Genetic diversity, stability of population structure and barriers to gene flow in a hybrid zone between two Sorex araneus chromosome races

We investigated the population genetic structure in the hybrid zone between the Guzowy Mlyn (Gu) and Legucki Mlyn (Lg) chromosome races of the common shrew in northern Poland, using 31 allozyme loci. Eight polymorphic allozyme markers showed significant genetic structuring among the 12 populations under study. Pairwise genetic differentiation (F-ST) between populations ranged from 0.000 to 0.129 (average 0.021) and was statistically significant (P < 0.01). Mantel tests revealed that the effect of geographic distance on gene flow was not important (R-2 = -0.21, ns). There were no significant differences in allele frequencies between temporal samples from a given population. The examination of principal component scores (PCA) derived from allelic frequencies among populations suggests that disturbed habitats (in particular, a town) represent a barrier to gene flow. This means anthropogenic changes to landscape structures might promote genetic differentiation among hybridising shrew populations. We conclude that in the studied hybrid zone landscape structure may have stronger effect on gene flow than chromosomal incompatibilities.