ANALYSIS OF A GENETIC HITCHHIKING MODEL, AND ITS APPLICATION TO DNA POLYMORPHISM DATA FROM DROSOPHILA-MELANOGASTER

Begun and Aquadro have demonstrated that levels of nucleotide variation correlate with recombination rate among 20 gene regions from across the genome of Drosophila melanogaster. It has been suggested that this correlation results from genetic hitchhiking associated with the fixation of strongly selected mutants. The hitchhiking process can be described as a series of two-step events. The first step consists of a strongly selected substitution wiping out linked variation in a population; this is followed by a recovery period in which polymorphism can build up via neutral mutations and random genetic drift. Genetic hitchhiking has previously been modeled as a steady-state process driven by recurring selected substitutions. We show here that the characteristic parameter of this steady-state model is alphanu, the product of selection intensity (alpha = 2Ns) and the frequency of beneficial mutations nu (where N is population size and s is the selective advantage of the favored allele). We also demonstrate that the steady-state model describes the hitchhiking process adequately, unless the recombination rate is very low. To estimate alphanu, we use the data of DNA sequence variation from 17 D. melanogaster loci from regions of intermediate to high recombination rates. We find that alphanu is likely to be > 1.3 x 10(-8). Additional data are needed to estimate this parameter more precisely. The estimation of alphanu is important, as this parameter determines the shape of the frequency distribution of strongly selected substitutions.