Resistance and tolerance to herbivory in Solidago altissima (Asteraceae): Genetic variability, costs, and selection for multiple traits
Premise of the study: Quantifying the genetic variability, fitness costs, and selection gradients associated with plant defense traits is necessary to understand their evolution and continued persistence in populations. Few studies have simultaneously examined the costs, benefits, and genetic variability in multiple traits related to plant resistance and tolerance to herbivory.Methods: Using 103 Solidago altissima (Asteraceae) genets from two populations previously studied in situ, we conducted a common garden experiment to assess genetic variability, costs, selection gradients, and correlations among resistance, tolerance, and various resistance and tolerance traits (i.e., lateral branching, relative growth rate, leaf addition and senescence rate, specific leaf area, and leaf toughness).Key results: We report evidence for significant genetic variability in resistance and various tolerance-related traits but low broad-sense heritability (H 2 < 0.14) for all traits. For all traits examined, no correlation existed between trait levels of parent ramets (measured in their field of origin) and daughter ramets (measured in the common garden), suggesting plasticity in goldenrod traits. We found a strong cost of resistance and selection gradient against high resistance. Conversely, we found no evidence of costs but did find significant selection gradients favoring increased tolerance and many tolerance trait levels.Conclusions: Our study suggests that herbivores impose selection favoring increased tolerance and reduced resistance in goldenrods. In this environment, we expect that over time, resistant genets will decrease in frequency. Despite strong selection pressures, the evolution of tolerance in this environment may be constrained by the low broad-sense heritability in tolerance traits. © 2011 Botanical Society of America.