7.11 8.55 3.33 6.55 4.12 8.33 8.11 10.11 2.75 HO 0.76 0.69 0.65 0.76 0.86 0.79 0.81 0.78 0.64 HE 0.86 0.80 0.75 0.88 0.88 0.88 0.89 0.87 0.81 FIS 0.11 0.14* 0.10* 0.02 −0.12 0.00 0.04 0.03 0.29 0.06* P 0.0681 0.0061 0.0000 0.4851 1.0000 0.5903 0.2649 0.1939 0.3461 0.0027 Table 2 Number of individuals (n) used in the analysis of micro- satellites (number of males is given in brackets), average allelic richness (A), observed (HO) and expected (HE) heterozygosity and inbreeding coefficient (FIS) for each of the geographical populations Locality Number (n) A from 0.57 to 0.86, and over all loci amounted to 0.75. Expected heterozygosities within samples (HS) ranged from 0.68 to 0.83 with an average value of 0.76, whereas over all samples (HT) ranged from 0.73 to 0.94 with the average value of 0.87 (Table 1). The average HO in different geographical populations ranged from 0.64 to 0.86, whereas HE fluctuated from 0.75 to 0.89 (Table 2). Genetic structure of the Łęgucki Młyn/Popielno hybrid zone FIS estimated over all loci revealed significant departure from HWE expectations (FIS =0.063, P< 0.01). The FIS observed in each geographical population ranged from 0.12 (KI) to 0.29 (KM) and departed significantly from HWE in two of them: L/K and L (P<0.01; Table 2). All other geographical populations showed no FIS values significantly different from zero. In the analysis of mean FIS over all loci, a very high and significant heterozygote deficit (FIS=0.13) was observed. FIS per locus over all populations ranged from 0.00 (L68) to 0.28 (L45). Results from the permutation procedure indicated that the FIS values were significant for three loci: L33, L45, and L97 (Table 1). The FST over all geographical populations was small and not significant (FST=0.02; Table 1). Locus-specific FST ranged from 0.008 (L68) to 0.038 (L97), and significant genetic differentiation was found only at locus L97 (Table 1). The mean FST was also calculated for both races separately. The Łęgucki Młyn race (populations P, L/K, L, NM, ZL) showed FST of 0.027, whereas the Popielno race showed FST of 0.003. For both races values of FST were small and non-significant (Table 1). The AMOVA analyses based on FST showed a lack of genetic differentiation between chromosomal races, both for autosomal microsatellite loci and for the Y-locus. For 120 P LK L NM KI K II ZL Z KM Total *P<0.01 10 (5) 15 (8) 3 (2) 7 (3) 10 4 (3) 11 (3) 13 (8) 2 75 (32) Acta Theriol (2011) 56:117–122 the autosomal loci, 0% (−0.26%) of the total variation was caused by variation between races, whereas for L8Y it was 3.03% of the total variation. Most of the variation was found within geographical populations as regards to both autosomal loci and L8Y (99% and 90%, respectively). For all genetic markers low and non-significant variation was found among geographical populations within the races. Discussion So far, detailed studies using DNA microsatellites have been carried out in three hybrid zones: two located in Sweden (Uppsala/Hällefors and Abisko/Sidensjö) (Wyttenbach et al. 1999; Andersson 2004), and one located in Poland (Drnholec/Białowieża) (Jadwiszczak et al. 2006). The Abisko/Sidensjö and Drnholec/Białowieża hybrid zones are the ones where hybridization of karyotypically diver- gent chromosome races takes place (races belong to distinct karyotypic groups: NEKG and WEKG, WEKG and EEKG, respectively) (Fredga 1996; Jadwiszczak 2002). The analysis of genetic differentiation revealed weak genetic structuring and in consequence unimpeded gene flow between the races. However, in the Abisko/ Sidensjö hybrid zone, significant reduction of male gene flow was found when the genetic marker linked to Y-chromosome (L8Y) was studied (Andersson 2004; Jadwiszczak et al. 2006). In the Uppsala/Hällefors hybrid zone, both races are not chromosomally distinct (they belong to the same evolutionary group, WEKG) (Narain and Fredga 1996). Based on genetic differentiation of autosomal microsatellite loci, weak genetic structuring over all localities was found (Wyttenbach et al. 1999). A comparison of the studied hybrid zones leads to the conclusion that the Łęgucki Młyn/Popielno hybrid zone strongly resembles the Uppsala/Hällefors hybrid zone. These two races (Łg and Po) are chromosomally quite closely related (both races represent EEKG) and in the hybrid zone complex heterozygotes are theoretically expected to form only short chains (CIV or CV) during meiosis. Molecular analysis of seven autosomal loci did not reveal significant differentiation between the Łęgucki Młyn and Popielno chromosomal races. The FST values were small and not significant, either over all geographical populations or within the races. Our results were comparable with the weak genetic structuring observed in the similar Uppsala/Hällefors hybrid zone. When we analyzed the level of genetic structure with regard to the Y-chromosome microsatellite, we found a level ten times higher compared to autosomal loci (FST=0.10). Our findings are comparable to results previously reported by different authors who observed differences in the estimates