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Background  Few models of genetic hitchhiking in subdivided populations have been developed and the rarity of empirical examples is even more striking. We here provide evidences of genetic hitchhiking in a subdivided population of the marine mussel  Mytilus edulis . In the Bay of Biscay (France), a patch of  M. edulis  populations happens to be separated from its North Sea conspecifics by a wide region occupied only by the sister species  M. galloprovincialis . Although genetic differentiation between the two  M. edulis  regions is largely non-significant at ten marker loci (average F ST ~0.007), a strong genetic differentiation is observed at a single locus (F ST  = 0.25). We validated the outlier status of this locus, and analysed DNA sequence polymorphism in order to identify the nature of the selection responsible for the unusual differentiation.    Results  We first showed that introgression of  M. galloprovincialis  alleles was very weak in both populations and did not significantly affect their differentiation. Secondly, we observed the genetic signature of a selective sweep within both  M. edulis  populations in the form of a star-shaped clade of alleles. This clade was nearly fixed in the North Sea and was segregating at a moderate frequency in the Bay of Biscay, explaining their genetic differentiation. Incomplete fixation reveals that selection was not direct on the locus but that the studied sequence recombined with a positively selected allele at a linked locus while it was on its way to fixation. Finally, using a deterministic model we showed that the wave of advance of a favourable allele at a linked locus, when crossing a strong enough barrier to gene flow, generates a step in neutral allele frequencies comparable to the step observed between the two  M. edulis  populations at the outlier locus. In our case, the position of the barrier is now materialised by a large patch of heterospecific  M. galloprovincialis  populations.    Conclusion  High F ST  outlier loci are usually interpreted as being the consequence of ongoing divergent local adaptation. Combining models and data we show that among-population differentiation can also dramatically increase following a selective sweep in a structured population. Our study illustrates how a striking geographical pattern of neutral diversity can emerge from past indirect hitchhiking selection in a structured population.    Note  Nucleotide sequences reported in this paper are available in the GenBank™ database under the accession numbers  EU684165  –  EU684228 .

Genetic hitchhiking in a subdivided population of Mytilus edulis