House mice Mus musculus domesticus (Schwartz and Schwartz, 1943) from Orkney are closely related on the basis of mandible morphology and allozyme variation. In three of the twenty islands where house mice occur populations have diverged from the standard 2n = 40 karyotype through fixation of Robertsonian (Rb) chromosomal mutations. Mice from Westray island carry 36 chromosomes and share one Rb fusion with those from Eday and Faray (2n = 34; 2n = 34-36). These islands are geographically very close to each other and to islands carrying standard mice. Behavioural and ecophysiological analyses were performed on the three chromosomally divergent populations of mice as well as on two nearby standard populations (Sanday and Papa-Westray). The aim of the study was to assess -whether divergences attributable to chromosomal changes occur, and to discuss whether behavioural divergences may explain the non-mixing of karyotypes. The study does not reveal any divergence between islands attributable to karyotype or habitat differences. The non-blending of the different karyotypes present in Orkney is discussed with reference to history of colonisation, human behaviour, and the consequences of site saturation. Particular characteristics displayed by the different island-populations suggest that in the near future the present pattern of karyotype variation observed in Orkney could change.
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