Evolution of phenotypic plasticity a comparative approach in the phylogenetic neighbourhood of Arabidopsis thaliana

The evolution of phenotypic plasticity has rarely been examined within an e xplicitly phylogenetic framework, making use of modern comparative techniqu es. Therefore, the purpose of this study was to determine phylogenetic patt erns in the evolution of phenotypic plasticity in response to vegetation sh ade (the 'shade avoidance' syndrome) in the annual plant Arabidopsis thalia na and its close relatives. Specifically, we asked the following questions: (i) Do A. thaliana and related species differ within or among clades in th e magnitude and/or pattern of plasticity to shade? (ii) Are the phenotypic variance-covariance matrices (phenotypic integration) of these taxa plastic to the changes in light quality induced by the presence of a canopy? (iii) To what extent does the variation in uni- and multivariate plasticity matc h the phylogeny of Arabidopsis? In order to address these questions we grew individuals from six taxa of known phylogenetic relationship in a greenhou se under full sun and under a grass canopy. Taxa differed in the magnitude, but not in the pattern, of plasticities for all traits. At the univariate level, the late flowering species, A. pumila and A. griffithiana, as well a s the late flowering Moscow ecotype of A. thaliana, showed greater plastici ty for allocation to vegetative and reproductive meristems. At the multivar iate level, several taxa displayed a very low stability of their variance-c ovariance structures to environmental change, with only one taxon sharing a s many as three principal components across environments. We conclude that both univariate and multivariate plasticities to vegetation shade can evolv e rapidly within a genus of flowering plants, with little evidence of histo rical constraints (phylogenetic inertia).