Growth-defense trade-offs in two major defense traits of the common milkweed Asclepias syriaca Tobias Züst and Anurag A. Agrawal

Costs of defense Key assumption of co-evolutionary theory Costs limit runaway selection Costs maintain genetic diversity

Asclepias syriaca

Defense traits of Milkweed Latex Cytoplasm Extracellular space Cardenolides

Defense traits of Milkweed Latex and cardenolides vary independently Agrawal et al. (in press) Agrawal (2005) Agrawal et al. (2012) Bingham and Agrawal (2010) Mooney and Agrawal (2008) Latex exudation (mg FM) Cardenolides (mg/g DM)

Defense traits of Milkweed Latex and cardenolides are inducible Agrawal et al. (in press)

Defense traits of Milkweed Constitutive and induced levels trade off Agrawal et al. (2012)

Questions 1.Do defense traits of milkweed convey allocation costs? 2.Is the presence of costs dependent on nutrient availability?

Growth as unit of costs A component of fitness in perennial plants Physiological measure of allocation Likely to show allocation costs

Plant growth rate

RGR: Relative growth rate NAR: Net assimilative rate SLA: Leaf density LMR: Proportion of photosynthetic tissue

Experimental design Plants grown for 45 days (24 genotypes x 2 nutrient levels) Frequent non-destructive size measurements Destructive harvests after 2, 4, and 6 weeks Quantification of defense traits at destructive harvests

Shoot cardenolides

Shoot latex exudation

Genotype-specific nutrient effects

Trait Low Fertiliser - Trait High Fertiliser Genotype-specific nutrient effects

Plant-size effect on latex Latex exudation reflects storage capacity, not production Big leaves contain larger laticifers, more latex is stored

Conclusions Leaf cardenolide levels trade-off with growth at two nutrient levels, especially photosynthetic efficiency Nutrient-driven change in cardenolide synthesis also affects growth rate Latex correlates positively with growth and plant size  simple exudation measure insufficient to quantify allocation costs

Acknowledgments Sergio Rasmann Elizabeth Davidson-Lowe Chris Stieha Yann Hautier Collin Edwards Georg Petschenka Rayko Halitschke Amy Hastings

Root cardenolides