The complex hybrid zone between the Abisko and Sidensjö chromosome races of Sorex araneus in Sweden

Six chromosome races of the common shrew occur in Sweden, each with its characteristic arm combination of metacentric chromosomes. G-banded karyotypes were analysed from 201 common shrews in 14 localities of the northern hybrid zone in Sweden. Analyses from another 64 shrews from seven localities outside the hybrid zone w ere included for comparison. The shrews were classified with respect to karyotype into any of five categories: (1) Abisko race, (2) Sidensjö race, (3) hybrids between the parental races, (4) pseudohybrids (a type of hybrid), and (5) AT with all race-specific chromosomes (h, i, n, o, p, r) present as telocentrics. Hybrids occurred at a frequency close to Hardy-Weinberg expectation in the centre of the hybrid zone. Chromosome polymorphism of Robertsonian type was common and 43 different karyotypes were found among the specimens studied. The polymorphism involved six metacentric pairs in the Abisko and three in the Sidensjö race. The frequency of the Sidensjö race-specific metacentric hi decreased and the frequency of the Abisko race-specific hn increased from south-west to north-east along a transect across the hybrid zone. The number of race-specific telocentrics reached a peak 13 km north-east of the hi-hn cline centre. The estimated standardized cline width for chromosomes hi and hn was 16.0 km. The extension of the Sidensjö race is comparatively narrow [c. 50 km in the region of the investigation), and it is regarded to be a 'hybrid race' between the Uppsala race, which colonized Sweden from the south-west, and the Abisko race which arrived from the north-east after the most recent glaciation. The origin of the Sidensjö race is thus less than 10 000 years old, because earlier this area was covered by glacial ice.