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 variatio n 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 fixa tion of strongly selected mutants. The hitchhiking process can be desc ribed as a series of two-step events. The first step consists of a str ongly selected substitution wiping out linked variation in a populatio n; this is followed by a recovery period in which polymorphism can bui ld up via neutral mutations and random genetic drift. Genetic hitchhik ing has previously been modeled as a steady-state process driven by re curring selected substitutions. We show here that the characteristic p arameter of this steady-state model is alphanu, the product of selecti on intensity (alpha = 2Ns) and the frequency of beneficial mutations n u (where N is population size and s is the selective advantage of the favored allele). We also demonstrate that the steady-state model descr ibes the hitchhiking process adequately, unless the recombination rate is very low. To estimate alphanu, we use the data of DNA sequence var iation from 17 D. melanogaster loci from regions of intermediate to hi gh recombination rates. We find that alphanu is likely to be > 1.3 x 1 0(-8). Additional data are needed to estimate this parameter more prec isely. The estimation of alphanu is important, as this parameter deter mines the shape of the frequency distribution of strongly selected sub stitutions.