Chromosomal speciation and phenotypic evolution in the house mouse

Chromosomal races of the house mouse (Mus musculus domesticus) in Valtellina and Orobian Alps (Northern Italy) are known for their very fast raciation. Here we present a study using geometric morphometries on size and shape changes in the skull and the mandible of three races (Orobian, Upper Valtellina, Poschiavo) occurring in this area and forming a presumably monophyletic group. One of the races (Upper Valtellina) went extinct recently in an area of sympatry with the Poschiavo race, so that data on genetics (chromosomes and allozymes), behaviour and morphology were available to investigate causes of phenotypic divergence during speciation with a recent extinction event. The evaluation of partial warp scores and the uniform component shows that morphological changes have been fast and that these races can be recognized on the basis of skull shape. Patterns of evolution in shape changes were visualized by combining the chromosomal phylogeny and shape space, summarizing therefore both the phenetic and cladistic relationships. Shape changes follow the cladogenetic sequence depicted by chromosomal fusions. The examination of Procrustes distances shows that the different parts of the skull evolved at different rates after speciation, with shifts in the integration of the various structures (olfactory, auditory, feeding, visualization, etc.). Among the possible causes, aggressive behaviour was advocated for sudden changes in the shape of the skull.