The Genetics of Adaptation in an Invasive Plant (Ambrosia artemisiifolia)

Introduced species represent opportunities to observe evolution over contemporary timescales, and as exotics encounter new environments adaptive responses can occur, potentially contributing to invasion. We compared native North American populations and introduced European populations of common ragweed (Ambrosia artemisiifolia) in common gardens, and found evidence of more rapid growth in Europe at the expense of reduced drought tolerance. Moreover, we found parallel latitudinal clines in flowering time, and correlations between fitness components and the local climate of the source populations in both ranges. Together these data provide evidence for adaptation to local environmental conditions in the native and introduced range of common ragweed. To identify the molecular and genetic basis of invasiveness in ragweed, we sequenced transcriptomes of native and introduced populations and conducted genotyping-by-sequencing. Using microarrays were able to identify differentially expressed genes between the native and introduced ranges across a number of common gardens, providing a list of candidate genes with a diverse set of functions, including genes relating to secondary metabolism and stress response. To detect episodes of positive selection repeated across multiple introductions and species, we conducted comparisons between native and introduced genotypes from common ragweed as well as five other Asteraceae species using transcriptome data and identified genes with elevated rates of amino acid change (dN/dS). Our findings provide little evidence for common genomic responses in invasive taxa of the Asteraceae and hence suggest that multiple evolutionary pathways may lead to adaptation during introduction and spread in these species.