Understanding Climate Adaptation through Genome-Wide Patterns of Differentiation and Local Selection in Populus balsamifera. How Important Are Peripheral Populations?

In the light of projected climate change, detection of environmental adaptation caused by local climate-driven selection has become an important research focus in ecological genomics.  In long-lived plants such as forest trees, populations located at southern range margins may be particularly vulnerable to climate change if they exist at their current physiological limits; yet these populations may also harbor the adaptive variants to warmer, longer growing seasons that will be needed in the central core of the range to maintain adaptation under future climate. We address this question by studying range-wide populations of Populus balsamifera, a model system for studying climate adaptation in forest trees.  A total of 619 trees from 34 southern range-edge and 25 range-core populations were sequenced using genotype by sequencing (GBS) to study genome-wide patterns of single nucleotide polymorphism (SNP).  These range-edge populations likely represent the ancestral pool of standing genetic diversity and evolutionary potential present in southern refugial populations, and may contain unique adaptations to warmer climates.  We scanned the genome for signals of local adaptation, manifest as outliers with extreme population structure (F_ST, using BAYESCAN) in core vs peripheral populations.  We also tested for SNP-environment correlations across the range using contemporary methods (BAYENV and LFMM) as well as the non-linear multivariate methods Gradient Forests and Generalized Dissimilarity Modeling. Our results yield novel insights into the landscape patterns of local adaptation in core and peripheral populations, and have implications for forest management, and conservation under climate change.