Plant-herbivore interactions in natural Brassica oleracea communities

Co-evolutionary interactions between plants and herbivores are suggested to be the driving force behind the high diversity observed in plant secondary metabolites. These compounds play an important role in herbivore resistance mechanisms in many plant species. An individual plant can produce and store a number of structurally different secondary compounds. Variation in plant chemical profiles is commonly observed within and between natural populations across a wide range of taxa, yet the ecological importance of this variation is still a major question in the area of plant-herbivore interactions. In this thesis I use wild cabbage (Brassica oleracea var. oleracea) plants in twelve naturally established populations to investigate plant-herbivore interactions mediated by structural variation in aliphatic glucosinolates, a class of secondary metabolites produced by the Brassicaceae. Overall, the results showed that several herbivore species respond to the genetically determined variation in glucosinolate profile, indicating that the structure of the local herbivore community can be influenced by variation in plant defence chemistry. In addition, the direction of herbivore responses to different plant chemical phenotypes differed between species. A finer scale study which focused on the interactions between an herbivore and aliphatic glucosinolate variation supported the general trend observed in the large scale study. Glucosinolate profile was also found to have an impact on plant seed set. The findings show that glucosinolate profiles may be under selection in these natural plant populations and provide some support for the role of herbivores in the maintenance of secondary metabolite diversity.ES