Instability of Sex-Determining Systems in Frogs

All of the anuran amphibians examined so far have genetic sex-determining systems, which include female heterogametic ZZ/ZW and male heterogametic XX/ XY types. For example, the Japanese wrinkled frog Glandirana rugosa has both types. Most of frog species including the African clawed frog Xenopus laevis possess homomorphic sex chromosomes, while most mammalian and avian species have heteromorphic sex chromosomes. Thus, there should be a variety of sexdetermining genes and sex chromosomes in frogs, although only X. laevis W-linked gene dm-W has been reported as a sex-determining gene. Interestingly, estrogen or androgen can induce sex reversal in many frog species, suggesting a vital role of sex steroid hormones on sex identity. In other words, frogs in the same order are good examples for the understanding of diversity of sex-determining systems. In this chapter, I summarize the diversity of frog sex-determining systems and discuss why sex-determining genes and systems have been unstable in frogs.