Genetic structuring and gene flow in a hybrid zone between two chromosome races of the common shrew (Sorex araneus, Insectivora) revealed by microsatellites

Hybrid zones between chromosome races are of interest because they offer the opportunity to investigate the role of chromosomes in speciation processes. Here we study gene flow and fine-scale genetic structure of a chromosome hybrid zone in the common shrew (Sorex araneus, Insectivora, Mammalia) in central Sweden using six polymorphic microsatellite loci. The centre of the hybrid zone closely follows a river towards which race-specific chromosomes sharply decrease suggesting a physical barrier to introgression. We examined 285 animals from 22 different localities and performed a hierarchical analysis for F and R statistics. Genetic structuring within and between the two chromosome races (Hallefors and Uppsala) was found to be weak, but significant. No positive relationship between geographical and genetic distance was detected, suggesting the absence of isolation by distance. Gene flow between the races is not reduced. Therefore, it does not seem that the river is an important barrier reducing introgression between races. The fact that the width of the hybrid zone is narrow and that its centre follows the river despite high levels of migration is discussed.