Do riverine barriers, history or introgression shape the genetic structuring of a common shrew (Sorex araneus) population?

The common shrew (Sosex araneus) is subdivided into numerous chromosome rac es. The Valais and Cordon chromosome races meet and hybridize at a mountain river in Les Houches (French Alps). Significant genetic structuring was re cently reported among populations found on the Valais side of this hybrid z one. In this paper, a phylogenetic analysis and partial Mantel tests are us ed to investigate the patterns and causes of this structuring. A total of 1 85 shrews were trapped at 12 localities. All individuals were typed for nin e microsatellite loci. Although several mountain rivers are found in the st udy area, riverine barriers do not have a significant influence on gene flo w. Partial Mantel tests show that our result is caused by the influence of the hybrid zone with the Cordon race. The geographical patterns of this str ucturing are discussed in the context of the contact zone, which appears to extend up to a group of two rivers. The glacier they originate from is kno wn to have cut the Arve valley as recently as 1818. The recent history of t his glacier, its moraine and possibly rivers, may therefore be linked to th e history of this hybrid zone.