sweeps at linked loci
Hartfield M & Otto SP 2011 Recombination and hitchhiking of deleterious alleles. Evolution, in press.
doi:10.1111/j.1558-5646.2011.01311.x
- once established, the frequency of A1B1 can be modeled by the standard deterministic equation for haploid selection
- p(t) = p0(1 + snet)t / [p0(1 + snet)t + 1 − p0] ... (1)
- among those alleles that succeed in fixing, the trajectory of the A1B1 haplotype is slightly faster, on average, than given by equation (1)
- this initial acceleration is taken into account in the diffusion model developed below
- it turns out to have little effect
- because rare recombination events that break apart the A1B1 haplotype are most likely to occur when the A1B1 haplotype is intermediate in frequency and not when it initially occurs
- comparison to the case of a linked neutral allele
- the dynamics of neutral loci are likely to be affected by the spread nearby of a beneficial allele whenever r is approximately less than sa
- this rule cannot be used to compare to equation (13) directly
- our criteria for being "affected" is now quite strict
- the linked B1 allele must fix due to the sweep
- if they remain associated, deleterious alleles can hitchhike to fixation as an advantageous allele sweeps through the population
- Williamson et al. (2007) found possible evidence of such hitchhiking causing the high prevalence of the hereditary hemochromatosis mutation C282Y, due to a selective sweep occurring 150 kb away from the HFE gene where the deleterious C282Y allele is located
- the hitchhiking of tightly linked deleterious alleles reduces the region in which the sweep is likely to fix surrounding sites
- (compare eq. 15 to eq. 13)
- it implies that deleterious hitchhiking can alter experimental estimates of the strength of such sweeps
- a potential example of these effects was reported by Clegg et al. (1980), who found that linkage disequilibrium in D. melanogaster broke down more quickly than expected (geometric decay at a ratio 1 − r), based on the surrounding markers being neutral and on measured recombination rates between the selected and neutral markers
- this observation could be explained by recombination untangling advantageous alleles from deleterious backgrounds