A historical perspective on a Sapsucker hybrid zone.
Hybrid zones can move. These regions, where species come into contact and interbreed, are not always stationary. For example, the North American hybrid zone between Black-capped Chickadee (Poecile atricapillus) and Carolina Chickadee (P. carolinensis) has moved about 10 kilometers to the north over the past decade (see here for more information on these birds). Detailed studies of this hybrid zones revealed that this movement was driven by climate change: higher winter temperatures allow for Carolina Chickadees to expand northwards. A recent study in the journal Molecular Ecology explored whether a hybrid zone between two woodpeckers species is also moving due to climate change.
Red-naped Sapsucker (Sphyrapicus nuchalis) and Red-Breasted Sapsucker (S. ruber) hybridize along a narrow contact zone that runs from northern California to southern British Columbia. Previous work indicated very low genetic divergence, although they are morphologically quite distinct. This hybrid zone has been studied for several decades. Shawn Billerman (University of Wyoming) and his colleagues collected museum specimens and recent samples to investigate whether the hybrid zone has changed over a 40-year time period.
Tail of Introgression
Using a genotyping-by-sequencing approach, they discovered that the hybrid zone has moved. Red-breasted Sapsuckers have been expanding eastward and replacing Red-naped Sapsuckers at particular locations. However, the broad-scale patterns of gene flow remained relatively stable.
The latter finding is a bit surprising. Hybrid zone movement is often accompanied by a “tail of introgression” in which genetic markers of one species are left behind in the wake of the moving hybrid zone. This has been shown for Setophaga warblers and Hippolais tree warblers. This “tail” is probably more difficult to detect in the woodpeckers because of the mosaic nature of their hybrid zone. Forest patches are scattered within a matrix of arid shrubs. An introgression tail is easier to document in a continuous and more gradual hybrid zone.
Contrary to the Chickadee hybrid zone, this movement of the woodpecker hybrid zone is not driven by climate. Instead, patterns of genetic divergence correlated most strongly with plumage characteristics. This suggests that assortative mating based on plumage is an important factor in reproductive isolation. This conclusion is also supported by the bimodal genetic distribution of this hybrid zone: the species form two distinct groups with little hybrids in between.
The authors nicely summarize their findings in the concluding section: “We found weak neutral genetic divergence between species, consistent with other studies of sapsuckers, extensive admixture, and maintenance of phenotypic boundaries, possibly through positive assortative mating.”
Billerman, S.M., Cicero, C., Bowie, R.C.K. & Carling, M.D. (2019) Phenotypic and Genetic Introgression Across a Moving Woodpecker Hybrid Zone. Molecular Ecology.
This paper has been added to the Piciformes page.