Genetic evidence for co-occurrence of chromosomal and thermal sex-determining systems in a lizard

Abstract

An individual's sex depends upon its genes (genotypic sex determination or GSD) in birds and mammals, but reptiles are more complex: some species have GSD whereas in others, nest temperatures determine offspring sex (temperature-dependent sex determination). Previous studies suggested that montane scincid lizards (Bassiana duperreyi, Scincidae) possess both of these systems simultaneously: offspring sex is determined by heteromorphic sex chromosomes (XX-XY system) in most natural nests, but sex ratio shifts suggest that temperatures override chromosomal sex in cool nests to generate phenotypically male offspring even from XX eggs. We now provide direct evidence that incubation temperatures can sex-reverse genotypically female offspring, using a DNA sex marker. Application of exogenous hormone to eggs also can sex-reverse offspring (oestradiol application produces XY as well as XX females). In conjunction with recent work on a distantly related lizard taxon, our study challenges the notion of a fundamental dichotomy between genetic and thermally determined sex determination, and hence the validity of current classification schemes for sex-determining systems in reptiles.

Figure 1

Agarose gel showing identification of chromosomal sex for two females and two males. Upper half of gel: duplex PCR amplification of 356 bp C-mos fragment (males and females) and 185 bp Y-chromosome fragment (males only) from genomic DNA extracted by high-salt method. Lower half of gel: duplex PCR amplification of 356 bp C-mos fragment (females only) and 92 bp Y-chromosome fragment (males only) from genomic DNA extracted by Chelex method. The Y-chromosome fragment is amplified preferentially over the positive-control C-mos fragment for the Chelex-extracted DNA. Lane 1 shows molecular weight marker.