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The purpose of this communication is to revise and clarify the position regarding house mouse populations with Robertsonian chromosomes in the northern Scottish county of Caithness.Several years ago Adolph and Klein (1981) described three Robertsonian translocations in nine mice caught near Castletown in the north of Caithness. All were homozygous for Rb(4.1O) and Rb(9.12); eight were homozygous for Rb(6.13), the other was heterozygous for this fusion. Hence, eight of the mice had a diploid chromosome number of 34 (2n = 34), the other was 2n = 35. Shortly afterwards Brooker (1982) confirmed this finding and reported also 12 other Robertsonian translocations in 31 Caithness house mice from 18 locations in the county.

The house mice, Mus musculus, of N. F. Scotland and some of the neighbouring Orkney islands carry Robertsonian translocations (centromeric fusions). By comparing the karyotypes, mandible morphology and biochemical variation of samples from four of these populations we suggest that despite chromosonial differences the populations are closely related. Available evidence suggests that the mice may have arrived with the Vikings as early as 600 A.D. and that the chromosomal changes have occurred since then. We found no evidence that the normal, 2n = 40, mice are, by other measures, different from those with Robertsonian translocations. The evidence suggests that these populations have been isolated for a considerable period of time during which there has been a marked reduction of polymorphism and heterozygosity in the island samples, which we tentatively suggest may explain differences observed in the chromosomal evolution of the four populations.

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