Variation in the response of Mimulus guttatus (Scrophulariaceae) to herbivore and virus attack
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Plants are often simultaneously attacked by insect herbivores and plant pathogens, yet relatively few studies have investigated the potential interactive effects of herbivores and pathogens on plant fitness. We studied the effects of simultaneous attack by meadow spittlebugs, Philaenus spumarius (Homoptera: Cercopidae), and a plant virus, cucumber mosaic virus (CMV), on the fitness of Mimulus guttatus (Scrophulariaceae). We wanted to determine: (1) if trade-offs in defense against meadow spittlebugs and CMV exist, (2) if meadow spittlebugs and CMV interact to affect plant fitness, and (3) if genetic variation is associated with these interactive effects. We found no evidence of trade-offs in defense against meadow spittlebugs and CMV in M. guttatus in a greenhouse experiment. Thus, the ability of M. guttatus to defend itself against one of these enemies is unlikely to preclude the evolution of adequate defenses against the second enemy. We did, however, find strong evidence that spittlebugs and CMV interacted to affect plant fitness and that genetic variation underlies the degree and direction of this interaction. This suggests that selection can act on the genetic variation underlying the interaction between the two enemies and that strong selection imposed by one will alter the response of M. guttatus populations to the second. To our knowledge this is the first study to demonstrate genetic variation associated with the non-additive effect of an herbivore and a pathogen on plant fitness. We suggest that future studies of the mechanisms underlying the defensive properties of plants need to consider variation associated with defense mechanisms and the potential effect of this variation on the response of plant populations to selection by multiple enemies. Springer 2005.