The view genotypic sex determination (GSD) and environmental sex determination (ESD) are mutually exclusive states has been contradicted by the discovery that chromosomal sex and environmental influences can co-exist within the same species, hinting at a continuum of intermediate states. Systems where genes and the environment interact to determine sex present the opportunity for sex reversal to occur, where the phenotypic sex is the opposite of that predicted by their sex chromosome complement. The skink Bassiana duperreyi has XX:XY sex chromosomes with sex reversal  of the  XX genotype to a male phenotype in response to exposure to cold incubation temperatures. Here we studied the frequency of sex reversal in B. duperreyi in response to climatic variation, using elevation as a surrogate for environmental temperatures. We demonstrate sex reversal for the first time in free-ranging adults of a reptile species with XX/XY sex determination. The highest frequency of sex reversal occurred at the highest elevation location, Mount Ginini (18.64%) and decreases in frequency with elevation. We model the impact of this under Fisher's frequency dependent selection to show that, only at the highest elevations, populations risk the loss of the Y chromosome and a transition to TSD. This study contributes to our understanding of the risks of extinction from climate change in species subject to sex reversal by temperature, and will provide focus for future research to test on-the-ground management strategies to mitigate the effects of climate in local populations.

Tail tissue samples of a total of 100 B. duperreyi from the 10 populations (10 samples per location) were passed to Diversity Arrays Technology Pty Ltd (Canberra) (DArT) for reduced representational sequencing. Genomic DNA (gDNA) was extracted using a NucleoMag® 96 Tissue kit (Macherey‐ Nagel, Düren, Germany) coupled with NucleoMag SEP (Ref. 744900) to allow automated separation of high‐quality DNA on a Freedom Evo robotic liquid handler (TECAN, Männedorf, Switzerland). Four combinations of restriction enzymes were evaluated for B. duperreyi (PstI enzyme combined with either HpaII, SphI, NspI, and MseI) and the restriction enzyme combination of PstI (recognition sequence 5’-CTGCA| G-3’) and SphI (5’ -GCATG|C-3’) was selected for complexity reduction by double digestion. A full account of the process of generating SNP genotypes for these samples is given by Georges et al. 2018, Molecular Ecology 27:5195-5213. The data comprised a matrix of SNP loci by individuals, with the contents stored as 0, homozygote, reference state; 1, heterozygote; and 2, homozygote, alternate state.

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