Acta Theriol (2011) 56:117–122 of genetic structure based on the Y-microsatellite variation compared to the autosomal microsatellite (Andersson et al. 2004; Balloux et al. 2000). In our study, FIS values over all loci and over all populations were high and significantly different from zero (FIS = 0.13 and FIS = 0.06, respectively). A closer look at FIS for each geographical population revealed that populations LK and L showed a high and significant heterozygote deficit (FIS = 0.14 and FIS = 0.10, respectively; P < 0.01). The high FIS may be partly cased by the Wahlund effect (White and Searle 2007). In one geographical population (LK), shrews were caught on two meadows separated by a busy road, so a deficit of heterozygotes could be caused by pooling independent populations in HWE. Previous studies have shown no indications of alleles at any of the loci used in this study, indicating that high FIS was probably not caused by presence of alleles (Wyttenbach and Hausser 1996; Wyttenbach et al. 1999; White and Searle 2007). However, the analysis of the data used in our study with the use of Micro-Checker software identified alleles in three loci (L33, L45, and L97) where a significant (P < 0.01) inbreeding coefficient was estimated. This is probably the most reasonable explanation of the lack of heterozygosity in those loci. The deficit of heterozygotes observed in our study may be also explained by the small sample sizes (particularly from populations L, KII, and KM, where samples of fewer than five individuals were taken). In conclusion, the results described in this paper give insight into the speciation process which may take place in hybrid zones. Our study, with the use of microsatellite markers, did not reveal significant differences between the genetically close Łęgucki Młyn and Popielno chro- mosomal races, which was proven by small chromosomal differentiation. It seems that the evolutionarily important genetic structuring does not occur in interracial zones, where races which are not genetically distinct come into contact. Acknowledgements The field assistance of Professor S. Fedyk and Dr. W. Chętnicki is gratefully acknowledged. We wish to thank M. Kowalska and A. Jonkisz for their laboratory help. Thanks also go to anonymous referees for their useful comments. Declaration concerning ethical standards Our experiments comply with all current laws in Poland. Conflict of interest The authors declare that they have no conflict of interest. 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