Genomic signatures of adaptation along an expansion route in the colonizing plant <i>Leontodon longirostris</i> (Asteraceae)

Species' range shifts are common in nature, often involving the colonization of new habitats. Identifying the evolutionary processes responsible for the colonization of novel environments is then fundamental to understanding the current distribution of organisms and predicting responses to ongoing global change. Leontodon longirostris is a short-lived plant native of the Western Mediterranean Basin that expanded its range through adaptive changes in key phenotypic traits along a south-to-north environmental gradient in the Iberian Peninsula. Here, we provide some insights into the genomic basis of adaptation underlying the colonization of novel environments during range expansion. In particular, we were interested in the role of preexisting genetic diversity to facilitate adaptation, and the identification of genes that have responded to selection in the novel colonized environments. We combined genomic, phenotypic, and environmental information to analyse (i) the prevalence of hard versus soft selective sweeps in ancestral and expanded populations, and (ii) the association of 168,733 SNPs with phenotypic traits and environmental variables. Our results suggest that adaptation during range expansion primarily occurred through selection on standing genetic variation already present in the ancestral populations. We also identified a small set of candidate genes involved in signalling pathways that may underlie changes in life-history traits related to colonization. Although functional validation is still needed, these findings highlight the potential role of regulatory networks in facilitating adaptation during range expansion.