Collaborative Research: Plant Tolerance of Herbivory: A Test for Constraints on Adaptation

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9815550SimmsHumans have long been concerned about the impact of insect pests on plant performance. To date ecologists and agronomists have concentrated on plant resistance to attack, that is, the plant's ability to deter potential enemies. However, insect attack is rarely fatal and so plants have the opportunity for a second way to deal with enemy attack, to recover and regrow. The ability to survive and prosper after attack is called tolerance. Compared to the research progress in understanding the genetics and ecology of resistance, very little is known about plant recovery. This knowledge void hampers understanding of the total plant defense system. Not only is it important to study the cost-benefit structure of tolerance in it own right, we also need to understand tolerance to understand the adaptive value of resistance. The interdependence of resistance and tolerance emerges from two considerations. First, resistance and tolerance can be redundant-if a plant can completely recover from attack, there is little profit in also resisting attack. The second consideration arises from optimal defense theory. It is widely hypothesized that resistance factors are costly since resources devoted to resistance cannot also be allocated to growth. Numerous experiments have tested this hypothesis, but with mixed results. Recent models suggest that resistance costs are hard to detect because they are obscured by tolerance costs. But because little is known about tolerance, little is known about tolerance costs. Thus, there is little basis for evaluating these important theoretical predictions. These experiments will look for tolerance costs using a wild mustard species, Brassica campestris, which is the wild progenitor of canola, the important oil seed crop. Artificial selection will be perform to determine if genetic trade-offs impose a fitness cost that constrains the ability of B. campestris to tolerate defoliation. This experiment involves selecting for performance in plant lines that experience sever damage every generation, lines that never experience damage, and lines that experience both. Selection will be by sib-selection. Following several generations of selection, tolerance differences among the lines will be measured by growing them across all damage environments. These experiments, thus, will test several major hypotheses about genetic constraints on tolerance, and will initiate a long-term effort to identify the physiological traits, and the underlying genetic loci, that contribute to plant recovery. In addition, by examining selection response across two environments, these experiments will offer insights on the more general problem of constraints on selection for phenotypic plasticity.H:\popbio\abstracts\fy1999\9815550 (Simms abstract)