Unusual sex-ratios and developmental mortality in the rice frog Fejervarya kawamurai

In the Japanese rice frog, Fejervarya kawamurai, we identifi ed two unusual features of the sex ratio using artificial crosses at laboratory. Firstly, inbreeding such as sibling-sibling crosses, backcrosses, and diploid gynogenesis resulted in a male-biased sex ratio with a high rate of developmental mortality. Secondly, outbreeding reduced the mortality, and particularly outbreeding of females with males from a geographically separate population restored the sex ratio. Established mechanisms of genetic and environmental sex determination in vertebrates do not easily explain these results. Ecologically, this mechanism favors expanding populations that invade new habitat, because frogs must move continuously between populations to produce enough daughters and reduce embryonic mortality.