Sex determination and sex chromosome evolution: insights from medaka

Among fish that exhibit a variety of sex-determining systems, medaka (Oryzias latipes) has been used as an ideal model to study sex determination and differentiation. Medaka sex differentiation starts with germ cell migration and proliferation during development, and the earliest sexual difference can be seen in germ cell number at stage 38 (1 day before hatching). Differentiation continues in young larvae. Medaka has an XX-XY genetic system, and the male sex-determining gene dmrt1bY (dmy) has been identified. Along with a sister species O. curvinotus, these species are the only 2 fish species whose sex-determining gene has been identified so far. The medaka sex chromosome is of recent origin, and the Y chromosome was formed through generation of the Y-specific region by duplication and insertion of an autosomal region into the proto-Y chromosome. Thus, the dmrt1bY gene is a duplicate of the autosomal dmrt1a gene. This event is estimated to have occurred about 10 million years ago. Sex-determining systems of other Oryzias species are under investigation, and a variety of sex-determining systems seem to exist. These findings may provide new insights into the sex-determining system of medaka.