House mice are believed to be, unlike rats, only reluctant swimmers. If water is a barrier to mouse dispersal water bodies and streams can have a substantial impact on the genetic structure of populations. Previous studies revealed influence of rivers on the position and structure of the European hybrid zone between two house mouse subspecies, Mus musculus musculus and M. m. domesticus. In this study, we used a simple motivation experiment to test the disposition of both wild and inbred mice representing the subspecies to overcome a water barrier. As the more dispersing sex, males were chosen for the experiment. Mice were tested under two air/water temperature regimes, 20 °C and 10 °C. Contrary to a common belief tested animals entered water rather easily, often even engaging in repeated swimming. We found significant differences in scored behavioural parameters between the subspecies. Under the 20 °C regime, both wild and inbred domesticus males entered and crossed the water earlier and more often swam even when satiated. Strikingly, under the 10 °C regime, the results were rather equivocal but with the opposite tendencies, with musculus males being more willing to swim. We discuss implications of these findings for the hybrid zone dynamics.
Despite the long-term study of the house mouse hybrid zone in Europe knowledge of its course in some areas is still rather vague. Comparisons of different portions of the zone showed some common patterns, however, several discordances were also revealed, the most remarkable being introgression of the Y chromosome. We sampled mice along the presumed course of the secondary contact zone between two subspecies, Mus musculus musculus and M. m. domesticus, from Schleswig-Holstein to southern Bavaria, in order to localize more precisely its position. A second aim was to reveal whether introgression shows some general rules obscured until now by studies of geographically isolated transects of the zone. We employed maternally (mtDNA), paternally (Y), and biparentally inherited markers and related their introgression patterns to the hybrid index (HI) based on five X-linked loci. While transition of autosomal loci across the zone was congruent with changes in HI, mtDNA showed bidirectional introgression with alien alleles occurring far behind the zone. Finally, the Y chromosome displayed asymmetric unidirectional introgression of the musculus type into domesticus background. We discuss evolutionary forces shaping these patterns.