Many plant defence traits (e.g., secondary metabolites, leaf hairs, spines) have been shown to have a genetic basis, which means that processes such as mating system, gene flow and population size should affect their expression, function and distribution in nature. However, few studies have explicitly linked these processes with plant defence or patterns of herbivory. This study focuses on mating system (inbreeding vs. outcrossing), which is a crucial life-history variable for many plant species. I tested the hypothesis that host plant inbreeding should reveal recessive, deleterious mutations at loci linked to plant defence, thereby altering the expression of defence traits, and exacerbating the fitness consequences of herbivory relative to outcrossing. Using horsenettle, Solanum carolinense, and its herbivorous insect fauna as a study system, I tested the effects of three different mating strategies (self-fertilization, biparental inbreeding and outcrossing) on the expression of nine putative resistance traits (levels of secondary metabolites and defensive proteins) in a large field study.
Results/Conclusions
Measurements of foliar chemistry and damage by the predominant herbivore (Epitrix flea beetles) revealed widespread alterations to defence traits that led to a significant 61% increase in herbivory as a result of inbreeding. Genetic covariance of inbreeding effects on defence traits and damage may underlie this substantial ecological inbreeding depression. My results suggest that variation in plant defences and herbivory, within and among both populations and species, may be explained to a significant extent by plant mating strategy. These findings also support classical theory on inbreeding depression (as applied to plant defences) and provide the first trait-level, mechanistic analysis for herbivory-mating system interactions. Preliminary results indicate that increased herbivory could depress plant performance in inbred plants, and thereby constitute an additional selective force in the evolution and maintenance of plant mating systems.