COMPETITION BETWEEN MITOCHONDRIAL HAPLOTYPES IN DISTINCT NUCLEAR GENETIC ENVIRONMENTS - DROSOPHILA-PSEUDOOBSCURA VS D-PERSIMILIS

A test for coadaptation of nuclear and mitochondrial genomes was perfo rmed using the sibling species, Drosophila pseudoobscura and D. persim ilis. Two lines of flies with ''disrupted'' cytonuclear genotypes were constructed by repeated backcrossing of males from one species to fem ales carrying mitochondrial DNA (mtDNA) from the other species. Each ' 'disrupted'' strain was competed in population cages with the original stock of each species from which the recurrent males were obtained du ring the backcrossing. As such, the two species' mitochondrial types w ere competed reciprocally in the nuclear genetic environments of each species. The trajectories of mtDNA haplotypes were followed in discret e-generation population cages using a PCR-four-cutter approach. A sign ificant increase in the frequency of D. pseudoobscura mtDNA was observ ed in each of four replicate cages with a D. pseudoobscura nuclear bac kground. In the D. persimilis nuclear background, one cage actually sh owed an increase in frequency of D. pseudoobscura mtDNA, although toge ther the four replicate cages show little change in frequency. These r esults were repeated after frequency perturbations and reinitiation of each cage. An analysis of fitness components revealed that fertility selection greatly outweighed viability selection in these cytonuclear competition experiments. The asymmetry of the fitnesses of the mtDNA h aplotypes on the two genetic backgrounds is consistent in direction wi th the previously reported asymmetry of female fertility in backcrosse s between these two species. While our experiments do not allow us to identify mtDNA as the sole source of fitness variation, at a minimum t he data indicate a fitness association between nuclear fertility facto rs and the D. pseudoobscura mtDNA on its own genetic background.