Ecology and evolutionary history determine plastic responses to environmental variation in two closely related species

Phenotypic plasticity can maintain population fitness in novel or changing environments if it allows the phenotype to track the new trait optimum. Understanding how adaptation to contrasting environments determines plastic responses can identify how plasticity evolves, and its potential to be adaptive in response to environmental change. We sampled 79 genotypes from populations of two closely related but ecologically divergent ragwort species (Senecio, Asteraceae), and transplanted multiple clones of each genotype into four field sites along an elevational gradient representing each species9 native range, the edge of their range, and in conditions outside their native range. At each transplant site, we quantified differences in survival, growth, leaf morphology, chlorophyll fluorescence and gene expression for both species. Overall, the two species differed in their sensitivity to the elevational gradient. As evidence of plasticity, leaf morphology changed across the elevational gradient, with changes occurring in orthogonal directions for the two species. Differential gene expression across the four field sites also revealed that the genetic pathways underlying plastic responses were highly distinct in the two species. Despite the two species having diverged recently, adaptation to contrasting habitats has resulted in the evolution of distinct sensitivities to environmental variation, underlain by distinct forms of plasticity.