The macroevolution of defensive traits in the genus Asclepias

Given that a plant’s defensive strategy to herbivory is never likely to be a single trait, we have been developing the concept of “plant defense syndromes”, where associations with specific abiotic conditions or ecological interactions can result in convergence on suites of covarying defensive traits. Defense syndromes can be studied within communities of diverse plant species as well as within clades of closely related species. In either case, theory predicts that plant defense traits may consistently covary across species due to shared evolutionary ancestry or due to adaptive convergence.

We have examined potential defense syndromes in 24 species of milkweeds (Asclepias spp.) in a preliminary field experiment, and we continue to study these patterns in about 45 species. Employing phylogenetically independent contrasts, we have thus far found few correlations between seven defensive traits (cardenolides, latex, water content, trichome density, leaf toughness, C-N ratio, specific leaf area), no bivariate trade-offs, and notable positive correlations between trichome density and latex production, and between C:N ratio and leaf toughness. We then used a hierarchical cluster analysis to produce a phenogram of defense-trait similarity among the 24 species. This analysis revealed three distinct clusters of species. The defense syndromes of these species clusters are associated with either low nutritional quality or a balance of higher nutritional quality coupled with physical or chemical defenses. The phenogram based on defense traits was not congruent, however, with a molecular phylogeny of the group, suggesting convergence on defense syndromes.

Schematic depiction of the lack of congruence between the molecular phylogeny of Asclepias and the defense-trait phenogram (above). The discovery of convergent plant defense syndromes can be used as a framework to ask questions about how abiotic environments, communities of herbivores and biogeography are associated with particular defense strategies of plants. Furthermore, we have proposed that the defenses of plants fall along a continuum of three core areas along too axes.

The plant defense syndrome triangle hypothesis (Agrawal and Fishbein 2006). Low nutritional defense syndrome is consistent with that outlined for apparent plants by Feeny (1976); a similar group was found in our preliminary analysis of Asclepias species. Tolerance follows the fast growth and high edibility pattern outlined by Coley et al. 1985 and Kursar and Coley 2003. Nutrition and defense is a strategy that couples a toxic defense or barrier to feeding with relatively high edibility and digestibility.