The western house mouse (Mus musculus domesticus) and common shrew (Sorex araneus) show dramatic chromosomal variation, with around 100 chromosomal races in each species distinguished by different sets of metacentric chromosomes that have arisen by Robertsonian fusion and whole-arm reciprocal translocation.
Numerous hybrid zones have been described in the two species at the contact of different races. In some cases the hybrids are characterised by extremely long chain or ring configurations at meiosis I, associated with high levels of gametogenic abnormality and reduced fertility. In accordance with hybrid zone theory, hybrid zone width varies with the complexity of the meiotic chromosomal configuration; where the hybrids are expected to have low fertility the hybrid zone is narrower.
Other features of the hybrid zones fit the classic tension zone model, e.g. localization of zones in regions of low population density. There is evidence of behavioral isolation between chromosomal races in some zones, which may be a response to selection against unfit hybrids (i.e. reinforcement). However, selection appears to have favored novel homozygous forms in other hybrid zones, thereby de-escalating rather than escalating the speciation process.
While the role of hybrid unfitness in determining hybrid zone structure in terms of chromosomal clines is relatively straightforward, the relationship between hybrid unfitness, recombination suppression and gene flow is more difficult to untangle.
I will describe recent data recording recombination using MLH1 foci, and examining gene flow using mapped microsatellites and SNPs. All-in-all, the western house mouse with its access to all the cell biological and genomic tools of the laboratory mouse, and the common shrew with its extraordinary diversity of chromomosomal hybrids from numerous hybrid zones, are wonderful models to examine the role of chromosomal rearrangements in speciation.
Giménez, M.D., Förster, D.W., Jones, E.P., Jóhannesdóttir, F., Gabriel, S.I., Panithanarak, T., Scascitelli, M., Merico, V., Garagna, S., Searle, J.B. & Hauffe, H.C., 2017. A half-century of studies on a chromosomal hybrid zone of the house mouse. Journal of Heredity, 108, 25-35.