Endocrine and Environmental Control of Sex Differentiation in Gonochoristic Fish

Abstract

Sex in vertebrates, including fish, is usually determined by genotype. In medaka (Oryzias latipes), a gonochoristic fish with the XX/XY sex determination system, a gene that encodes the DM domain on the Y chromosome is identified as the master sex-determining gene. However, the sex-determining genes in many nonmammalian vertebrates remain unclarified. In contrast, sex determination in some reptiles, amphibians, and fish is influenced greatly by environmental factors. For example, although the genotypic sex determination mechanism in Japanese flounder (Paralichthys olivaceus) is basically the XX/XY type, genotypic females can be sex reversed to phenotypic males by rearing the larvae at high or low water temperatures during gonadal sex differentiation. In addition, the phenotypic sex of many teleost fish, including flounder, can be experimentally altered by treatment with sex steroid hormones, suggesting an important role for sex steroid hormones in gonadal sex differentiation in fish. In this chapter, we review general information and recent knowledge on the basic mechanisms of sex determination and gonadal sex differentiation, and present the effects of sex steroid hormones and water temperature on gonadal sex differentiation in gonochoristic fish.