Genetic variation for tolerance to insect herbivory in Boechera stricta (Brassicaceae): Influence of type of damage, nature of glucosinolates, and costs

Background/Question/Methods Tolerance to damage is a well-known mechanism of plant defense to insect herbivory, but the ecological basis underlying intra-specific variation in such tolerance still is not clear. Although not always detected, ecological costs and the presence of trade-offs with resistance traits have been pointed out as major factors that maintain such intra-specific variation in tolerance. Likewise, other ecological factors such as the interactive ecological context where the plants grow, the availability of soil nutrients, and the effect of the ontogeny have been proven to play an important role in the origin and maintenance of the natural variation in plant tolerance to damage by herbivores. In contrast to research in resistance, the potential role of the type of damage yielded by different insect herbivores and/or the effect of different constitutive and induced secondary compounds is much less known. Here we present the results of two lab/common garden experiments to investigate (i) the degree of genetic variation for tolerance in Boechera stricta, (ii) the effect of different herbivores (specialists versus generalists lepidoptera) on B. stricta plants with a distinct chemical background (Valine or Methionine derived glucosinolates) and (iii) the presence of ecological costs and trade-offs with resistance traits. Results/Conclusions Insect herbivory had an important effect on all of the fitness components analyzed (flowering time, number of flowers and seeds, and reproductive biomass), however, both the magnitude and sign of effects depended largely on the plant genotype. We recorded significant genetic variation for tolerance, ranging between genotypes that were able to fully compensate after the herbivory damage, and others where herbivory damage had a dramatic reduction in fitness. The type of damage caused by different insect herbivores did not affect fitness, although herbivores differed significantly in their patterns of vegetative damage. We did no detect any ecological cost for tolerance. Glucosinolate induction depended on type of insect herbivore: after 24 hours plants damage by specialists lepidoptera presented higher total amount of glucosinolates than plants undamaged, damaged by generalist lepidoptera or plants wounded manually. Effect of the type of damage on induction was similar across genotypes, suggesting that there is no genetic variation for induction. However, we recorded significant genetic variation for constitutive glucosinolates. Induction level was not dependent on the nature of the glucosinolates. Finally, the presence of trade-offs with constitutive glucosinolates depended on the nature of the glucosinolates.