Sex Determination and Differentiation in Fish

Sexual dimorphism is probably the most penetrant but plastic feature of animal physiology, morphology and behavior. Despite the quasi-universality of that phenomenon itself, the different mechanisms of how sex is specified and determined are very diverse among various organismic groups. In contrast to the other vertebrates, in which the same genetic sex determination systems are shared by most species, the diversity of sex determination systems is especially obvious in fish where, within groups of closely related species, a wide spectrum of different systems can be found. Classically, sex determination mechanisms are divided in two major categories, either (i) genetic sex determination (GSD) or (ii) environmental sex determination (ESD). Interestingly, among each type, a multitude of mechanisms of how to spark either male or female gonadal development have been described. While it appears that the phenotypic expression of sex might be better seen as a threshold trait, for which very plastic and modular networks of interactions are influenced by a variety of "master" or "minor" triggers, practically, combinations (of various degrees) of the different systems (GSD and ESD) are also frequently observed. Because the amazing diversity of sex triggers in fish emphasizes the many options possible at the sex determination stage to switch and supervise over the destiny of the gonads, fish on the whole are an attractive system for studying the evolution of sex determining genes and regulatory networks, in relation to the emergence and turnover of master sex-determining genes.