GENETIC AND KARYOTYPIC STRUCTURE OF A HYBRID ZONE BETWEEN THE CHROMOSOMAL RACES CORDON AND VALAIS IN THE COMMON SHREW, SOREX-ARANEUS

Hybrid zones between genetically differentiated populations provide ma terial to study evolutionary processes. Since the discovery of chromos omal races in Sorex araneus, contact zones have attracted attention of scientists. So far, studies on genetic markers in Sorex hybrid zones are missing. The acrocentric chromosomal race Cordon and the highly me tacentric race Valais meet and hybridize at Les Houches in the Western Alps. On a transect through the hybrid zone, 273 shrews were caught a t 15 localities over 4 years. Karyotype as well as the nuclear protein loci Alb and Pg were analyzed. F-st and F-is values were calculated b y F-statistics. An analysis on pooled samples revealed the genetical d ifferences between the hybridizing races as the only cause of populati on structuring. Genetical markers show dines with very strong frequenc y shifts at a mountain torrent, but behave differently through the zon e. The performance of the torrent in maintaining the hybrid zone, sele ction against hybrids, possible assortative mating and linkage of the Valais Pg allele to a diagnostic chromosome arm, are discussed.