Governing sex determination in fish: regulatory putsches and ephemeral dictators

In contrast to the rather stable regulatory regimes established over more that 100 million years in birds and mammals, sex determination in fish might frequently undergo evolutionary changes bringing the sex-determining cascade under new master sex regulators. This phenomenon, possibly associated with the emergence of new sex chromosomes from autosomes, would explain the frequent switching between sex determination systems observed in fish. In the medaka Oryzias latipes, the Y-specific master sex-determining gene dmrt1bY has been formed through duplication of the autosomal gene dmrt1 onto another autosome, thus generating a new Y chromosome. Dmrt1bY emerged about 10 million years ago and is restricted to several Oryzias species, indicating that the Y chromosome of the medaka is evolutionarily much younger than mammalian and bird sex chromosomes. Fertile males without dmrt1bY have been detected in some medaka populations, and this gene might even have been inactivated in one Oryzias species, indicating the existence of sexual regulators already able to supplant dmrt1bY. Studies on other models have confirmed that fish sex chromosomes are generally young and occurred independently in different fish lineages. The identification of new sex-determining genes in these species will shed new light on the exceptional evolutionary instability governing sex determination in fish.