The effect of selective sweeps on the variance of the allele distribution of a linked multiallele locus: hitchhiking of microsatellites.

Microsatellite variation and the mechanisms which are responsible for this variation have received much attention in the last few years. Most theoretical studies of microsatellite allele distributions, however, did not incorporate the evolutionary dynamics of linked sites. The dynamics is usually modeled by invoking a special mutation mechanism such as stepwise mutation, which leads to a stepwise increase or decrease of the number of motif repeats on the occasion of mutation. It is shown here that selection at a locus, which itself is not subject to mutation, but which is adjacent to a microsatellite locus has an influence on statistics of the microsatellite allele distribution, provided that mutation rates are low to intermediate, when compared to 1/t1, the inverse of the time to fixation of a linked favorable substitution. If mutation rates are high, as for example in humans, a selective effect upon the microsatellite locus, such as hitchhiking, will quickly be obscured by mutations. In particular, in the latter case, the model shows that no correlation is to be expected between recombination rates and variability of microsatellites--such as had been predicted and experimentally demonstrated for nucleotide variability and recombination rates in Drosophila. The present model is a generalization of the two locus two allele hitchhiking model which had been studied by Stephan and co-workers.