Novel sex-determining genes in fish and sex chromosome evolution

Although the molecular mechanisms underlying many developmental events are conserved across vertebrate taxa, the lability at the top of the sex-determining (SD) cascade has been evident from the fact that four master SD genes have been identified: mammalian Sry; chicken DMRT1; medaka Dmy; and Xenopus laevis DM-W. This diversity is thought to be associated with the turnover of sex chromosomes, which is likely to be more frequent in fishes and other poikilotherms than in therian mammals and birds. Recently, four novel candidates for vertebrate SD genes were reported, all of them in fishes. These include amhy in the Patagonian pejerrey, Gsdf in Oryzias luzonensis, Amhr2 in fugu and sdY in rainbow trout. These studies provide a good opportunity to infer patterns from the seemingly chaotic picture of sex determination systems. Here, we review recent advances in our understanding of the master SD genes in fishes.