A TEST OF THE BACKGROUND SELECTION HYPOTHESIS BASED ON NUCLEOTIDE DATA FROM DROSOPHILA-ANANASSAE

We estimated DNA sequence variation within and between four population s of Drosophila ananassae at Om(ID) and vermilion (v) by using single strand conformation polymorphism analysis and direct DNA sequencing, O m(1D) is located on the X chromosome in a region with a normal recombi nation rate; v is in a region of low recombination. In each population , levels of nucleotide diversity at v are reduced 10- to 25-fold relat ive to those at Om(ID). Divergence between D. ananassae and its siblin g species D. pallidosa, however, is comparable for both loci. This lac k of correlation between levels of polymorphism and divergence led to the rejection of a constant-rate, neutral model. To distinguish among alternative models, we propose a test of the background selection hypo thesis based on the observed pattern of differentiation between popula tions, Although the degree of differentiation (measured by F-ST) among all I,airs of subpopulations is similar at Om(1D), we found substanti al differences at v. The two northern populations from Burma and Nepal are very homogeneous, whereas comparisons between northern and southe rn populations (e.g., between Nepal and middle India) produced large F -ST values. A coalescent-based simulation of the background selection model (in a geographically structured species with a finite number of demes) showed that the observed homogeneity among the northern populat ions is inconsistent with the background selection hypothesis, Instead , it may have been caused by a recent hitchhiking event that was limit ed to the northern species range.