Comparative studies of reaction norms in Arabidopsis. I. Evolution of response to daylength

The evolution of reaction norms (the genotype-specific manifestation of dif ferent phenotypes in response to environmental variation) has rarely been a ddressed from within a phylogenetic comparative framework, despite general agreement that we need a better understanding of how historical and determi nistic processes (e.g. selection and constraints) interact to yield a parti cular pattern of variation of phenotypic plasticity within and across speci es. In this study, we compare the reaction norms to daylength in eight acce ssions of Arabidopsis thaliana from Scandinavia and three closely related s pecies, A. arenosa, A. lyrata subsp. petraea and A. suecica. We find that a cross-environment means evolved continuously and very rapidly within this g roup, while plasticity (calculated as the difference between the character value under short photoperiod minus the expression of the same trait under long photoperiod) changed only rarely and especially at the base of the A. thaliana clade. Character means co-evolved in a fashion that identifies two functional sets of traits, one during the vegetative phase and the other d uring the reproductive phase, with trade-offs between characters expressed across the two phases. With a few exceptions, plasticities of different tra its tended to evolve largely independently of each other. Several plasticit ies evolved in concert with their corresponding across-environment means, b ut by following a pattern of negative correlations that cannot be explained by simple geometrical considerations in environment-phenotype space. Neith er the mean nor the plasticity of a focal trait, flowering time, were corre lated with differences in phylogenetic relatedness, geographical distance o r latitudinal differential. This suggests that neither genetic drift nor ad aptation to large-scale geographical factors occurred.