Dynamics of the mtDNA Haplotype Variability in a Drosophila subobscura Population Over a Two-Year Period

Restriction site analysis of mitochondrial DNA (mtDNA) was carried out on 607 isofemale lines, corresponding to monthly samples obtained over a two-year period, from a single geographic population of Drosophila subobscura to evaluate the possible changes in the action of the evolutionary forces with respect to the variation of the seasonal enviromental conditions. The haplotype distribution pattern was: (1) two highly frequent haplotypes and (2) sporadic haplotypes that were almost never found again after a population bottleneck; similar to that observed for the entire range of this species on the European continent. We detected significant negative D-values for the entire population, reflecting an excess of rare haplotypes. The results confirm that the coexistence of the main haplotypes in nature, and the rare ones, could be mainly caused by seasonal population expansions. However, the fact that the frequencies of the two major haplotypes are exchanged in some months, could indicate that the action of selection cannot be discarded, possibly acting upon gene arrangement associated with mtDNA haplotypes.