Robertsonian polymorphism in the common shrew (Sorex araneus L.) and selective advantage of heterozygotes indicated by their higher maximum metabolic rates.

Some cases of Robertsonian (Rb) polymorphism in the common shrew (Sorex araneus L.) are believed not to be associated with hybrid zones. One of the hypotheses explaining the persistence of such Rb polymorphism is that they are maintained by some form of selection for Rb heterozygotes. To test this hypothesis, we compared several parameters between homozygotes and Rb heterozygotes for the mp chromosome pair. We used shrews from Jurowce population in Poland, situated within the range of the Białowieza race, where Rb polymorphism persists far from any known hybrid zone. We found no differences between the two karyotypic classes in maximum metabolic rate during running (forced activity). However, the Rb heterozygotes showed significantly higher maximum metabolic rate during swimming (forced activity combined with thermal stress). The levels of fluctuating asymmetry (FA) of homozygous and Rb heterozygous shrews were indistinguishable, indicating no effect of chromosomal heterozygosity on developmental stability of shrews. We suggest that selective advantages, such as the higher metabolic performance in activity combined with cold stress, may outweigh the expected negative effects of Rb heterozygosity upon fertility, and help to maintain huge areas of the Rb polymorphism in this species.