118 Moska 2003). In hybrid zones, detailed analysis has been done of either modifications enabling gene flow between neighboring races (acrocentric peak, recombinant peak, or elimination of acrocentrics from the zone) or factors responsible for the formation of isolation barriers between races (Searle and Wójcik 1998; Fredga and Narain 2000; Moska 2003). Different molecular markers have been used in studies of the evolutionary processes of S. araneus (allozymes, mtDNA, microsatellites), of which microsatellites have been found a sensitive measure to detect patterns of hybridization. Up to now, the presence of over 40 highly polymorphic autosomal microsatellite loci and one polymorphic Y-chromosome microsatellite (L8Y) of this species have been reported (Wyttenbach et al. 1999; Balloux et al. 2000; Lugon-Moulin et al. 2000; Brünner et al. 2002a, b; Lugon-Moulin and Hausser 2002; Andersson 2004; Basset et al. 2006). L8Y provides an opportunity for investigating the level of male gene flow between populations and comparing it with the gene flow of both sexes (Lugon-Moulin and Hausser 2002). The aim of the present study was to analyze the level of genetic differentiation in the Łęgucki Młyn/Popielno hybrid zone using microsatellite markers. Moreover, by analyzing the genetic structure within and between chromosome races, we tried to contribute to efforts of elucidating the role of interracial hybrid zones in the evolutionary process of speciation. Acta Theriol (2011) 56:117–122 Material and methods Study area The hybrid zone between the Łęgucki Młyn and Popielno karyotypic races is located in the northeast of Poland (Fig. 1). The Łęgucki Młyn race (Łg) is characterized by chromo- somes: g/r, h/k, i/o, j/l, m/n, p, q while the Popielno race (Po) by: g/r, h/q, i/k, j/l, m/n, o, p (Wójcik 1986; Moska 2003). Shrews were collected over the period 1996–1999 from nine sites: Prawdowo (P), Lisunie-Kulinowo (LK), Lipowo (L), Nowy Most (NM), Krutyń I (KI), Krutyń II (KII), Zielony Lasek (ZL), Zakręt (Z), and Karwica Mazurska (KM) (Fig.1). Sites P, LK, L, NM, and ZL were situated in the area of the Łęgucki Młyn race (southern side of the hybrid zone) and KI, KII, Z, and KM in the area of the Popielno race (northern side of the hybrid zone) (Moska 2003). Sampling and molecular methods For the microsatellite analysis, 75 individuals of two races were used, 46 of the Łęgucki Młyn race and 29 of the Popielno race. Seven polymorphic autosomal microsatel- lites (L9, L14, L33, L45, L67, L68, L97) and a Y-linked microsatellite (L8Y) were used for the analysis. Amplifica- tion conditions for each locus were carried out according to Balloux et al. (1998) and Lugon-Moulin et al. (2000). In order to improve estimation of the size of the amplification Fig. 1 The location of the sam- pling sites in the Łęgucki Młyn/ Popielno hybrid zone: P Praw- dowo (53°48′N, 21°32′E), LK Lisunie-Kulinowo (53°46′N, 21°33′E), L Lipowo (53°47′N, 21°25′E), NM Nowy Most (53°44′N, 21°31′E), KI Krutyń I (53°42′N, 21°26′E), KII Krutyń II (53°41′N, 21°26′E), ZL Zielony Lasek (53°40′N, 21°27′E), Z Zakręt (53°39′N, 21°26′E), KM Karwica Mazurska (53°37′N, 21°27′E)