Restriction analysis had revealed only M u domesticus mtDNA in house mouse populations in northern Denmark, Sweden, and Finland, where the nuclear genomes are Mus mutculus. The goals of the present study were to (1) test the proposal that these Scandinavian musculus mice arose by a series of founder and island-hopping events from one or a few populations near the domesticus-musculus hybrid zone on the East Holstein Peninsula of northern Germany and (2) see whether more than the two types of domesticus mtDNA detected by restriction analysis existed among these musculus populations. Sequences of the 1 -kilobase mtDNA segment encompassing the entire control region and the flanking tRNAs were gathered from 217 domesticus and musculus mice. Included were 104 mice from 12 localities across a 120-km transect of the Holstein hybrid and adjacent zones, 56 animals from 12 localities in the musculus range of Scandinavia, 56 domesticus mice representing 46 localities in Europe, North Africa, the Middle East, and the Americas, and one musculus mouse from Czechoslovakia. Save for the Czech mouse, all individuals had domcslicus mtDNA, a total of 56 types being resolved and 10 of these being found among the Holstein mice. Electrophoresis of proteins encoded by nuclear loci along with assessment of external morphology and of skeletal, cranial, and dental traits revealed a transition from domesticus to musculus nuclear genomes over a distance of about 20-40 km in East Holstein. Ofthe 216 mtDNAs, 149 fall into a clade of 17 types characterized by the addition of an 1 l-bp direct repeat; included are all Swedish, Finnish, and northern Danish mtDNAs as well as 82 from the Holstein transect. Support for the previously proposed migration pathway from the European mainland to Scandinavia came from finding that on the domesticus side of the hybrid zone in Holstein a high proportion of mice carry the types of mtDNA found in the mice 85 0024-4066/93/010085 + 38 SOS.OO/O 0 1993 The Linnean Society of London 86 E.M. PRAGER ETAL.with rnwculw nuclear genomes. Sequencing revealed 1 I different mtDNAs in the + I I-bp clade among rnuscufus mice, one of these being widespread, seven others being confined to single localities, and one Danish collecting locality yielding five types. The mitochondria1 diversity uncovered among these mice leads to a reexamination ofsome aspects of the original model, notably with respect to the number of founding females, the possible role of selection, and the possibility of an accelerated rate of mtDNA evolution. This study also showed that for the mouse control region the transition/ transversion ratio is significantly lower than for the human control region and that small length mutations occur about as frequently as transversions during mouse control region evolution.

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SummaryA total of 357 house mice {Mus domesticus) from 83 localities uniformly distributed throughout Switzerland were screened for the presence of a homogenously staining region (HSR) on chromosome 1. Altogether 47 mice from 11 localities were HSR/ + or HSR/HSR. One sample of 11 individuals all had an HSR/HSR karyotype. Almost all mice with the variant were collected from the Rhone valley (HSR frequency: 61 %) and Val Bregaglia (HSR frequency: 81 %). For samples from most of the area of Switzerland, the HSR was absent. There was no strong association between the geographic distribution of the HSR and the areas of occurrence of metacentrics. However, at Chiggiogna the HSR was found on Rb (13). Possible explanations for the HSR polymorphism are discussed.

The present status of Robertsonian karyotype variation in populations of wild mice from Belgium is presented. Two fusions, Rb(4.12)1Nam and Rb(5.10)3Nam, were identified in the central plain of this flat country. Surrounding this region only mice with the usual 2n = 40 karyotype occurred. From the distribution pattern some possible relationships to other Rb populations from Europe are discussed.