GENETICS OF BRASSICA-RAPA .3. COSTS OF DISEASE RESISTANCE TO 3 FUNGALPATHOGENS

Genetic costs of resistance to pathogens may be an important factor ma intaining heritable variation for resistance in natural populations. P leiotropic fitness trade-offs occur when genetic resistance causes red uction in other components of fitness. Although costs of resistance ha ve 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 potent ial problem, we performed artificial selection experiments on resistan ce to two fungal pathogens, Leptosphaeria maculans, and Peronospora pa rasitica, and compared growth rates of resistant and susceptible genot ypes of Brassica rapa in the absence of pathogens. Leptosphaeria resis tance had no effect on growth rate, indicating cost-free defense. In c ontrast, Peronospora-resistant genotypes grow 6% slower than Peronospo ra-susceptible genotypes in pathogen-free environments, indicating a s ignificant genetic fitness cost to Peronospora resistance. Such geneti c trade-offs could maintain genetic variation in the wild. Another fac tor that might explain heritable variation for resistance is ecologica l trade-offs, in which genetic resistance to one species causes suscep tibility to another. Such ecological trade-offs do not exist for the p athogens studied in this system.