Recombination in yeast mitochondrial DNA

When haploid yeast strains containing mitochondrial DNAs (mtDNAs) of different buoyant densities are mated, the resulting zygotes contain a mixed population of mitochondria and mitochondrial DNAs. During vegetative growth of diploid cells formed from such a cross between a petite strain with mtDNA of density 1.677 g cm−3 and a respiratory competent strain with mtDNA of density 1.684 g cm−3, mtDNAs with intermediate buoyant densities are obtained. Virtually all newly synthesized mtDNA in diploid ρ− progeny has the intermediate buoyant density. Therefore, within 2 generations of growth of the diploid cells, the intermediate buoyant density species predominate. In crosses between a respiratory competent strain and other petite strains with different values of genetic suppressiveness, it was found that the amount of recombination yielding mtDNAs of intermediate buoyant densities roughly parallels the degree of suppressiveness. Individual clones of respiratory deficient cells from such crosses were also isolated to confirm that stable mtDNAs with intermediate buoyant densities were obtained. Thus, it is apparent that some form of recombination takes place within the mtDNAs of yeast cells that results in stable mtDNA species.