Pervasive non-random mitochondrial DNA heteroplasmy in the hybrid water frog Pelophylax esculentus

Heteroplasmy, the presence of more than one type of mitochondrial DNA (mtDNA) within an individual, is an exception to the maternal transmission of mtDNA and has been observed in several animal species. A central question is whether heteroplasmy among individuals and across generations is mainly influenced by genetic drift or by selection.

We quantified heteroplasmy in eight males, eight females and eight unfertilized eggs per female from a natural population of the hybrid frog species Pelophylax esculentus (between P. ridibundus and P. lessonae). After excluding sequencing error and potential sources of contamination, we found that all individuals and most of the eggs were heteroplasmic, containing 2-5 different haplotypes, from which one was very common and the rest appeared at very low frequencies (at maximum 2%). We observed a single lessonae haplotype, which was present in females and in their eggs but absent from all males. On the other hand, we observed four different ridibundus haplotypes that were present in males, females and eggs. Eggs had significantly lower heteroplasmy levels than their mothers.

The distribution of haplotypes between males and females, the difference of heteroplasmy levels between mothers and their eggs, and results from simulations suggest that drift alone is not sufficient to explain the observed patterns of heteroplasmy.