Genomic study uncovers candidate genes for adaptive introgression in saltmarsh and Nelson’s sparrow.
Hybridization can work as an evolutionary stimulus. For example by transferring beneficial genetic variants from one species to another. This process, adaptive introgression, has been described for numerous taxa, such as butterflies and snowshoe hares. However, examples in birds are quite rare. A recent study in the journal Evolution provides evidence for adaptive introgression between two sparrow species.
Hybrid Zone
Jennifer Walsh (Cornell University) has published a nice series of papers on saltmarsh sparrow (Ammodramus caudacutus) and Nelson’s sparrow (A. nelsoni) that hybridize along the coast of New England (see the Emberizidae page for an overview). The saltmarsh sparrow is a specialist of – you guessed it – saltmarshes, whereas Nelson’s sparrow has a broader ecological niche that includes grasslands and brackish marshes. Using microsatellites, she was able to document gene flow between these species. But could this exchange of genetic material be adaptive? Time to bring out the big guns: genome sequences!
A saltmarsh sparrow (from:Â https://www.audubon.org/)
Salty Genes
By comparing the genomes of 36 individuals, Jennifer and her colleagues were able to pinpoint several genomic regions that have been exchanged between the sparrows. Further exploration of these regions uncovered several genes that could be important for life in the saltmarshes.
[Sixteen] of these 24 putative candidate genes confer potential adaptations to tidal marsh environments, including genes with links to osmotic regulation, response to salt stress, response to water deprivation, and muscle development. The remaining 8 candidates include additional regions, including four genes related to DNA repair and several genes with a range of putative adaptive functions (visual perception, response to pH).
It can be tempting to tell an adaptive story for each gene, but it is important to keep in mind that these are “candidate genes” for adaptation. They should be the starting point for further research, not the final answer. So, you can expect more sparrow studies in the near future.
References
Walsh, J., Kovach, A.I., Olsen, B.J., Shriver, W.G. & Lovette, I.J. (2018) Bidirectional adaptive introgression between two ecologically divergent sparrow species. Evolution
This paper has been added to the Emberizidae page.
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Avian Hybrids 
Thoughts on this possible Chestnut-sided/Golden-winged Warbler hybrid? Photographed in central Pennsylvania
https://ebird.org/view/checklist/S48100159
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[…] introgression of beneficial alleles might have facilitated adaptation to salt marshes. Indeed, a previous study on Nelson’s Sparrows identified several candidate genes for adaptive introgression. Whether […]
[…] caudacuta) and the Nelson’s Sparrow (A. nelsoni). Despite high levels of interspecific gene flow, these species remain largely distinct. What reproductive isolation mechanisms could prevent these […]
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