Widespread unidirectional transfer of mitochondrial DNA: a case in western Palearctic water frogs

Interspecies transfer of mitochondrial (mt) DNA is a common phenomenon in plants, invertebrates, and vertebrates, normally linked with hybridization of closely related species in zones of sympatry or parapatry. In Central Europe, in an area north of 48° N latitude and between 08° and 22° E longitude, western Palearctic water frogs show massive unidirectional introgression of mtDNA: 33.7% of 407 R. ridibunda possessed mtDNA specific for R. lessonae . In contrast, no R. lessonae with R. ridibunda mtDNA was observed. That R. ridibunda with introgressed mitochondrial genomes were found exclusively within the range of the hybrid R. esculenta and that most hybrids had lessonae mtDNA (90.4% of 335 individuals investigated) is evidence that R. esculenta serves as a vehicle for transfer of lessonae mtDNA into R. ridibunda . Such introgression has occurred several times independently. The abundance and wide distribution of individuals with introgressed mitochondrial genomes show that R. lessonae mt genomes work successfully in a R. ridibunda chromosomal background despite their high sequence divergence from R. ridibunda mtDNAs (14.2–15.2% in the ND2/ND3 genes). Greater effectiveness of enzymes encoded by R. lessonae mtDNA may be advantageous to individuals of R. ridibunda and probably R. esculenta in the northern parts of their ranges.