epistatic selection

Chevin L-M, Bastide H, Montchamp-Moreau C & Hospital F 2009 Molecular signature of epistatic selection: interrogating genetic interactions in the sex-ratio meiotic drive of Drosophila simulans. Genet Res 91:171-182.

  • the linkage disequilibrium (LD) within each of the candidate regions, and also most notably between them, is very strong
  • this feature may have emerged as a result of positive epistasis between the drive loci included in each region
  • in spite of this (putatively strong epistatic) selection involving two loci only 1 cM apart, the diversity at markers located in between the two regions is high
  • Kim & Stephan (2003) showed that selective sweeps at two closely linked loci have on average less than additive effects on the reduction of heterozygosity at a neutral locus
  • Chevin et al. (2008) further showed that the fine-scale polymorphism pattern around two partially linked loci with independent (multiplicative) effects on fitness can exhibit a spike in diversity in the interval delimited by the selected loci
  • the pattern may be different if there is epistasis between the loci under selection
  • in a region where several genes have been identified as candidates for selection on a phenotype, the polymorphism pattern can provide more information than the simple presence/absence of selection
  • to study the influence of selection and epistasis on the polymorphism pattern along the recombining region of the X chromosome that includes the SR1 and SR2 distorter loci, we used forward individual-based stochastic simulations
  • we chose to study two realistic cases of interest
  • the first case is obligate interaction of the meiotic drive elements, whereby none has an effect of its own (k1 = 0⋅5 and k2 = 0⋅5), as observed in the lab
  • in the second case, SR1 is a meiotic drive locus, whose effect is possibly enhanced by SR2 (otherwise neutral)
  • in this scenario, we chose k1 = 0⋅75 (and still k2 = 0⋅5)
  • this other scenario is consistent with the fact that many meiotic drive systems are thought to evolve by recruiting interacting elements at linked loci during their spread in a population
  • several values of the epistasis parameter e were simulated
  • from e = 0⋅33 to e = 0⋅89
  • corresponding under obligate interaction to k12 = 0⋅6 and k12 = 0⋅9, respectively
  • the presence of several different haplotypes carrying the SR2 mutation at the beginning of the sweep makes the selective sweep at SR2 similar to a 'soft sweep' from the standing genetic variation
  • contrary to the usual 'soft sweep' models, selection of the SR2 allele is controlled by its genetic background rather than by the environment
  • only the copies of SR2 associated with a SR1 allele become selected, while the others remain effectively neutral