International Journal of Modern Botany 2013, 3(2A): 2633 DOI: 10.5923/s.ijmb.201310.04 Genetic Architecture of Tolerance to Foliar Damage in a Salix Hybrid System Cris G. Hochwender 1,* , Mary Ellen Czesak 2 , Crystal Harmon 1 , Bre nt Mock 1 1 Department of Biology, University of Evansville, 1800 Lincoln Ave, Evansville, IN 47722, USA 2 Department of Biology, Vassar College, 124 Raymond Ave, Poughkeepsie, NY 12604, USA Abstract The release of apical dominance following the browsing of stems can alter plant architecture and provide tolerance to herbivore damage. Meristem changes are not associated with foliar damage, so tolerance responses to foliar feeding may not be robustly expressed. In this study, tolerance to foliar damage was characterized for six genetic classes of willows in an interspecific hybrid system (pure Salix eriocephala plants, pure S. sericea plants, F 1 hybrids, F 2 hybrids, backcrosses to S. sericea, and backcrosses to S. eriocephala). This characterization allowed the genetic architecture of tolerance to be explored. Plagiodera versicolora, a small leaf beetle, was used to inflict foliar damage. Cuttings ofgenetically identical plants were matched to create a metric ofbiomass tolerance (defined here as the ratio of biomass for a damaged plant relative to an undamaged plant). Initial size differences between cuttings influenced the relative performance of damaged/undamaged pairs, so a ratio of the cuttings (damaged/undamaged) was calculated using initial wet weights. The ratio of cutting weight explained meaningful variance in biomass tolerance (F 1,68 = 71.4; P = 0.0001; r 2 = 0.51). Residual variance in biomass tolerance (the variance remaining in biomass tolerance following removal of variance explained by the ratio of cutting weight) was used to reduce the effect of differences in initial plant size for damaged versus undamaged plants. Residual variance in biomass tolerance differed significantly among genetic classes (F 5,64 = 2.7; P = 0.03). Using a Tukey posthoc test, F 1 hybrids had significantly greater tolerance to foliar damage than backcrosses to S. sericea. Using line cross analysis,the model expressing the genetic architecture for tolerance in this hybrid systemincluded additive genetic effects (a) + dominance–dominance epistasis (dd). For this willow system, tolerance to damage appears to be a fundamental response, whether damage is associated with browsing or foliar damage. Given the epistatic interactions observed in the current study, coupled together with the potential complexity of growth/storage traits associated with the mechanisms of tolerance, the trajectory for the evolution of tolerance challenges easy interpretations. Keywords Compensation, Compensatory Response, Gene Interaction, Herbivory, Heterosis, Hybridization, Speciation 1. Introduction Genetic variation in plant tolerance to damage often exists, suggesting that evolution of tolerance is a possible response to herbivores.[1,2,3,4,5] Although the majority of tolerance research has centered on herbaceous plants (see citations above), the greater apparency of woody perennials[6] should favor the evolution of increased plant tolerance to damage for woody species.[7] Moreover, woody plants accumulate and store resources over multiple years, favoring the use of stored resources to compensate for damage. Studies have documented compensatory responses across a range of woody plant species.[8,9,10] However, type of herbivory may influence whether woody plants show complete compensation, overcompensation, or * Corresponding author: Ch81@evansville.edu (Cris G. Hochwender) Published online at http://journal.sapub.org/ijmb Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved undercompensation for damage.[11] For example, compensatory patterns may differ depending on whether damage is due to browsing of stems or restricted to foliar damage. Commonly, overcompensation to damage has been associated with a release of meristems after browsing. [2,12,13,14,15] Damage to apical meristems changes plant architecture by releasing apical dominance and increasing growth of axillary meristems, thereby increasing branching. [16] In contrast to apical browsing, foliar damage is not associated with the same change in meristem growth, so the compensatory response in such cases may not be as great for this damage type. Speciesspecific effects for tolerance have also been documented for woody;[17,18] however, information is lacking on the genetic architecture of tolerance. Populations or species can differ from each other through additive, dominance, or epistatic genetic effects on a trait, and the relative contribution of these genetic differences (i.e., genetic architecture) can be estimated by comparing means among genetic classes (i.e., pure species and hybrids[19]).