Evolution of pest-induced defenses in Brassica plants: Tests of theory
Theory on the evolution of pest-induced defenses in plants predicts (1) a negative genetic correlation between induced and constitutive (basal) levels of secondary metabolites, and (2) costs of maintaining high constitutive levels of secondary metabolites. We tested these predictions with genetically diverged populations created by artificial selection on myrosinase and glucosinolate levels in Brassica rapa. Glucosinolates and their breakdown products from the action of the enzyme myrosinase are putative defensive compounds in brassicas. Theory also suggests that effects of genetic changes in secondary metabolites may depend on resource availability, so nitrogen, a main constituent of glucosinolates and myrosinase was added in fertilizer treatments to assess costs. We used the fungal pathogen Leptosphaeria maculans and diamondback moth larvae Plutella xylostella as induction agents in comparisons of the diverged myrosinase populations. We found pleiotropic effects among constitutive myrosinase levels and pathogen-induced levels of myrosinase, glucosinolates, and resistance to diamondback moth larvae. In field experiments, genetic increases in myrosinase production were associated with significant decreases in estimated seed production, despite potential benefits from increased resistance to flea beetles Phylotreta cruciferae. No genotype-by-nitrogen interaction was found. Although costs were detected, our results indicate, in contrast to theory, mainly positive pleiotropic effects between constitutive levels of secondary metabolites and induced responses.
Siemens, DH; Mitchell-Olds, T
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