Comparative studies of reaction norms in response to light in Arabidopsis

The evolution of reaction norms has rarely been addressed from within a phylogenetic comparative framework, despite general agreement that we need a better understanding of how historical and deterministic processes (e.g., selection and constraints) interact to yield a particular pattern of variation of phenotypic plasticity within and across species. In this study the reaction norms to daylength and foliage shade(changes in light quality) were compared in eight accessions of iArabidopsis thaliana from Scandinavia and three closely related species, A. arenosa, A. lyrata subsp. petraea,and A. suecica. There was a high correlation among the trait means in response to daylength (a coarse-grained environment for this plant) and a high correlation among the trait plasticities in response to light quality (a fine-grained environment). This supports expectations of the evolution of a specialist strategy in response to daylength and of a generalist phenotype in reaction to light quality. The existence of some degree of genetic constraint between reaction norms to daylength and foliage shade was also predicted because both aspects of light are detected by an overlapping set of photoreceptors and because the plant has to integrate the two signals in order to make crucial life history decisions. Accordingly, some trait means and plasticities were correlated across experiments for characters related to life history. However, the overall multivariate phenotype was extremely labile, thereby indicating a potentially large effect of light environments on the shape of evolutionary trajectories.