Morphological clines across a karyotypic zone of house mice in Central Italy

Multivariate morphometric differentiation between chromosomal races of the mouse Mus domesticus in Central Italy was investigated using a population of 2n = 22 “CB ” karyotype, three populations of standard 2n = 40 karyotype, five populations of 2n = 22 “CD ” karyotype and three populations from the hybrid zone between the latter two karyotypes. Whilst populations of different karyotype generally have significantly different morphometry, canonical analysis does not reveal that the populations ordinate into distinct aggregations based on karyotype, largely because the 2n = 22 “CD ” populations are so diverse. Nevertheless, canon-ical analysis does reveal a significant cline in morphology across the contact zone between the 2n = 40 and 2n = 22 “CD ” mice. The nature of this transition, i. e. a cline 1. within the 2n = 40 range, 2. within the hybrid range (but unrelated to chromosome number) and 3. within the 2n = 22 “CD ” range, tends to indicate that the morphometric divergence is due to adaptation to the different ecological regimes across which these mice are distributed rather than the phylogenetic divergence of the karyotypic races