Do inbred plants exhibit less compensation than outcrossed ones in response to simulated herbivory?

Background/Question/Methods Inbred plants frequently display reduced measures of fitness compared to outcrossed ones. Plants also exhibit plasticity when confronted with damage by herbivores, ranging from inducible chemical defenses to overcompensation. These factors have rarely been examined in combination, although increased plasticity may be an important factor in determining the ecological advantages of outcrossed plants. We conducted a greenhouse experiment to discover if plastic responses in female fitness and corolla traits depended on simulated herbivore damage to Mimulus guttatus (Phrymaceae). Inbred and outcrossed siblings within each of four populations from Northern California were used, and vertebrate herbivory was simulated by removing the apex of each plant as they flowered. Total flower number, corolla width, height, and tube length were all measured 2 months after the damage treatment.

Results/Conclusions There were large significant effects of population, simulated herbivory, and mating type on total flowers produced (all P < 0.001), but no significant interactions among them. One population produced significantly more flowers than the others, simulated herbivory significantly decreased flower number, and inbred plants produced significantly fewer flowers versus outcrossed plants. For the floral traits, there was a significant population X mating type interaction for corolla width because in two populations outcrossed plants made bigger flowers but within the other two populations there was no difference between the mating types. Corolla height was not significantly affected by any of the variables or their interactions, and population significantly affected corolla tube length. We did not observe compensation and there were no differences in plastic responses due to mating type, suggesting that outcrossed plants may thrive due to other factors, like bigger overall size and seed number. It would be useful to determine if mating type affected other traits, like chemical defense or response to drought, as theory suggests that plasticity may be an important, although overlooked, trait favoring outcrossed versus inbred populations in nature.