Non-metric morphological divergence in the western house mouse, Mus musculus domesticus, from the Barcelona chromosomal hybrid zone

The effect of hybridization on morphological variation was investigated in 120 western house mice, Mus musculus domesticus, from the hybrid zone between the Barcelona and standard chromosomal races. The incidence of 37 nonmetric cranial traits was calculated for standard mice (2n = 40) and Barcelona-standard hybrids (2n = 27-39). Subsequent analyses were conducted on several karyological subgroups, established by grouping the animals according to either their diploid number or their degree of chromosomal heterozygosity. Results revealed no significant difference by sex, asymmetry, or geographical distance. Significant phenetic divergences were found between the karyotypes studied in relation to several variants. Differences were especially substantial between the standard race and hybrid mice, even with respect to those hybrids with karyotypes close to that of the standard race. Within the hybrids, the maximum divergence corresponded to the 28-chromosome homozygotes, chromosomally close to the Barcelona race, and to the heterozygotes with more than two fusions. Since differences in non-metric trait frequencies are generally considered a measure of genetic divergence, the results suggest the occurrence of a barrier to gene flow in the Barcelona hybrid zone. The decrease of genetic exchange between the chromosomally differentiated mice might be due to reduced fertility in hybrids, associated with chromosomal heterozygosity. (C) 2003 The Linnean Society of London.