Humans and other mammals have an XX female: XY male system of sex determination, in which a gene SRY on the Y chromosome kick-starts testis-differentiation in the embryo. The testis makes male hormones, which induce male development of the fetus. Birds have a different system of ZZ male: ZW female, in which dosage of a gene DMRT1 on the Z controls sex; the ZW pair is not homologous to the mammal XY. Other reptiles, frogs and fish have different sex chromosome systems and we now know of many distinct sex determining genes which act at different points of the conserved sex determining pathway. This astonishing variety of sex determining genes and chromosomes is the result of the rapid birth and death of sex chromosomes.
Many reptiles and some fish have no sex chromosomes. Sex is determined by environmental factors such as temperature (TSD), through epigenetic changes whose nature has been a longstanding mystery. We work with an Australian dragon lizard, which has a ZW system driven by yet another sex determining gene. However, when it’s hot, all the eggs hatch as females. We have used this system to investigate how TSD works. We found that the transcriptome of ZZ females contains upregulated stress markers and unique transcripts of two epigenetic markers. This suggests that temperature acts, via the stress pathway, to activate epgenetic modifications involved in male determination. Have we discovered the mechanism of TSD at last?