Further studies of a staggered hybrid zone in Mus musculus domesticus (the house mouse)

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Abstract
In the extreme north-east of Scotland (near the village of John o'Groats) there is a small karyotypic race of house mouse (2n=32), characterized by four metacentric chromosomes 4.10, 9.12, 6.13 and 11.14. We present new data on the hybrid zone between this form and the standard race (2n=40) and show an association between race and habitat. In a transect south of John o'Groats we demonstrate that the clines for arm combinations 4.10 and 9.12 are staggered relative to the clines for 6.13 and 11.14, confirming previous data collected along an east-west transect (Searle, 1991). There are populations within the John o'Groats-standard hybrid zone dominated by individuals with 36 chromosomes (homozygous for 4.10 and 9.12), which may represent a novel karyotypic form that has arisen within the zone. Alternatively the type with 36 chromosomes may have been the progenitor of the John o'Groats race. Additional cytogenetic interest is provided by the occurrence of a homogeneous staining region on one or both copies of chromosome 1 in some mice from the zone.