The phylogenetic tree of reptiles is a mosaic of species in which sex determination is genetic and closely related ones in which it is temperature dependent. Sex reversal is thought to have a key role in the transition between these mechanisms, and several species with genetic systems have been shown to have a temperature override. Now, Holleley et al. report that the first sex reversed individuals have been found in the wild and demonstrate the transition from genetic to temperature-dependent mechanisms in these individuals in the laboratory in a single generation.
Genetic sex determination occurs in the Australian bearded dragon (Pogona vitticeps), in which males are ZZ and females are ZW, but with a temperature override. Holleley et al. collected 131 individuals from across locations in eastern Australia during 2003, 2004 and 2011. Of these, they identified 11 as being sex reversed ZZ females using a PCR sex-specific molecular marker that they developed. These females came from the warmer, northern end of the species' range, and there was a non-significant increase in the proportion of sex reversed individuals from 2003 to 2011.
The authors then mated a subset of ZZ females with normal ZZ males. The offspring had a hatching rate comparable to that of controls, and the ZZ female fecundity was actually greater than controls. As expected, the offspring of sex reversed females were all male when incubated at low temperatures and predominantly female at high temperatures compared with controls, which showed a 50/50 ratio at low and intermediate temperatures and a female bias at high temperatures. Interestingly, the ZZ offspring of sex reversed females were themselves more likely to reverse than the ZZ offspring of ZW females, suggesting a heritable component to reversal propensity.
“The study ... suggests considerable fluidity of sex determination systems in the face of climate change”
This study demonstrates that the W chromosome can be lost and temperature-dependent sex determination established in a single generation, although in the wild, it is likely to be a more gradual process. It could, however, be far faster and more prevalent than previously thought, without the requirement postulated in the Charnov–Bull model for specific changes in the fitness of individuals producing different offspring sex ratios. The study also suggests considerable fluidity of sex determination systems in the face of climate change, although it is unclear how often this will be advantageous and how often it will be maladaptive.
Holleley, C. E. et al. Sex reversal triggers the rapid transition from genetic to temperature dependent sex. Nature http://dx.doi.org/10.1038/nature14574 (2015)