Population genomics of Populus trichocarpa identifies signatures of selection and adaptive trait associations

Stephen DiFazio and colleagues report the genome sequences and population genomic analyses of 544 black cottonwood trees ( Populus trichocarpa ) along the Northwest coast of North America. They find evidence for climate-driven selection on adaptive traits, including genes related to drought, photoperiod and stress. Forest trees are dominant components of terrestrial ecosystems that have global ecological and economic importance. Despite distributions that span wide environmental gradients, many tree populations are locally adapted, and mechanisms underlying this adaptation are poorly understood. Here we use a combination of whole-genome selection scans and association analyses of 544 Populus trichocarpa trees to reveal genomic bases of adaptive variation across a wide latitudinal range. Three hundred ninety-seven genomic regions showed evidence of recent positive and/or divergent selection and enrichment for associations with adaptive traits that also displayed patterns consistent with natural selection. These regions also provide unexpected insights into the evolutionary dynamics of duplicated genes and their roles in adaptive trait variation.