Sex Determination and Differentiation in Frogs

Abstract

In amphibians, it is believed that sex is genetically determined. The genetic sex-determining systems of amphibians include female (ZW) and male (XY) heterogamety. Interestingly, the Japanese Wrinkled Frog (Glandirana (Rana) rugosa) has both types of heterogamety, which was caused by geographic variation. Although almost all mammalian and avian species have heteromorphic sex chromosomes, the majority of amphibians, including the African Clawed Frog Xenopus laevis, possess homomorphic sex chromosomes. Thus, there should be a variety of sex-determining genes in amphibians. However, little is known about the molecular mechanisms underlying sex determination, although a W chromosome-linked gene dm-w in X. laevis was reported in 2008 to be responsible for a case of female sex determination. In contrast to the heterogamety, gonadal sexual differentiation follows a more conservative system. In many frog species, exposure of tadpoles with undifferentiated gonads to estrogen or androgen can induce male-to-female or female-to-male sex reversal, respectively. These findings suggest that sex steroid hormones have important roles in early sex differentiation. Estrogen- and androgen-synthesizing genes cyp19a1 and cyp17a1 show sexually dimorphic expression in early differentiating gonads in some frog species. In X. laevis, the structure called ‘mass-in-line,’ consisting of cyp17a1/cyp19a1-expressing cells, is involved in ovarian cavity formation. This chapter describes these situations in detail, and co-evolution between sex-determining genes and sex chromosomes is discussed. Germ cell development including gametogenesis and its endocrine control are also described.