Genetic variation in expression of defense phenotype may mediate evolutionary adaptation of Asclepias syriaca to elevated CO 2

How species interactions may modify the effects of environmental change on evolutionary adaptation is poorly understood. Elevated CO 2 is known to alter plant–herbivore interactions, but the evolutionary consequences for plant populations have received little attention. We conducted an experiment to determine the effects of elevated CO 2 and herbivory by a specialist insect herbivore ( Danaus plexippus ) on the expression of constitutive and induced plant defense traits in five genotypes of Asclepias syriaca , and assessed the heritability of these traits. We also examined changes in relative fitness among plant genotypes in response to altered CO 2 and herbivory. The expression of plant defense traits varied significantly among genotypes. Elevated CO 2 increased plant growth and physical defenses (toughness and latex), but decreased investment in chemical defenses (cardenolides). We found no effect of elevated CO 2 on plant induction of cardenolides in response to caterpillar herbivory. Elevated CO 2 decreased the expression of chemical defenses (cardenolides) to a different extent depending on plant genotype. Differential effects of CO 2 on plant defense expression, rather than direct effects on relative fitness, may alter A. syriaca adaptation to changing climate.