Causes and consequences of phenotypic plasticity across the milkweeds (Asclepias spp)

Phenotypic plasticity allows organisms to enhance their fitness in variable environments and it is predicted that more plastic species will have broader ecological distributions because they can better cope with environmental heterogeneity. Traditionally, phenotypic plasticity has been measured by growing multiple genotypes in two environments to determine reaction norms for each genotype. We have expanded upon this classic design by growing several species in multiple environments and examining several phenotypic responses. We investigated the plasticity of 12 milkweed (Asclepias) species belonging to the phylogenetically well-resolved Incarnatae clade. Phenotypic plasticity was determined across four environments within a growth chamber and nine phenotypic traits (physiological, defense, growth) were measured. We then calculated global means to collapse the plastic responses of each species into a single mean plasticity value across traits and environments. Specifically, we addressed the following hypotheses: 1) Do species display variable levels of plasticity to different environmental cues? 2) Is the overall level of plasticity phylogenetically conserved? 3) Does plasticity predict ecological success (i.e. biogeographic distribution and environmental tolerance)? and 4) Can the level of plasticity be predicted by a single factor such as growth rate? We found that all traits responded plastically, but varied in the magnitude of their responses depending on the species, environment and traits. Correlations indicate weak associations between plasticity and ecological distribution and plant growth rate. We conclude by suggesting that global analyses across traits and environments are required to generally understand the causes and consequences of phenotypic plasticity.