The evolution of plant response to herbivory: simultaneously considering resistance and tolerance in Brassica rapa |
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Authors: | Diana Pilson |
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Institution: | (1) School of Biological Sciences, University of Nebraska, Lincoln, NE, USA |
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Abstract: | Although the evolution of plant response to herbivory can involve either resistance (a decrease in susceptibility to herbivore
damage) or tolerance (a decrease in the per unit effect of herbivory on plant fitness), until recently few studies have explicitly
incorporated both of these characters. Moreover, theory suggests these characters do not evolve independently, and also that
the pattern of natural selection acting on resistance and tolerance depends on their costs and benefits. In a genotypic selection
analysis on an experimental population of Brassica rapa (Brassicaceae) I found a complex set of correlational selection gradients acting on resistance and tolerance of damage by
flea beetles (Phyllotreta cruciferae: Chrysomelidae) and weevils (Ceutorhynchus assimilis: Curculionidae), as well as directional and stabilizing selection on resistance to attack by weevils. Evolution of response
to flea beetle attack is constrained by a strong allocation cost of tolerance, and this allocation cost may be caused by a
complex correlation among weevil resistance, weevil tolerance, flea beetle resistance, and flea beetle tolerance. Thus, one
important conclusion of this study is that ecological costs may involve complex correlations among multiple characters, and
for this reason these costs may not be detectable by simple pairwise correlations between characters. The evolution of response
to weevil attack is probably constrained by a series of correlations between weevil resistance, weevil tolerance, and fitness
in the absence of weevil damage, and possibly by a cost of tolerance of weevil damage. However, the nature of these constraints
is complicated by apparent overcompensation for weevil damage. Because damage by both flea beetles and weevils had non-linear
effects on plant fitness, standard measures of tolerance were not appropriate. Thus, a second important contribution of this
study is the use of the area under the curve defined by the regression of fitness on damage and damage-squared as a measure
of tolerance.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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Keywords: | Brassica campestris Brassica rapa Ceutorhynchus assimilis compensation correlational selection herbivory multiple herbivores natural selection overcompensation Phyllotreta cruciferae plant fitness resistance tolerance |
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