Comparative aspects of gonadal sex differentiation in medaka: a conserved role of developing oocytes in sexual canalization.

Medaka is a differentiated gonochoristic species with a male heterogametic sex determination. Here, we review recent studies on gonadal sex differentiation in medaka, as well as the experimental sex-reversal models available for this organism. The accumulated literature on teleost gonadal development facilitates comparative interspecies studies. Among these, comparison between medaka and zebrafish is of special interest, because zebrafish is an undifferentiated gonochorist that, nevertheless, shares many similarities with medaka and is also used as a small fish model for developmental biology. Accordingly, here we focus on the comparative aspects of gonadal development in medaka and zebrafish. In medaka, oogenesis begins in female gonads, whereas oogenesis is suppressed and germ cells remain in an undifferentiated state in male gonads. In zebrafish, oogenesis begins in all individuals, regardless of their future sex, while actual sex differentiation begins later in gonadal development, which means that degeneration of immature oocytes occurs in presumptive males, while oogenesis proceeds to completion in presumptive females. Despite these apparent differences between medaka and zebrafish, the process of gonadal development comprises similar stages: early oogenesis, early aromatase expression, later oocyte development or loss, sexually dimorphic expression of somatic genes, and spermatogenesis. We propose the concept of canalization as a key to gaining a comprehensive understanding of gonadal sex differentiation. In this respect, the possible role of the male-determining gene DMY/dmrt1bY is also discussed.