Abstract
The timing of growth and dormancy represent critical life-history trade-offs in perennial plants and often show strong local adaptation. Despite their importance, the genetic architecture of phenological traits remains poorly understood. Here we identify a ∼700 Kbp region mediating local adaptation in the timing of bud set in Populus tremula across a latitudinal gradient (∼56-66°N) in Sweden, where extensive gene flow has removed almost all traces of population structure. The strongest genomic associations with bud set are centered on a P. tremula homolog of FLOWERING LOCUS T2 (PtFT2) and explain 32-52% of the variation in bud set across sites and years. This region also shows multiple signs of a recent selective sweep restricted to the northernmost populations. Under field and greenhouse conditions, variation in bud set among accessions originating from different latitudes is strongly associated with variation in PtFT2 gene expression. Finally, transgenic down-regulation of PtFT2 yields a phenotype that, under field conditions, closely mimics variation observed between phenotypic extremes in natural populations. Our results thus provide evidence of one genomic region in P. tremula, centered on PtFT2, with major effects on adaptation to shorter growing seasons and colder climates following post-glacial colonization.