The evolution of tolerance to damage in Gentianella campestris: natural selection and the quantitative genetics of tolerance

In the framework of phenotypic plasticity, tolerance to browsing can be ope rationally defined as a norm of reaction comparing plant performance in und amaged and damaged conditions. Genetic variation in tolerance is then indic ated by heterogeneity in the slopes of norms of reaction from a population. We investigated field gentian (Gentianella campestris) tolerance to damage in the framework of phenotypic plasticity using a sample of maternal lines from natural populations grown under common garden conditions and randomly split into either a control or an artificial clipping treatment. We found a diversity of tolerance norms of reaction at both the population and famil y level: the impacts of clipping ranged from poor tolerance (negative slope ) to overcompensation (positive slope). We detected heterogeneity in tolera nce norms of reaction in four populations. Similarly, we found a variety of plastic architectural responses to clipping and genetic variation in these responses in several populations. Overall, we found that the most tolerant populations were late flowering and also exhibit the greatest plastic incr eases in node (meristem) production in response to damage. We studied damag e-imposed natural selection on plasticity in plant architecture in 10 of th e sampled populations. In general, there was strong positive direct selecti on on final number of nodes for both control and clipped plants. However, t he total selection on nodes (direct + indirect selection) within each treat ment category depended heavily on the frequency of damage and cross-treatme nt genetic correlations in node production. In some cases, strong correlate d responses to selection across the damage treatment led to total selection against nodes in the more rare environment. This could ultimately lead to the evolution of maladaptive phenotypes in one or both of the treatment cat egories. These results suggest that tolerance and a variety of architectura l responses to damage may evolve by both direct and indirect responses to n atural selection. While the present study demonstrates the potential import ance of cross-treatment genetic correlations in directing the evolution of tolerance traits, such as branch or node production, we did not find any st rong evidence of genetic trade-offs in candidate tolerance traits between u ndamaged and damaged conditions.