The evolution of compensation to herbivory in scarlet gilia, Ipomopsis aggregata: herbivore-imposed natural selection and the quantitative genetics of tolerance

Tolerance is the ability of plants to maintain fitness after experiencing herbivore damage. We investigated scarlet gilia tolerance to browsing in the framework of phenotypic plasticity using both an operational and candidate trait approach. Individuals from full-sib families were split into an artificial clipping treatment, a natural-damage treatment, or left as controls. We tested for genetic variation in tolerance by evaluating family x herbivory treatment interactions on fitness in a mixed model analysis of variance. In addition, we used selection analyses to assess the function of flowering phenology and compensatory regrowth (via branch production) as candidate tolerance traits. We found a strong detrimental fitness effect of browsing and considerable variation among sire half-sib families in levels of tolerance (25% to 63% of the fitness of controls). There was no evidence of overcompensation at either the population or family level and no additive genetic variation in operationally defined tolerance. Phenotypic selection analyses provide evidence that early flowering and compensatory regrowth function as tolerance characters. We found strong linear and correlational selection for early flowering and increased branch production for damaged plants and linear selection for apical dominance (reduced branchiness) and early flowering in control plants. Moreover, reduced phenological delay and increased plasticity in branch production were correlated with tolerance. We detected significant additive genetic variation in flowering phenology in both treatments and a positive genetic correlation between the phenology of control and damaged plants. We found significant additive genetic variation in branch production in undamaged and naturally damaged plants, but not in clipped plants. Damaged plants exhibited marginally significant additive genetic variance in fitness, although its heritability was very low (approximately 3.6%). We failed to find additive genetic variation in the fitness of control plants. Our results suggest that tolerance traits are under herbivore-imposed natural selection in this population, but that responses to selection are limited by available genetic variation and selective constraints.