Robertsonian chromosomal variation in the common shrew Sorex araneus L.
Three British Robertsonian chromosome races of common shrew, which are distinguished by different metacentric chromosomes composed of arms k, n, o, p, q, r, were found: the 'Aberdeen' race (northeastern Scotland: ko, np, qr), the 'Oxford' race (central and northern Britain: kq, no, pr) and the 'Hermitage' race (southern England: ko only). Cladistic analysis demonstrated a closer karyotypic relationship of these races to each other and to races closely located in Europe than to races found elsewhere in Eurasia. The three British races were successfully intercrossed in captivity and hybrids between the Oxford and Hermitage races were found in nature, near Oxford. A morphological and electrophoretic comparison of Oxford and Hermitage race shrews from the Oxford area and Aberdeen race shrews from the Aberdeen area did not reveal any diagnostic characteristics for any race. However, there were notable differences between the two areas in allele frequencies at three isoenzyme loci, which on the basis of preliminary evidence, could be a function of racial identity. A Robertsonian polymorphism of five arm combinations in the Oxford area was still present some 25 years after it was first discovered by Ford and coworkers. Foetal and meiotic studies on Robertsonian heterozygotes from this area of polymorphism indicate that a stable trivalent is formed at meiotic prophase I and that rates of anaphase I nondisjunction are low. The polymorphism appears to be the consequence of the hybrid zone between the Oxford and Hermitage races. Where those hybrids which are Robertsonian heterozygotes with monobrachial homology occur the frequency of race-specific metacentrics is low, presumably due to selection against the hybrids. Away from the area occupied by the hybrids race-specific metacentrics increase in frequency, presumably because of selection in their favour. Some evidence suggests that preferential transmission may operate as such a selective force.