A project undertaken at the University of Tasmania, and supervised by Dr Paul Saunders.
Temperatures have been on the rise in recent decades, threatening many species with extinction. To mitigate this threat, we urgently need to identify species that are the most at risk and understand why. Fish, amphibians and reptiles with temperature-dependent sex determination are particularly vulnerable, as climate change can cause skewed sex ratios, threatening population persistence through reduced population growth. An additional but presently unassessed risk exists for species where temperature over-rides a genetically programmed sex. This phenomenon, known as sex reversal, generates a discordance between the genetic makeup of an individual and its sex. For instance, in species with XY sex chromosomes, in which XX embryos would normally develop as females and XY embryos normally develop as males, sex reversal causes the emergence of XX males and/or XY females.
Sex reversal is thought to affect numerous cold-blooded animals, including many Australian reptiles, but has not been extensively studied so far. Our knowledge of sex reversal’s consequences on growth and survival, and potential contribution to extinction risk remains incomplete. Historical observations in mammals indicate sex reversal causes total sterility, while in a few fish and frogs it has little or no effect.
We propose that the degree of genetic differentiation between sex chromosomes—which varies greatly among species and sometimes even populations—can predict fitness consequences of sex reversal. Specifically, species or populations with greater sex chromosome differentiation would be at greater extinction risk from temperature-induced sex reversal because of greater genotype-phenotype conflict.
To test this hypothesis, our project focuses on the spotted snow skink Carinascincus ocellatus, a lizard restricted to Tasmania. Recent research on populations at the extremes of its climatic range has revealed both sex reversal and inter-population variation in XY sex chromosome differentiation. By integrating various methodological approaches, and comparing populations throughout the species range, we will assess the full extent of variation in sex chromosome differentiation, and test how it influences fitness in sex reversed individuals. The outcomes of this research will facilitate predictions of extinction risk in species where temperature can override genetic sex, using the nature and level of sex chromosome differentiation as an indicator for reduced fitness.