Structural Heterogeneity of Mitochondrial DNA Molecules within the Genus Drosophila

We have determined by electron microscopy the molecular weights of circular mitochondrial DNA (mtDNA) molecules from 39 species representing 13 groups of five subgenera of the genus Drosophila. mtDNA molecules of all species examined, other than members of the melanogaster group, had, with one exception, molecular weights in the rather narrow range 9.90 × 106to 10.34 × 106. The one exception was D. robusta, which had a molecular weight of 10.61 × 106. In contrast, mtDNA molecules from species within the melanogaster group had molecular weights covering the considerably greater range 9.92 × 106to 12.35 × 106. Applying the electron microscope denaturation mapping technique of Inman to mtDNA molecules of eight species of the melanogaster group, we found each of them to contain a region [presumably rich in adenine and thymine (A+T)] which denatured at a specific temperature (40 degrees) at which most of the remainder of the molecule remained undenatured. The size of the A+T-rich region was constant for mtDNA molecules of a species, but varied from 0.62 × 106to 3.41 × 106for mtDNA molecules of different species. It was demonstrated that the differences in molecular weights of the A+T-rich regions can almost completely account for the differences in total molecular weights of the mtDNA molecules from species of the melanogaster group.