An Evolutionary Witness: the Frog <i>Rana rugosa</i> Underwent Change of Heterogametic Sex from XY Male to ZW Female

There are two basic types of heterogamety for genetic sex determination in animals and plants: male heterogamety (XX/XY) and female heterogamety (ZZ/ZW). Although apparently in opposition, the two distinct types may in fact be interchangeable. For example, in amphibians it has been shown that the heterogametic sex was originally female and may have become male at some branching point in their phylogenetic evolution. In particular, there is evidence that the male heterogametic sex determination of the frog <i>Rana rugosa</i> returned to its previous female state during speciation that occurred when the distribution range of the frog expanded across Japan. This change is quite recent in the phylogenetic time scale. This paper presents a review of the sex chromosomes and sex determination in the frog <i>R. rugosa</i>, an evolutionary witness proving the viability of changing heterogametic sex, and introduces recent findings and on-going studies in the frog. Change of the heterogametic sex will also be discussed, relating data from frogs (<i>Rana</i>) and other animals to the replacement of a master sex-determining gene in the course of speciation.