laven, Kristoffer Wild, Daniel W.A. Noble

Abstract:
Sex reversal describes the process of external factors or genetic mutations overriding normal sex determination during sexual development. This phenomenon is widespread among vertebrates and occurs under a number of different developmental circumstances. Environmental factors such as temperature and stress, mutations in key sex determination genes, and exposure to exogenous hormones or endocrine-disrupting compounds may all cause sex reversal. Outcomes of the sex reversal process are incredibly varied. Some taxa are far more adept at switching sex than others, and different sex reversal triggers can lead to contrasting fertile, reproductively impaired, or intersex phenotypes, with subsequently divergent implications for both evolutionary and population-level dynamics. Sex reversal has received considerable scientific attention since early experiments with exogenous hormones revealed an inherent flexibility in the sex determination of vertebrates. However, this attention is spread over a variety of research disciplines, many of which use specialised terminology, and focus on specific aspects of sex reversal. This comes at a detriment to a rapidly-advancing field which would greatly benefit from increased cross-disciplinary study. Here, we conducted a systematic review of 589 studies to provide an overview of current research on vertebrate sex reversals. We summarise known taxonomic trends, common research applications, and contemporary theories of how and why sex reversal occurs. Drawing from the collected primary literature, we further examine specific triggers of sex reversal, affected taxa, and potential mechanisms. We then discuss the role of sex reversal as a natural phenomenon in wild populations, exploring current hypotheses of sex reversal as an adaptive trait. Finally, we conclude the review with a discussion of how projected future increases in anthropogenically-driven sex reversal triggers – namely extreme high temperatures and endocrine-disruptors – present a further challenge to vulnerable populations under global climate change.