Properties of a hybrid zone between highly distinct chromosomal races of the house mouse (Mus musculus domesticus) in northern Italy, and comparisons with other hybrid zones

Here we provide the first detailed description of the hybrid zone between the Cremona chromosomal race of house mouse (ICRE; 2n = 22) and the standard all-telocentric race (40ST; 2n = 40), with full karyotypes of 106 individuals from 17 localities along a transect between the 2 races to the west of Lake Garda in Northern Italy. The ICRE race is characterised by 9 pairs of metacentric chromosomes in a homozygous state and we use the metacentric frequency data along the transect to fit tanh metacentric clines. The clines are narrow (5-8 km, standardised width) suggesting low hybrid fitness. However, the lack of occurrence of ICRE × 40ST F(1) hybrids and presence of other hybrid types suggests that the F(1) hybrids initially produced in this hybrid zone were at least partially fertile, despite having 9 meiotic trivalent configurations. We apply the same cline-fitting methodology to 3 previously studied hybrid zones between metacentric races and the 40ST race. Taken together with published clinal data on 4 further metacentric-40ST hybrid zones, we are able to make objective generalisations on the characteristics of such zones in the house mouse. Zones involving 22-chromosome races are narrower, on average, than other metacentric-40ST hybrid zones and do not show a tendency towards the generation of new races as found with zones where the metacentric race has a higher 2n. It appears that metacentric-40ST zones are unlikely to be sites of speciation (even when a 22-chromosome race is involved), although a mosaic structure to the hybrid zone may enhance this possibility. We make a comparison between metacentric-40ST zones and contacts between 2 metacentric races, for a comprehensive perspective of chromosomal hybrid zones in the house mouse