MITOCHONDRIAL-DNA HAPLOTYPE FREQUENCIES IN NATURAL AND EXPERIMENTAL POPULATIONS OF DROSOPHILA-SUBOBSCURA

The evolution of Drosophila subobscura mitochondrial DNA has been stud ied in experimental populations, founded with flies from a natural pop ulation fern Esporles (Majorca, Balearic Islands, Spain). This populat ion, like other European ones, is characterized by the presence of two ver; common (>96%) mitochondrial haplotypes (called I and II) and rar e and endemic haplotypes that appear at very low frequencies. There is no statistical evidence of positive Darwinian selection acting on the mitochondrial DNA variants according to Tajima's neutrality test. Two experimental populations, with one replicate each, were established w ith flies having a heterogeneous nuclear genetic background, which was representative of the composition of the natural population. Both pop ulations were started with the two most frequent mitochondrial haploty pes, but at different initial frequencies. After 13 to 16 generations, haplotype II reached fixation in three cages and its frequency was 0. 89 by generation 25 in the fourth cage. Random drift can be rejected a s the force responsible for the observed changes in haplotype frequenc ies. There is not only statistical evidence of a linear trend favoring a mtDNA (haploid) fitness effect, but also of a significant nonlinear de deviation that could be due to a nuclear component.