A cloned ^-specific sequence (Bishop et al. 1985) was used as a diagnostic probe to distinguish between Mus musculus domesticus and Mus musculus musculus F-chromosomes. Analysis of the RFLPs obtained with genomic DNA isolated from wild mice caught along the contact zone between M. m. domesticus and M. m. musculus in Bulgaria and Denmark showed that the y-chromosome flow between the two semi-species is very limited. The degree of y-chromosome penetration was compared with that of seven diagnostic autosomal loci and the mitochondrial DNA. Breeding experiments showed that the lack of y-chromosome introgression from one semispecies to the other was not due to a major hybrid breakdown. The results suggest that the disruption of differentiated co-adapted gene systems could play a role in limiting y-introgression.

A palaeontological and archaeozoological survey has allowed us to establish the different steps in the colonization of western Eurasia and northern Africa by the house mouse Mus rnusculus. After successive immigration waves of the genus Mus into this zone from the late Pliocene to the upper Pleistocene, the house mouse appeared and remained confined to the easternmost Mediterranean Basin at the uppermost Pleistocene. The first progression of this species into the Mediterranean Basin occurred in the Middle East from the Epipaleolithic to the Neolithic. Subsequently, this species was found in the western Mediterranean Basin during the Bronze Age and in north-west Europe during the Iron Age. In comparison to this latter zone, north central Europe was colonized relatively early, from the Neolithic to the Bronze Age which may, in fact, not only correspond to a much earlier invasion of Europe by M. musculus musculus but also suggest that the distribution of this subspecies extended much further west than it does nowadays, at a time when M . musculus domesticus was restricted to the Mediterranean zone. This archaeological survey is in agreement with genetic data which provide indications as to the speed, steps and pathways of progression of house mouse populations in western Eurasia.

SummaryOne hundred and ninety-eight mice trapped along a south–north transect through the Danish hybrid zone between Mus musculus domesticus and M. m. musculus were typed for mitochondrial DNA (mtDNA), the Y chromosome and ten autosomal loci encoding diagnostic proteins. The southern (domesticus) populations display two mtDNA variants (S1 and S2) and the northern (musculus) have a third mtDNA variant (N) of domesticus origin. Across the hybrid zone defined by ten autosomal loci, there is a steep dine between the southern and northern types of mtDNA. As well as confirming an earlier finding that Danish musculus all have a domesticus mtDNA (Ferris et al. 1983a, & b), our results show that this mtDNA takeover is not the result of a persistent mitochondrial gene flow between the two subspecies. While the coincident dines for the ten autosomal loci and the abrupt dine for the Y chromosome can be explained by selection, it is less likely to be the case for the mtDNA exchanges. We discuss the possible role of sex-linked migration and genetic drift to account for the distribution of the mitochondrial variants.