SCHMITT, J; STINCHCOMBE, J; HUBER, H; Brown University; Brown University; University of Nijmegan: Adaptive evolution of shade avoidance responses in natural plant populations

Many plant species exhibit stem elongation and other developmental responses to the decreased ratio of red: far red wavelengths (R:FR) characteristic of foliage shade. This “shade avoidance” response, mediated by phytochrome photoreceptors, has been hypothesized to be advantageous because it allows plants to sense and avoid competition for light in dense stands. Shade avoidance responses provide an ideal system for investigating the mechanisms underlying the adaptive evolution of phenotypic plasticity in natural populations. For adaptive plasticity of a trait to evolve, selection on that trait must differ among environments, and genotypes within populations must vary in reaction norms of the trait to those environments. The evolutionary outcome may also depend upon genetic and physiological constraints and upon the relative frequency of selective environments. In the native annual plant Impatiens capensis (jewelweed or touch-me-not) phytochrome-mediated shade avoidance responses are an important component of plasticity to density in natural populations. Manipulative field studies of density-dependent selection support the hypothesis that this plasticity is adaptive, conferring high relative fitness in dense stands, but low fitness at low density. Shade avoidance responses are also genetically variable within populations, indicating evolutionary potential for response to natural selection. However, the costs and benefits of expressing the shade avoidance phenotype depend upon the environment, consistent with mechanistic predictions. Natural populations display adaptive genetic differentiation consistent with these selective pressures.