These studies centre on the ‘Barcelona’ karyotypic race of the western house mouse (Mus musculus
domesticus), first described by Adolph & Klein (1981). This is one of many races within M. m.
domesticus characterized by metacentric chromosomes that have originated by repeated
Robertsonian fusions, with perhaps further modification by whole-arm reciprocal translocations.
Data 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 with the standard 40-acrocentric karyotype were caught only
30 km from the race centre. Throughout the area of occurrence of metacentrics there is
polymorphism (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 widths of all clines (except 3·8) were determined. Likelihood ratio
tests showed that most of the cline 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 clines tended to be wider the further they were from the race
centre. There are various possible explanations for this hybrid zone structure and further data are
needed to distinguish between them.