Genetic variation underlying plasticity in physiological traits mediates response to climate in a Populus hybrid zone

Phenotypic plasticity can buffer the potential fitness consequences of environmental change, yet limited understanding of its genetic basis constrains its application to predicting population response to future climates. Hybrid zones provide powerful systems to study the genetic basis of plasticity because admixture can create novel allele combinations that generate new reaction norms for selection to act upon. Here, we combine two clonally replicated common gardens of Populus trichocarpa x P. balsamifera genotypes with whole-genome resequencing to identify genetic variation underlying plasticity for physiological traits. Admixed genotypes exhibited broader, and in some cases novel, reaction norms relative to parental genotypes, particularly for key stomatal traits. Admixture mapping of genotype-specific reaction norms identified ten candidate genes on chromosome 15, including TWIST, associated with plasticity in adaxial stomatal occurrence and density. Using random forest models, we projected allele-specific responses to climate warming within the hybrid zone to link genetic variation in plasticity with predicted warming. Random forest models forecast that future climates would favor P. trichocarpa alleles at TWIST, while P. balsamifera alleles would be maintained in heterozygous genotypes. These results suggest that hybridization can expand reaction norms and maintain genetic variation that may facilitate rapid phenotypic response needed to adapt to climate change.