Chromosome rearrangements do not prevent gene flow in the common shrew, Sorex araneus : Analysis of two chromosomal hybrid zones in Sweden

Abstract: Two chromosomal hybrid zones of the common shrew, Sorex araneus, have been investigated: the Hällefors-Uppsala hybrid zone in central Sweden and the Abisko-Sidensjö hybrid zone in northern Sweden. The Hällefors (Hä) and Uppsala (Up) chromosome races consisted of biarmed autosomes only (2nA=18), and differed in the Robertsonian fusions of two arms, ko, pq, and kp, oq, respectively. The Abisko (Ai) and Sidensjö (Si) races consisted of biarmed and uniarmed autosomes, and are mainly characterized by the fusions hn and hi, respectively. Robertsonian polymorphism was present in both races, affecting chromosomes g/m, h/n, i/p, j/l, kq, and j/l, k/q, n/r, respectively. The width of the Hällefors-Uppsala hybrid zone was 2.7 km and the center of the zone followed the Kolbäck river. The Abisko-Sidensjö hybrid zone had a width of 15.2 km in the area studied.In the center of the Hällefors-Uppsala hybrid zone, one type of "complex" heterozygotes occured that formed a ring of four elements during meiosis. In the Abisko-Sidensjö hybrid zone, "simple" and "complex" heterozygotes ocurred which formed trivalents and/or a chain configuration of four/six elements during meiosis, respectively. There is no evidence from fertility studies that heterozygotes are less fertile than homozygotes, and the minor differences found in certain reproductive characters might be due to different genetic background rather than to chromosomal rearrangements. Microsatellite data demonstrated that gene flow occurs across the Hällefors-Uppsala hybrid zone, which is in agreement with the results obtained from the fertility analysis.