Genetic- and chemical-based resistance to two mammalian herbivores varies across the geographic range of Eucalyptus globulus

Background: The lifetime fitness of a plant depends in part on how it responds to interspecific biotic interactions and these interactions can play major roles in shaping the evolution of many phenotypic traits. The interactions between a single plant and its many herbivores may occur independently, or the direction and rate of any evolutionary change in plant defensive traits may be a result of interacting selection pressures. Questions: Does a single genetically based defensive trait in Eucalyptus globulus confer resistance to two mammalian herbivores? Is there a genetically based correlation for resistance to browsing by alternative herbivores? Does the strength of the relationship between plant defensive chemistry and resistance to both herbivores differ across the geographic range of E. globulus? Methods: We offered foliage from a common garden trial (from different genetic hierarchies; race, population, and family) to two herbivores and analysed plants for defensive chemistry. We examined the relationship between relative plant resistance to both herbivores and plant secondary chemistry, both across and within races of E. globulus. Furthermore, we examined the genetic-based correlation in preference between herbivores. Results: Resistance to both herbivores is mediated by genetically based variation in formylated phloroglucinol compounds, particularly sideroxylonal. There was a positive correlation in herbivore resistance among plant populations reared under common conditions and a correlation in resistance within races, thus the preferences and subsequent selection imposed by alternate herbivores are not independent. We discuss these results in light of herbivores as possible agents of selection on plant secondary metabolites in E. globulus. In addition, the relationship between chemistry and foliage susceptibility varies across the geographic range of E. globulus, suggesting that the extant selection surface is not uniform within the species. © 2010 Julianne M. O'Reilly-Wapstra.