Mitochondrial DNA reveals formation of nonhybrid frogs by natural matings between hemiclonal hybrids.

The European water frog Rana esculenta (RL), a natural hybrid between R. ridibunda (RR) and R. lessonae (LL), reproduces by hybridogenesis: haploid gametes usually contain an intact chromosome set of R. ridibunda (R); the lessonae nuclear genome (L) is lost from the germ line. Hybridity is restored in the next generation, via fertilization by syntopic R. lessonae. Matings between two hybrids (RL x RL) usually give inviable R. ridibunda (RR) progeny. The adult R. ridibunda subpopulation of Trubeschloo, a gravel pit in northern Switzerland, consists only of females. Fragment patterns for mitochondrial DNA (mtDNA) of these R. ridibunda were identical with those of syntopic R. esculenta and of local populations of R. lessonae; they differed from the patterns in eastern European populations of R. lessonae and of R. ridibunda mtDNAs (3.7% and 9.3% estimated sequence divergence, respectively). In contrast, mtDNAs of two R. ridibunda from an introduced Swiss population with both sexes, although different (2.7% divergence) from each other, were typical R. ridibunda rather than R. lessonae mtDNAs. These data, together with unisexuality, demonstrate conclusively that the all-female R. ridibunda population at Trubeschloo originated from matings between two R. esculenta. The formation of independently reproducing R. ridibunda populations via such hybrid x hybrid matings is precluded because progeny of these matings are unisexual. Recombination in the regenerated fertile R. ridibunda females, followed by matings with R. lessonae, nevertheless provides a mechanism for meiotic reshuffling of genetic material in ridibunda haplotypes that is not typically available in hemiclonal lineages.