Phenotypic Variation across Chromosomal Hybrid Zones of the Common Shrew (Sorex araneus) Indicates Reduced Gene Flow

Abstract

Sorex araneus, the 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 chromosomal clines. The amount of phenotypic differentiation across the zones is greater than expected if it were dissipating due to unrestricted 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.

Geometric morphometric analyses of worn cheek teeth help identify extant and extinct gophers (Rodentia, Geomyidae)

“…Spitze ; Polly , Polly et al . ). Although the landmarking scheme may affect the comparisons between extant and fossil groups, the distances we calculated enable comparisons of morphological differences among taxa with differing morphologies.…”

Gene networks, occlusal clocks, and functional patches: new understanding of pattern and process in the evolution of the dentition

“…Adjacent (peripatric) karyotypic groups of the Common shrew are known to hybridize, presumably after coming back in geographic contact after expanding from isolated glacial refugia [68,70]. Drift is sufficient to explain differences observed between populations across such hybrid zones, but whether the seemingly small differences measured at the present time arose due to drift or whether they are remnants of historically larger differences that have already been lost through gene flow, thus erasing the effects of past selection during refugial isolation, is unclear [71]. Fossil evidence suggests the latter [56,72].…”

Spatial processes and evolutionary models: a critical review

“…Analyses of geographical variation in bones and teeth in extant and fossil species indicates that the ‘surfing’ phenomenon also in morphological traits in much the same way as it does in genetic markers (Polly , ; Szuma ; Polly et al . ). Spatially modified pure drift processes could therefore drive seemingly non‐random trait changes in palaeontological sequences, at least in principle, a possibility that should be considered among other alternative models of evolution.…”