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 (bas al) levels of secondary metabolites, and (2) costs of maintaining high constitutive levels of secondary metabolites. We tested these predict ions with genetically diverged populations created by artificial selec tion on myrosinase and glucosinolate levels in Brassica mpa. Glucosino lates and their breakdown products from the action of the enzyme myros inase are putative defensive compounds in brassicas. Theory also sugge sts that effects of genetic changes in secondary metabolites may depen d on resource availability, so nitrogen, a main constituent of glucosi nolates and myrosinase was added in fertilizer treatments to assess co sts. We used the fungal pathogen Leptosphaeria maculans and diamondbac k moth larvae Plutella xylostella as induction agents in comparisons o f the diverged myrosinase populations, We found pleiotropic effects am ong constitutive myrosinase levels and pathogen-induced levels of myro sinase, glucosinolates, and resistance to diamondback moth larvae. In field experiments, genetic increases in myrosinase production were ass ociated with significant decreases in estimated seed production, despi te potential benefits from increased resistance to flea beetles Phylot reta cruciferae. No genotype-by-nitrogen interaction was found. Althou gh costs were detected, our results indicate, in contrast to theory, m ainly positive pleiotropic effects between constitutive levels of seco ndary metabolites and induced responses.