Hybrid Zone between the Sok and Serov Chromosomal Races of the Common Shrew Sorex araneus (Lipotyphla, Mammalia) in European Russia

Abstract

Analysis of the frequencies of karyotypic variants in the hybrid zone between the Sok and Serov chromosomal races of the common shrew Sorex araneus revealed that the zone has a complex configuration. A large part of the contact zone is located in a monotonous mid taiga zone, where there are no significant barriers to limit dispersion of shrews. Another part of the zone runs along the Pechora River (Komi Republic). Analysis of distribution of the diagnostic chromosomes on a transect along the Pechora River showed that the hybrid zone could be described by opposing metacentric clines. Individuals of both pure Sok and Serov races, as well as F1 hybrids, were found only on the left bank of the Pechora River. The right bank was occupied only by the Serov race, and F1 hybrids were rare there. The observed number of interracial hybrids in the center of the zone does not differ from that expected in the case of random breeding. This allows us to define this zone as unimodal. The standard width of the zone is about 40 km. This is considerably higher than all known cases of hybrid zones with the same level of karyotypic differences between parental races: two pairs of diagnostic metacentric chromosomes with monobrachial homology. We discuss possible causes of this phenomenon.