Abstract
Male and female heterogameties are two distinct modes for genetic sex determination. In almost all mammals including humans, male is the heterogametic sex, while female is the heterogametic sex in all birds. The above fact has contributed to creating a long-standing idea among the researchers that “Heterogametic sex once fixed is not changed so easily to the other”. A marginally evolved recent idea, however, proposes that heterogametic sex could be changed to each other far more frequently than we ever expected. In fact, we can well see many cases of transitions in lower vertebrates. Among them, Japanese frog Rana rugosa is surprisingly unique, because it has already experienced the change of heterogametic sex from male to female three times, within its own lineage. The fourth change, moreover, seems to be on the verge of appearance at the central Japan stage. Why does heterogametic sex change so frequently in the frog? We review the sex determining systems and conduct a discussion on driving-force to change the heterogametic sex, particularly from a point of view of uniqueness of this situation in phylogeny of the frog and topography of Japanese Islands involved in the population dynamics.
