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Growth-defense trade-offs in two major defense traits of the common milkweed Asclepias syriaca
Tobias Züst and Anurag A. Agrawal
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Costs of defense
• Key assumption of co-evolutionary theory
• Costs limit runaway selection
• Costs maintain genetic diversity
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Asclepias syriaca
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Defense traits of Milkweed
Latex
Cytoplasm
Extracellular space
Cardenolides
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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)
0 0.5 1 1.5 2 2.5 30
1
2
3
4
5
Late
x ex
udati
on (m
g F
M)
Cardenolides (mg/g DM)
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Defense traits of Milkweed
• Latex and cardenolides are inducible
Agrawal et al. (in press)
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Defense traits of Milkweed
• Constitutive and induced levels trade off
Agrawal et al. (2012)
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Questions
1. Do defense traits of milkweed convey allocation costs?
2. Is the presence of costs dependent on nutrient availability?
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Growth as unit of costs
• A component of fitness in perennial plants
• Physiological measure of allocation
• Likely to show allocation costs
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Plant growth rate
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Plant growth rate
• RGR: Relative growth rate
• NAR: Net assimilative rate
• SLA: Leaf density
• LMR: Proportion of photosynthetic tissue
𝑅𝐺𝑅=𝑁𝐴𝑅×𝑆𝐿𝐴× 𝐿𝑀𝑅
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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
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Shoot cardenolides
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Shoot latex exudation
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Genotype-specific nutrient effects
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Trait Low Fertiliser- Trait High Fertiliser
Genotype-specific nutrient effects
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Plant-size effect on latex
• Latex exudation reflects storage capacity, not production
• Big leaves contain larger laticifers, more latex is stored
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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 sizesimple exudation measure insufficient to quantify allocation costs
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Acknowledgments• Sergio Rasmann• Elizabeth Davidson-Lowe• Chris Stieha• Yann Hautier• Collin Edwards• Georg Petschenka• Rayko Halitschke• Amy Hastings
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Root cardenolides