New metacentric population of the house mouse (Mus musculus domesticus) found in Valchiavenna, Northern Italy.

Although the standard karyotype of the western house mouse (Mus musculus domesticus) consists entirely of telocentric chromosomes, there are over 100 populations across western Europe and North Africa characterized by different sets of metacentrics formed by Robertsonian fusions and whole-arm reciprocal translocations. Here we report the discovery of a new metacentric population from Valchiavenna, northern Italy, that we have named the 'Lower Valchiavenna population' (abbreviated as ILVC). This metacentric population is found in villages and on farms over a 10-kilometer stretch comprising the southern half of Valchiavenna. ILVC is characterized by the metacentrics 1.18, 2.4, 3.8, 5.15, 6.7, 9.14, 10.12, 11.13 and 16.17, and appears to be closely related to the Chiasso population (CHCH), which possesses the same set of metacentrics except 1.18. We discuss the evolutionary origin of ILVC in relation to human occupation of the region. We also suggest that the geographical position of ILVC between 2 other metacentric populations with entirely different sets of metacentrics (Chiavenna, ICHI, and lower Valtellina, ILVA) may provide 2 additional chromosomal hybrid zones for the study of speciation.