Population structure and mitochondrial DNA gene flow in Old World populations of Drosophila subobscura

An extensive survey of mitochondrial DNA (mtDNA) restriction polymorphism in 156 isofemale lines from 29 different geographic populations of Drosophila subobscura distributed throughout the Old World was carried out. Ten restriction enzymes were used, five of which revealed restriction site polymorphism. Of the 31 restriction sites detected, 13 were found to be polymorphic. Comparisons with the mtDNA map of Drosophila yakuba indicate that the variable sites are mainly concentrated in protein genes, especially those corresponding to the NADH complex. A total of 13 different haplotypes were observed, two of which (haplotypes I and II) are quite frequent and widely distributed throughout the populations, whereas the other 11 with the exception of VIII, which deserves special attention, are each restricted to one population only and occur at low frequencies. The observed distribution of haplotypes, corroborated by a parsimonious unrooted tree, suggests an ancient origin of haplotypes I and II in the continent. In order to compare genetic structure according to mtDNA and allozymes, the 10 populations with higher population sizes were studied for 10 polymorphic allozymes also. One striking result is the high degree of population structure of the mtDNA when compared to that obtained for allozymes. If an island model is assumed, estimates of gene flow give values of 0.013 and 1.89 migrants per generation for mtDNA and allozymes, respectively. What is apparent from these estimates is that Drosophila subobscura populations are effectively subdivided for mtDNA genes at migration rates at which nuclear genes (allozymes) are almost panmictic.