In order to uncover the genetic basis of phenotypic trait variation, we used 448 unrelated wild accessions of black cottonwood (Populus trichocarpa) from much of its range in western North America. Extensive data from large-scale trait phenotyping (with spatial and temporal replications within a common garden) and genotyping (with a 34 K Populus single nucleotide polymorphism (SNP) array) of all accessions were used for gene discovery in a genome-wide association study (GWAS). We performed GWAS with 40 biomass, ecophysiology and phenology traits and 29,355 filtered SNPs representing 3518 genes. The association analyses were carried out using a Unified Mixed Model accounting for population structure effects among accessions. We uncovered 410 significant SNPs using a Bonferroni-corrected threshold (P<1.7×10(-6)). Markers were found across 19 chromosomes, explained 1-13% of trait variation, and implicated 275 unique genes in trait associations. Phenology had the largest number of associated genes (240 genes), followed by biomass (53 genes) and ecophysiology traits (25 genes). The GWAS results propose numerous loci for further investigation. Many traits had significant associations with multiple genes, underscoring their genetic complexity. Genes were also identified with multiple trait associations within and/or across trait categories. In some cases, traits were genetically correlated while in others they were not.

BioSciences Division Oak Ridge National Laboratory Oak Ridge TN 37831 USA

Department of Biology and Centre for Forest Biology University of Victoria Victoria BC V8W 3N5 Canada

Department of Botany University of British Columbia Vancouver BC V6T 1Z4 Canada

Department of Dendrology and Forest Tree Breeding Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague 165 21 Czech Republic

Department of Forest and Conservation Sciences Faculty of Forestry University of British Columbia Forest Sciences Centre 2424 Main Mall Vancouver BC V6T 1Z4 Canada

Department of Wood Science Faculty of Forestry University of British Columbia Forest Sciences Centre 2424 Main Mall Vancouver BC V6T 1Z4 Canada

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Sexual homomorphism in dioecious trees: extensive tests fail to detect sexual dimorphism in Populus †

Evolutionary Quantitative Genomics of Populus trichocarpa