Mitochondrial DNA sequence diversity and the colonization of Scandinavia by house mice from East Holstein

Abstract

Abstract Restriction analysis had revealed only Mus domesticus mtDNA in house mouse populations in northern Denmark, Sweden, and Finland, where the nuclear genomes are Mus musculus. 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 domesticus 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. Of the 216 mtDNAs, 149 fall into a clade of 17 types characterized by the addition of an 11-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 with musculus nuclear genomes. Sequencing revealed 11 different mtDNAs in the +11-bp clade among musculus mice, one of these being widespread, seven others being confined to single localities, and one Danish collecting locality yielding five types. The mitochondrial diversity uncovered among these mice leads to a reexamination of some 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.