GENETICS OF BRASSICA RAPA. 3. COSTS OF DISEASE RESISTANCE TO THREE FUNGAL PATHOGENS.

Genetic costs of resistance to pathogens may be an important factor maintaining heritable variation for resistance in natural populations. Pleiotropic fitness trade-offs occur when genetic resistance causes reduction in other components of fitness. Although costs of resistance have an important influence on plant-pathogen interactions, few previous studies have detected pleiotropic costs of resistance in the absence of confounding effects of linkage disequilibrium. To avoid this potential problem, we performed artificial selection experiments on resistance to two fungal pathogens, Leptosphaeria maculans, and Peronospora parasitica, and compared growth rates of resistant and susceptible genotypes of Brassica rapa in the absence of pathogens. Leptosphaeria resistance had no effect on growth rate, indicating cost-free defense. In contrast, Peronospora-resistant genotypes grow 6% slower than Peronospora-susceptible genotypes in pathogen-free environments, indicating a significant genetic fitness cost to Peronospora resistance. Such genetic trade-offs could maintain genetic variation in the wild. Another factor that might explain heritable variation for resistance is ecological trade-offs, in which genetic resistance to one species causes susceptibility to another. Such ecological trade-offs do not exist for the pathogens studied in this system.