| Summary: | <p>Throughout the range of the common shrew (<em>Sorex araneus</em> Linnaeus 1767), repeated Robertsonian fusion mutations have led to a karyotypic polymorphism dividing the species into chromosome races. Studies of fertility were undertaken in the male, both of homozygotes and of heterozygotes forming meiotic multivalents of varying complexity.</p> <p>Observations made at pachytene, diakinesis/metaphase I and metaphase II did not provide evidence for fertility impairment in homozygotes or simple heterozygotes. Males forming a chain of seven chromosomes during meiotic prophase I were produced through a program of captive breeding, and were brought to premature sexual maturity through photoperiod manipulation. In these more complex heterozygotes, incomplete pachytene pairing was frequently observed (68% of cells), germ cell death was found to be elevated (23%), and data from analysis of metaphase II spreads are indicative of an increase in nondisjunction above background of approximately 10%.</p> <p>At the interface between two chromosome races, a complex of clines of chromosome frequency can be found. Field studies determined the routes and widths of four such clines (<em>ko</em>, 8.6km; <em>kg</em>, 28.4km; <em>no</em>, 37.1km; <em>pr</em>, 47.9km), and concluded that the courses taken are entirely consistent with maintenance through a mechanism of heterozygote disadvantage. Estimates of disadvantage made from cline width were in close agreement with those derived from the fertility studies.</p>
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