Hakes, A. S. and J. T. Cronin. 2011. Resistance and tolerance to herbivory in Solidago altissima (Asteraceae): genetic variability, costs and selection for multiple traits. American Journal of Botany 98: 1446-1455.
ABSTRACT
To understand how diverse types and levels of plant defense traits evolve, it is necessary to evaluate the contribution of genetics and the environment to trait expression and examine the adaptive value of these traits in response to a gradient of herbivory (i.e., in terms of costs and benefits). We conducted a common garden study to assess genetic variability, costs, selection gradients, and correlations among resistance, tolerance, and various putative defense traits (e.g., relative growth rate, leaf addition and senescence rate, specific leaf area, and leaf toughness) of 103 Solidago altissima genets. To date, no studies have quantified tolerance or examined the adaptive value of any goldenrod defense traits. Genets for this study originated from either an early (ca. 3-years old) or late-successional field (ca 15-years old) that had been studied previously in situ. We found significant genetic variability in resistance, tolerance, and various putative defense traits, but low broad-sense heritability estimates (H2 <0.17) for all traits. For resistance and all other goldenrod traits examined, there was no correlation between trait levels in the common garden and those measured from the same genets in their field of origin. This finding suggests that there is much plasticity in goldenrod defense traits. There was no evidence for costs or benefits (i.e., selection gradient) for resistance. However, there was strong selection favoring increased tolerance and putative tolerance trait levels, and there was a significant fitness cost to tolerance. Our study suggests that tolerance has a high adaptive value in the common garden environment and may increase over time under continued herbivore pressures. However, we expect the evolution of tolerance to be constrained by the low broad-sense heritability estimates in tolerance traits and a strong fitness cost when herbivores are absent.
Keywords: Adaptive landscape; plant defense strategies; phenotypic plasticity; trait evolution
