Plant-herbivore interactions in domesticated crops: Does selection for increased fruit size in apples affect chemical defense and herbivore performance?
The domestication and breeding of crops can be viewed as the longest running selection experiment in history. Most crops have been dramatically altered from their wild ancestors, with one of the most impressive changes involving gigantism of edible parts and an overall increase in yield. However, plant defense theory suggests that selection for faster growth and higher yields may result in resource allocation trade-offs that reduce plant investment in other functions, such as the production of secondary metabolites involved in defense. We used apples and their insect herbivores as a case study to test whether selection for increased fruit size has affected herbivore resistance. We examined variation in fruit size, production of phenolics, and resistance to codling moth (Cydia pomonella) across 50 genotypes of wild apples (Malus sieversii) and 50 genotypes of domestic apples (Malus x domestica), utilizing the USDA germplasm collection in Geneva, NY.
Our preliminary results show that fruit phenolic production decreases with increasing fruit size, supporting the hypothesis of resource allocation costs associated with increased yield. Furthermore, larval performance, as estimated by pupal weight, decreased with increasing phenolic content in fruits. However, we found variable patterns for the relationship between fruit size and resistance. Contrary to expectation, larval survival rates decreased with increasing fruit size. Larval performance, as estimated by pupal weights, increased with increasing fruit size, but only for wild apples. This suggests that although herbivores do perform better on fruits with lower phenolic content, other factors mediate larval survival and may be more important than fruit size in determining overall variation in resistance to codling moth. These results provide an improved understanding of the mechanisms by which domestication, and in particular selection for increased yield, can influence the trajectory of plant-herbivore interactions in agroecosystems.