Evolution of sex determining genes in fishes

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Sexual reproduction is one of the most highly conserved processes in evolution and implicates many fields of biology but also breeding or human health and society. The genetic and cell biological mechanisms making the decision whether the undifferentiated gonad of the embryo develops either towards male or female are manifold and quite different. Sex determining (SD) mechanism range from environmental to simple or complex genetic mechanisms and have evolved repeatedly and independently. In species with genetic sex determination, master SD genes on sex chromosomes drive gonadal differentiation by switching on a developmental program that ultimately leads to testis or ovary differentiation. Very few sex-determining genes have been so far identified in fish, and in animals in general. Fish are uniquely suited to study the evolution of sex determination. Comprising about half of the about 60.000 species of vertebrates, fish show the greatest variety of sex determination mechanisms including species with either environmental or genetic sex determination. Even those few known SD genes are apparently not conserved over a larger number of related orders, families, genera or even species. This frequent evolutionary turnover of SD genes may be explained by a large diversity of master sex regulators. This project has thus two major objectives: (1) To screen for potential sex-determining genes in many fish species; (2) To demonstrate the role of identified genes as master sex determinants in a few selected species. To address these questions, we have developed a strategy (Rad-Sex) that makes use of Next Generation Sequencing technology to identify markers that define sex specific segments of the male or female genome. The obtained markers will be used to isolate candidates for SD genes from these regions. This approach will be carried out by screening in a first round 35 species representing major branches of the fish tree of life. From these we will then take the most promising 15 species to find candidate SD genes. Those will be further evaluated and finally a selection of 3 genes will be taken to functional characterization. The finding of new master SD genes will be of major importance taking into account the very low number of vertebrate species, in which such genes have been characterized. From a comparative point of view, this project will also have a great impact on what is already known on the evolution of genetic sex determination in vertebrates in general and more specifically in fishes, in which the questions about the evolutionary significance of the high diversity of master sex-determining genes and the genomic mechanisms allowing this rapid turnovers are largely unanswered. This project will also have important practical outcomes for molecular sexing as a tool for a better control of sex determination in aquaculture and for ecology and ecotoxicology research in economically or environmentally very important species investigated here.