Maximization of the frequency of recombinants in the hybrid zones of Sorex araneus in northern Poland

We studied populations of Sorex araneus Linnaeus, 1758 in the upper Drwęca River valley and in the vicinity of Lake Szeląg Wielki. We found that the cline of frequency of metacentric np decays in the Drwęca River valley, which results in a smooth transition between the Stobnica race (karyotype hi, ko, gm, np) and the Laska race (hi, ko, gm). However, on the west shore of Lake Szeląg Wielki the cline of frequency of metacentric gm disappears. Reduction of frequencies of autosomes np and gm makes possible broadening of the ciines of metacentrics gr and mn, originating from the Łęgucki Mtyn race. This results in a reduction of the number of hybrids and maximization of frequency of recombinants (hi, ko, gr, mn), which reaches the value of 1.0 in the populations located on the west shore of the lake. We compared studied populations with those in the Pasłęka River valley, and those between the Pasłęka and Drwęca Rivers valleys. We also discussed the maximization of the frequency of recombinants as a mechanism enhancing fertility of hybrid populations.