Ignition Grant Round 6 (July 2016)

• Clare Holleley, CSIRO (pictured, R)
• Kerensa McElroy, CSIRO
• Dan Rosauer, ANU
• Craig Moritz, ANU
• Arthur Georges, University of Canberra (pictured, L)
• UC Students Caitlin Cherryh and Meghan Castelli

This project poses fundamental questions about how changes in the climate can rapidly alter the biology and genome of organisms. Specifically, how gene-environment interactions affect the rapid evolution of new sex determining modes. 

Reptiles can switch from a sex determination mode that is predominantly genetic (using sex chromosomes) to a system where sex is determined by egg incubation temperature (without sex chromosomes). This is achieved through the occurrence of sex reversal in wild populations - where chromosomal males are feminised at high incubation temperature. 

The bearded dragon (Pogona vitticeps) is the first terrestrial vertebrate species where environmental sex reversal has been identified in wild populations. Initial data suggests that in some populations, rates of sex reversal have been increasing over time (from 7% in 2003 to 22% in 2011). Populations experiencing high rates of reversal co-occur with geographical regions that have experienced a rapid increase in diurnal temperature range (Holleley et al., 2015).

This study aims to more fully characterise temporal changes in sex reversal and to identify climatic variables that might impact on rates of sex reversal in the wild (e.g. mean temperatures, temperature variability, frequency of extreme high temperature events). Using historical climate data to identify biologically relevant climatic variables in this model system will ultimately allow us to generate a sophisticated spatially explicit predictive model, linking environmental change with rapid evolutionary responses. 

This project will also develop new robust genomic approaches to interrogate historical specimens for evidence of sex reversal. We will develop a custom capture array that targets the recently characterised 219 genes on the Pogona Z chromosome. This whole-chromosome approach will be a much more sensitive and accurate approach to identify genotypic sex than the existing single-locus PCR based method and will facilitate the use of ancient and poor-quality DNA template. In the custom capture array we will also include a selection of fast and slow evolving autosomal regions to generate a fine resolution agamid phylogeny, and estimate population structure within Pogona vitticeps. Information about population connectivity is essential for future predictive modelling of the dynamics of W-chromosome loss. 

This work addresses questions of broad scientific interest regarding our capacity to predict evolutionary responses to climate change and thus the impact on biodiversity globally. 

Outcomes

This study has successfully tripled the spatial and temporal sampling of bearded dragons and tested for occurrences of sex reversal. We now have genotyped a total of 398 specimens spanning a 37-year period (1980 – 2017); whereas our last published report included just 131 individuals spanning a 9-year period (2003 – 2011) [Holleley et al., 2015]. Sex reversal occurrences are geographically clustered and temporally stable, with the highest-incidence occurring on the border of NSW and QLD. This suggests that it is possible for mixed-model systems of sex determination (GSD + thermal override) to exist stably in the short term, with low rates of sex reversal. It also suggests that loss of the heterogametic sex chromosome could be buffered by immigration from surrounding populations. 

To understand how the occurrence of sex reversal relates to real-world climate data, we developed models to probabilistically estimate egg incubation date and nest location based on adult capture data. We used this framework to query publically available climate datasets and estimate the likely egg incubation temperatures in the field. Despite having an increased empirical dataset, we did not detect a significant correlation between predicted nest environment (mean maximum temperature and mean diurnal temperature range) and sex reversal. This suggests that occurrence of sex reversal in the wild might be more closely associated with variability in the physiological threshold for sex reversal (temperature sensitivity) rather than an absolute temperature experienced during development. However, the statistical power of this model could be improved with new information on the age structure of wild populations, life-span in the wild and territory size of female individuals. 

DNA extractions from formalin preserved specimens have proved to be challenging and thus we have not yet been able to include these samples in the analyses mentioned above. However, efforts continue and we are optimistic that we will have successful workflow for formalin preserved specimens in 2018, adding another potential 933 specimens from museum collections nationally.

Holleley CE, O'Meally D, Sarre SD, Marshall Graves JA, Ezaz T, Matsubara K, Azad B, Zhang X, Georges A: Sex reversal triggers the rapid transition from genetic to temperature-dependent sex. Nature 523:79-82 (2015).