Phenotypic variation across chromosomal hybrid zones of the Eurasian common shrew (Sorex araneus) indicates reduced gene flow

Sorex araneus, the Eurasian common shrew, is a species with more than 70 karyotypic races, many of which form parapatric hybrid zones, making it a model for studying chromosomal speciation. Hybrids between races have reduced fitness, but microsatellite markers have demonstrated considerable gene flow between them calling into question whether the chromosomal barriers actually do contribute to genetic divergence. We studied phenotypic clines across two hybrid zones with especially complex heterozygotes. Hybrids between the Novosibirsk and Tomsk races produce chains of nine and three chromosomes at meiosis, and hybrids between the Moscow and Seliger races produce chains of eleven. Our goal was to determine whether phenotypes show evidence of reduced gene flow at hybrid zones. We used maximum likelihood to fit tanh cline models to geometric shape data and found that phenotypic clines in skulls and mandibles across these zones had similar centers and widths as metacentric clines. The amount of phenotypic differentiation across the zones is greater than expected if it were dissipating due to gene flow given the amount of time since contact, but it is less than expected to have accumulated from drift during allopatric separation in glacial refugia. Only if heritability is very low, Ne very high, and the time spent in allopatry very short, will the differences we observe be large enough to match the expectation of drift. Our results therefore suggest that phenotypic differentiation has been lost through gene flow since post-glacial secondary contact, but not as quickly as would be expected if there was free gene flow across the hybrid zones. The chromosomal tension zones are confirmed to be partial barriers that prevent differentiated races from becoming phenotypically homogenous. The data archived here underpin these results.INTAS Cooperation Project, the Russian Foundation for Basic Research (RFBR)Morphometric landmark coordinates -- XY coordinates of landmarks in TPS or CSV format. Data from the Novosibirsk-Tomsk (NT) and Moscow-Seliger (MS) hybrid zones include landmarks from the ventral cranium (k=25), the lateral mandible (k=17, and the medial mandible (k=23). Data from other hybrid zones include landmarks from the cranium only. Associated data for individual specimens -- Data for the individual animals included in the NT and MS morphometric files. These data include an identifier for each individual (the file name from which the coordinates were captured), the race of the individual based on its karyotype, its site number, and other information. Data for the sites, which are referred to as sublocalities in the published paper, include the site number, the latitude, longitude, and elevation, and the proportion of individuals from each race at the site.Files ending in .tps are plain-text files in TPS format containing the x,y coordinates of morphometric landmarks. Files ending in .csv are plain-text, comma-delimited files containing associated data for the specimens in the TPS files