Chromosomal and allozyme analysis of a hybrid zone between parapatric Robertsonian races of the house mouse: a case of monobrachial homology

A detailed chromosomal and allozyme analysis of Robertsonian (Rb) populations of the house mouse in Alsace (France) was performed to evaluate the model of speciation by monobrachial centric fusions. The karyological analysis confirmed the existence of a hybrid zone between two Rb races differentiated by monobrachially homologous Rb fusions. The clinal distribution of Rb fusions showed that almost no overlap occurred between the Rb fusions specific to each race, indicating the presence of an acrocentric peak in the center of the hybrid zone. Thus, only chromosomal heterozygotes carrying one to three trivalents were present, instead of the expected highly unfit interracial hybrids, none of which were observed. The effect of karyotypic differentiation on genic divergence was tested across the hybrid zone by comparing levels of genetic structure for chromosomal and allozyme markers according to the isolation by distance model. Results confirmed the highly significant spatial structure for chromosomes, but revealed none for allozymes, indicating that genic diversity was not structured according to karyotype. These data suggest the absence of a strong chromosomal barrier to gene flow, which does not conform with predictions of the model of speciation by monobrachial centric fusions. In addition, the relatedness of these Rb races to neighboring ones, as well as the nonrandom contribution of several chromosomes to the Rb fusions, was analyzed.