Integration of functional traits in twelve milkweed (Asclepias) species: A macroevolutionary approach

Phenotypic integration refers to the pattern of correlation among different functional traits in a given organism. In a growth chamber experiment, we exposed twelve milkweed (Asclepias) species to four environments (damage by a natural herbivore, nutrient addition, rocky soil, and an unmanipulated control) and measured the plasticity of nine growth, defensive, and ecophysiological traits. We evaluated whether these traits respond to the environments in a correlated fashion and, if so, represent potential constraints or synergisms. All twelve species are part of the well-resolved Incarnatae clade, allowing us to make phylogenetically controlled comparisons and to ask whether correlations between traits are conserved or evolutionarily labile. We found that all traits were plastic in at least some environments. The magnitudes of differences in trait values among species were generally greater in the herbivory and the nutrient addition treatments compared to the rocky soil treatment. We found some evidence for a consistent pattern in the direction of trait plasticities among species, and several correlations between traits indicating strong phenotypic integration. We present evidence for limited phylogenetic conservatism of phenotypic integration in this clade. By taking a comparative approach and including numerous species, traits and environments, this work represents one of the first attempts to understand the macroevolution of phenotypic plasticity in a broad context.