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ABSTRACT. We provide the first description of a hybrid zone between chromosome races of Sorex araneus in Central European Russia. In the area near Lake Seliger (Upper Volga), the locally-distributed Seliger race (diagnostic chromosomes hn, ik, mq, pr, g, o) meets and hybridises with the more widespread Moscow race (gm, hi, kr, no, pq). This hybrid zone appears to be narrow and constrained to the inter-lake isthmus between Lake Seliger and the River Volga system. The hybrid zone may be centred on a minor road. Hybrids occur between the two races but at low frequency. There were three F1 hybrids out of a total of 87 specimens. These were expected to form the maximally long meiotic chain that can be found in the common shrew: a chain-of-eleven (CXI). Such a configuration was observed in meiotic preparations from one adult male F1. Two other complex heterozygote karyotypes were found. The existence of these backcross individuals reflects Robertsonian polymorphism for certain race-specific chromosomes. It is presumed that these various hybrids have reduced fertility, consistent with the narrowness of the hybrid zone and bimodal structure. This is likely to be a “strong” hybrid zone with a greater probability of reduced gene flow and evolution of reproductive isolation than other hybrid zones in the common shrew.
KEY WORDS: chromosome race, hybrid zone, Robertsonian fusion, Sorex araneus.
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