Differences in genetic backgrounds affecting gonadal differentiation between two local populations of the Japanese wrinkled frog (Rana rugosa).

Gonadal differentiation in two local populations - Hamakita and Hiroshima - of the Japanese wrinkled frog (Rana rugosa) was examined from hatching to the end of metamorphosis. The Hamakita male has X and Y chromosomes distinguishable morphologically from each other, while X and Y chromosomes in the Hiroshima male cannot be distinguished morphologically. In the two populations, primordial gonads differentiated into testes or ovaries by stage I of Taylor and Kollros, and larvae at stage XXV - the end of metamorphosis - possessed definitive testes or ovaries. However, immature testes at stages II-III in the Hiroshima population showed a higher incidence of testes with ovarian characteristics - meiotic figures and gonadal cavities - than those in the Hamakita population. In addition, proliferative activities in the testicular cells of the immature testes in the Hiroshima population were very low. On the other hand, there were no remarkable differences in the histological processes of ovary development between the two populations. Intraspecific hybridization between females of the Hamakita population and males of the Hiroshima population retarded testis differentiation after stage XV, while the reciprocal hybridization showed normal gonadal sex differentiation and development. These observations suggest that R. rugosa, possessing heteromorphic sex chromosomes in the male, establishes firm gonochorism, where the feminization involves the suppression of testis differentiation, and that masculinization is performed by resistance to this suppression as well as active proliferation of testicular cells.