Acta Theriol (2011) 56:117–122 DOI 10.1007/s13364-011-0029-x ORIGINAL PAPER A microsatellite study in the Łęgucki Młyn/Popielno hybrid zone reveals no genetic differentiation between two chromosome races of the common shrew (Sorex araneus) Magdalena Moska & Heliodor Wierzbicki & Anna Macierzyńska & Tomasz Strzała & Robert Maślak & Marcin Warchałowski Received: 13 May 2010 / Accepted: 8 November 2010 / Published online: 22 February 2011 # The Author(s) 2011. This article is published with open access at Springerlink.com Abstract This study investigated a chromosome hybrid zone between two chromosomal races of the common shrew (Sorex araneus). Gene flow and genetic structure of the hybrid zone, located in the northeast of Poland, were studied using seven polymorphic autosomal microsatellite loci (L9, L14, L33, L45, L67, L68, L97) and a Y-linked microsatellite locus (L8Y). Seventy-five animals (46 of the Łęgucki Młyn race and 29 of the Popielno race) from nine different localities were examined and the data were analyzed using hierarchical AMOVA and F-statistic. The studied microsatellite loci and races (divided into nine geographical populations) were characterized by observed heterozygosity (HO), expected heterozygosities within (HS), and between (HT) populations, inbreeding coefficient (FIS), fixation index (FST), and average allelic richness (A). We found that genetic structuring within and between the two chromosome races were weak and non-significant. This finding and unconstrained gene flow between the races indicates a high level of migration within the Łęgucki Communicated by: Jan M. Wójcik M. Moska (*) : H. Wierzbicki : A. Macierzyńska : T. Strzała : M. Warchałowski Department of Genetics and Animal Breeding, Wrocław University of Environmental and Life Sciences, Kożuchowska 7, 51-631 Wrocław, Poland e-mail: magdalena.moska@up.wroc.pl R. Maślak Institute of Zoology, Wrocław University, Sienkiewicza 21, 50-335 Wrocław, Poland Młyn/Popielno hybrid zone, suggesting that evolutionarily important genetic structuring does not occur in interracial zones where races which are not genetically distinct come into contact. Keywords DNA polymorphism . Gene flow . Genetic structure . Microsatellites . Sorex araneus Introduction The common shrew (Sorex araneus), a small insectivore, seems to be an ideal object for the study of evolutionary processes, especially to investigate the role of chromo- somes in speciation. It is a unique species, first of all, due to its great chromosome variation (both within and between populations) (Searle and Wójcik 1998). The basis of the quantitative and qualitative variation of the common shrew’s autosomes is chromosome aberrations in which the main cause is believed to be centric fusions (Robertsonian fusions) (Searle and Wójcik 1998). However, the recent study by White et al. (2010) taking into account different types of mutation (fusions, fissions, whole-arm reciprocal translocations, and zonal raciation events) revealed that fusions may be much less important than previously imagined. As an effect of karyotype variation, S. araneus has been divided into almost 70 chromosome races, ten of which are found in Poland (Wójcik et al. 2003; Fedyk et al. 2008). In many cases, the neighboring races created hybrid zones, and several of them have already been described (Searle and Wójcik 1998; Jadwiszczak 2002;