Clinal analysis of a chromosomal hybrid zone in the house mouse

These studies centre on the 'Barcelona' karyotypic race of the western hous e mouse (Mus musculus domesticus), first described by Adolph & Klein (1981) . This is one of many races within M. m. domesticus characterized by metace ntric chromosomes that have originated by repeated Robertsonian fusions, wi th perhaps further modification by whole-arm reciprocal translocations. Dat a on 111 mice from 20 sites show that the race is centred 24 km to the west of Barcelona city and has a homozygous metacentric karyotype of 2n = 28 (3 .8, 4.14, 5.15, 6.10, 9.11, 12.13). The race has a small range, and mice wi th the standard 40-acrocentric karyotype were caught only 30 km from the ra ce centre. Throughout the area of occurrence of metacentrics there is polym orphism (i.e. presence of acrocentrics in the population), although all six metacentrics approach fixation close to the race centre. Thus, there is a hybrid zone between the Barcelona and standard races. The centres and width s of all clines (except 3.8) were determined. Likelihood ratio tests showed that most of the dine centres differed significantly in position (i.e. the clines were staggered) and the clines for metacentrics 6.10 and 9.11 were significantly narrower than those for 4.14, 5.15 and 12.13. Overall, the cl ines tended to be wider the further they were from the race centre. There a re various possible explanations for this hybrid zone structure and further data are needed to distinguish between them.