Amphidiploidy recovers the viability of hybrids of European and east Asian water frogs.

Reciprocal diploid hybrids, artificially produced from crosses between Rana nigromaculata and Rana lessonae, die at the stage from neurula to tailbud. We found in a previous study that triploid hybrids having two R. nigromaculata genomes and one R. lessonae genome in R. nigromaculata cytoplasm grow into mature frogs, whereas triploid hybrids composed of the other combinations of genome and cytoplasm arrest before hatching. In this study, we made amphidiploid hybrids with two R. nigromaculata and two R. lessonae genomes in the cytoplasm of either species and examined their viability to discover the interaction between parental genomes in each cytoplasm. The amphidiploids with R. nigromaculata cytoplasm developed into mature frogs, whereas ones with R. lessonae cytoplasm arrested at almost the same embryo stage as the lethal triploid hybrids. These findings suggest that the arrest of the reciprocal diploid hybrids is not caused only by the incompatibility between the R. nigromaculata genome and the R. lessonae genome. Taking these together with the results of the triploid hybrids, we suppose that the lethality of the hybrids is mainly due to the incompatibility between the egg cytoplasm and foreign genome(s). With the R. nigromaculata cytoplasm, doubling the maternal genome can ease this incompatibility, but it cannot with the R. lessonae cytoplasm.