Mitochondrial DNA Polymorphisms in the Two Subspecies

Recent empirical and theoretical studies on mitochondrial DNA (mtDNA) variation in higher animals have suggested that the extent of mtDNA polymorphism is largely affected by spatial population subdivision. To examine this we studied mtDNA polymorphism in two subspecies of Drosophila sulfurigaster: D. s. albostrigata and D. s. bilimbata. Drosophila sulfurigaster albostrigata is mainly distributed on the mainland of Southeast Asia. In contrast, D. s. bilimbata forms discontinuous populations on many islands scattered in the Pacific Ocean. Because of the difference in their distribution patterns, the two subspecies are thought to be different in the extent of spatial population subdivision. mtDNA was isolated from >50 isofemale strains for each subspecies and were analyzed by eight restriction endonucleases. Nucleotide diversity within a population was higher in D. s. albostrigata than in D. s. bifimbata. However, haplotype diversity was 1.6 times greater in D. s. bilimbata (0.85) than in D. s. albostrigata (0.53). The large difference in overall heterogeneity was attributed to the difference in interpopulational nucleotide diversity. For the two subspecies the proportion of interpopulational gene diversity in a subdivided population was calculated to be 0.54 in D. s. bilimbata and 0.40 in D. s. albostrigata. These observations indicate that spatial population subdivision is a major factor in determining mtDNA polymorphism in these subspecies. The extent of mtDNA divergence between the subspecies was very high. The average nucleotide divergence between them was 7.6%, which is almost the interspecific level reported for other Drosophila species. The cause of the high degree of mtDNA divergence is discussed.