Mammals and birds both have well defined sex chromosome systems containing sex determining genes that determine gonad type with apparently no input from the environment. Yet we have long known that there are systems in which sex is determined without the benefit of sex chromosomes. Crocodilians and marine turtles are famous for having no sex chromosomes. Instead, sex of hatchlings is determined by the temperature at which the egg is incubated. Males and females have identical genomes, so the choice of sex must be effected by epigenetic means. The mechanism has been a longstanding mystery.
We now know that there are several species with perfectly respectable sex chromosomes, but sex can be reversed by temperature. The Central bearded dragon has well defined Z and W chromosomes, but at a high temperature ZZ as well as ZW embryos develop as females. The half-smooth tongue sole also has ZZ males and ZW females and DMRT1 is the sex determining gene (as in birds). The difference is that the DMRT1 locus is repressed by methylation at moderate temperatures; incubation at higher temperatures releases this inhibition and results in ZW “pseudomales”.
The interaction of genes and the environment to precipitate male or female development gives us a chance to explore the epigenetic basis of temperature dependent sex determination. A breakthrough has been the identification of unique transcripts in sex reversed dragons that establish links between stress and sex determining pathways.