Genetic constraints and selection acting on tolerance to herbivory in the common morning glory Ipomoea purpurea

Tolerance to herbivory minimizes the effects of herbivory on plant fitness. In the presence of herbivores, maximal levels of tolerance may be expected to evolve. In many plant species. however, tolerance is found at an interm ediate level. Tolerance may be prevented from evolving to a maximal level b y genetic constraints or stabilizing selection. We report on a field study of Ipomoea purpurea, the common morning glory in which we measured three ty pes of costs that may be associated with tolerance and the pattern of selec tion acting on tolerance to two types of herbivore damage: apical meristem damage and folivory. We used genetic correlations to test for the presence of three types of costs: a trade-off between tolerance and fitness in the a bsence of herbivore damage, a trade-off between tolerance and resistance, a nd genetic covariances among tolerance to different types of damage. We fou nd no evidence that tolerance to apical meristem damage or tolerance to fol ivory was correlated with resistance, although these two types of tolerance were positively correlated with one another. Tolerance to both types of da mage involved costs of lower fitness in the absence of herbivory. Selection acting on tolerance to either type of herbivory was not detected at natura l levels of herbivory. Selection is expected to act against tolerance at re duced levels of herbivory and favor tolerance at elevated levels of herbivo ry. In addition, significant correlational selection gradients indicate tha t the pattern of selection acting on tolerance depends on values of resista nce. Although we found no evidence for stabilizing selection, fluctuating s election resulting from fluctuating herbivore loads may be responsible for maintaining tolerance at an intermediate level.