Cost of glandular trichomes, a "resistance" character in Datura wrightii Regel (Solanaceae)

Models regarding the evolution of plant resistance to herbivory often assum e that the primary mechanism maintaining resistance polymorphisms is the ba lance between benefits of increased resistance to herbivores and costs asso ciated with the production of a resistance character. However, rarely has i t been demonstrated that genetically based resistance traits are costly. He re, we document costs associated with the production of glandular trichomes , a resistance character in Datura wrightii that is predominantly under the control of a single gene of large effect. In the absence of herbivores, pl ants with glandular trichomes (sticky) produced 45% fewer viable seeds than plants with nonglandular trichomes (velvety). Although both plant types fl owered with similar frequency, sticky plants matured fewer capsules and few er of their seeds germinated. The fitness difference between the types in h erbivore-free conditions was not mitigated by the addition of water, a pote ntially limiting resource for sticky plants. Under herbivore pressure, ther e was no significant fitness difference between the types, although the fit ness of velvety plants was still higher than that of sticky plants. This oc curred even though velvety plants sustained more herbivore damage than stic ky plants and were more likely to be attacked by most herbivore species pre sent. The fitness difference between the plant types was especially reduced when herbivore-attacked plants were watered, which indicates that sticky p lants may have higher tolerance for damage than velvety plants when supplie d with a potentially limiting resource. Yet, the maintenance of a fitness d eficit (albeit small and nonsignificant) for sticky plants when attacked by herbivores indicates no net benefit associated with the production of glan dular trichomes in this first year of our study. These results add to our c urrent understanding that herbivore resistance characters can be costly and raise the question of how this genetic polymorphism is maintained in wild populations.