Extensive variation and heteroplasmy in size of mitochondrial DNA among geographic populations of Drosophila melanogaster

Size variation and heteroplasmy in mitochondrial DNA (mtDNA) are relatively common in natural populations of Drosophila melanogaster. Of 92 isofemale lines of flies obtained from various geographic regions throughout the world, 75 lines were homoplasmic and showed a total of 12 different mtDNA size classes. The remaining 17 lines were heteroplasmic, each line carrying two different mtDNAs, and, in all but one case, the mtDNAs in these heteroplasmic lines differed in size; a total of nine size classes was represented among them. In cases where one type was predominant within an individual, it was usually the smaller mtDNA. This finding parallels what was observed in homoplasmic lines, in that the smaller mtDNAs were much more common than the larger variants in most populations. The data suggest a high rate of mutational occurrence of mtDNA size variants and some natural selection against them.