Copyright 2001 by the Genetics Society of America Regions of Lower Crossing Over Harbor More Rare Variants in African Populations of Drosophila melanogaster

A correlation between diversity levels and rates of recombination is predicted both by models of positive selection, such as hitchhiking associated with the rapid fixation of advantageous mutations, and by models of purifying selection against strongly deleterious mutations (commonly referred to as “background selec-tion”). With parameter values appropriate for Drosophila populations, only the first class of models predicts a marked skew in the frequency spectrum of linked neutral variants, relative to a neutral model. Here, we consider 29 loci scattered throughout the Drosophila melanogaster genome. We show that, in African populations, a summary of the frequency spectrum of polymorphic mutations is positively correlated with the meiotic rate of crossing over. This pattern is demonstrated to be unlikely under a model of background selection. Models of weakly deleterious selection are not expected to produce both the observed correlation and the extent to which nucleotide diversity is reduced in regions of low (but nonzero) recombination. Thus, of existing models, hitchhiking due to the recurrent fixation of advantageous variants is the most plausible explanation for the data. IT has been known for a decade that levels of nucleo- pie 2000). This occurs because the physical length of thehitchhiking region depends on the strength of selectiontide diversity in Drosophila melanogaster, but not levels relative to the recombination rate (Kaplan et al. 1989).of divergence with its sibling species, are correlated wit