6 Plant hybridization and secondary metabolite expression: a case study of pyrrolizidine alkaloids in the genus

Among other consequences in plants, hybridization may influence the expression and evolution of resistance to natural enemies. Plant resistance to parasites, including both microbial pathogens and herbivores, is often mediated by the composition of second-ary metabolites expressed by the plant. Here, we focus on pyrrolizidine alkaloids (PAs), a class of secondary metabolites that exhibits high structural diversity among species within the plant genus Senecio. We examine PA composition in Senecio jacobaea, S. aquaticus, artificially generated F1 hybrids, and also later gener-ation natural hybrids between these two species. We test the hypothesis that hybridization may contribute to PA diversity with-in plants, by comparing PA expression in hybrids to that in parents across a range of water and nutrient treatments. We report that hybrids produce a putatively novel PA, and that this PA is con-served in natural hybrids, which are highly backcrossed to S. jacobaea. Also, the range of PA concentration and diversity is more extreme in the roots and shoots of artificial hybrids over var-ious environmental conditions, and high ranges are preserved in the shoots of natural hybrids. These results suggest that hybridiza-tion may partially explain the diversity of PAs found within the Senecio genus. Also, hybridization may increase the variation upon which selection can act in hybrid populations, such that some hybrids have the potential to be more resistant to natural enemies than parental individuals