Timing for success: Expression phenotype and local adaptation related to latitude in the boreal forest tree, Populus balsamifera

Timing of seasonal dormancy is among the most important traits allowing forest tree populations to span latitudinal gradients of temperature and growing season length. We used RNA-sequencing (RNA-seq) data to identify gene expression differences between latitude-adapted populations and search for elevated genetic differentiation, a signature of adaptation, between northern and southern populations of Populus balsamifera. RNA was extracted from leaf tissues collected from trees originating from two northern (65° N) and two southern (50° N) populations and growing in a common garden. Leaves for RNA-seq were collected on the same day in late July, when southern trees were actively growing and northern trees had set bud. Transcripts of 2,594 genes were more abundant in actively growing leaves, whereas 1,424 were more abundant in leaves of trees that had set bud. The actively growing southern trees exhibited overexpression of genes in the biological process gene ontology (GO) domain, whereas dormant northern trees exhibited overexpression of genes in the cellular component domain. We identified relatively few genes (641) bearing signatures consistent with local adaptation to latitude for three or more SNPs. Exons from differentially expressed (DE) genes were not more likely than non-DE genes to exhibit patterns of latitudinal differentiation consistent with local adaptation. These results indicate that genes associated with differences in the timing of bud set, which is correlated with initiation of dormancy, were not more likely than other categories to exhibit latitudinal differentiation in exonic regions.