Abstract

Background/Objectives: Closely related species provide an opportunity to study evolutionary processes that underpin new species formation. Speciation driven by adaptation to distinct environments, such as adaptation to high and low elevation in Senecio aethnensis and Senecio chrysanthemifolius, respectively, on Mount Etna, Sicily, is particularly informative about the interplay between natural selection and ongoing interspecific gene flow. Methods: In this study, we analysed the relative contribution of adaptive introgression and local adaptation in these Senecio species. We used genome resequencing with short read sequence data, and analysed patterns of polymorphism within and between the two Senecio species. Results: Genome-wide scans for selection identified numerous putative selective sweep regions, many of which overlap with selected regions detected in cline-based analysis in the same system. Our results indicate the prevalence of local over introgressive sweeps, consistent with local adaptation to contrasting environments driving divergence of these species. However, many putative introgressive sweeps were also detected, possibly driven by shared selective forces, such as adaptation to volcanic soils and disease resistance, as indicated by GO terms linked to defence responses for some of the introgressive sweeps. Conclusions: The results suggest that even though the two species are now adapted to different environments, introgression has facilitated their adaptation by sharing adaptive alleles between species, which is likely an important factor for adaptation in closely related species.

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