PHYSIOLOGY AND COSTS OF RESISTANCE TO HERBIVORY AND DISEASE IN BRASSICA

We used artificial selection experiments to study genetic allocation c osts and physiological mechanisms of resistance to herbivory and funga l disease. Genetic costs to resistance were present in some instances and absent in others. Genetic resistance to the fungal pathogen, Lepto sphaeria maculans was cost-free, while resistance to Peronospora paras itica showed a negative genetic correlation between disease resistance and growth rate. Leptosphaeria resistant genotypes had 13% higher chi tinase activity. Genetic increases in myrosinase activity were correla ted with increased resistance to flea beetles (Phyllotreta cruciferae) , but resulted in lower plant fecundity, presumably due to production costs of myrosinase. Genetic costs of resistance may maintain genetic variation in natural plant populations. These studies demonstrate the predictive and explanatory power of a functional approach to plant-her bivore and plant-pathogen interactions.