Interspecific cytoplasmic gene flow in the absence of nuclear gene flow: evidence from Drosophila.

mtDNA polymorphism has been studied by restriction endonuclease site variation in Drosophila pseudoobscura and its sibling species D. persimilis. Eight enzymes have been used to study 54 isofemale strains from areas where the two species are sympatric and D. pseudoobscura is allopatric. Where sympatric, 75-80% of the strains have mitochondrial genomes found in both species. Where allopatric, D. pseudoobscura has diverged to the point where none of the strains have mtDNA in common with D. persimilis. The most likely explanation for this observation is that where sympatric the two species hybridize frequently enough to keep their mtDNA from diverging. However, hybridization has not prevented their nuclear genomes from diverging, perhaps due to selection against nuclear gene introgression contrasted with little or no selection against mtDNA introgression. These observations suggest that nuclear and cytoplasmic genomes have different evolutionary dynamics.