Pyrrolizidine alkaloid variation in Jacobaea hybrids : influence on resistance against generalist and specialist insect herbivores

Plants produce a vast array of secondary metabolites (SMs). Some hypotheses were put forward to explain the diversity of structurally related SMs within the framework of plant defense against herbivores:1) SMs could be Selectively Neutral; 2) SM diversity is a result of the __Arms Race__ between plants and the herbivores; 3) Plants benefit from the SM diversity because of the Synergistic Effects among the SMs; 4) The SM diversity may be a response to the Selection from Multiple Herbivores. In this thesis, a hybrid family originated from an artificial cross between Jacobaea aquatica and Jacobaea vulgaris was used as a model system to study the selective forces from insect herbivores on pyrrolizidine alkaloids (PAs) evolution. The Jacobaea hybrid family showed great variation in PA variation and herbivore resistance. By means of three bioassays, one with a specialist (Tyria jacobaea) and two with generalist insect herbivores (Frankliniella occidentalis and Liriomyza trifolii), I could show that Jacobaea hybrid genotypes differed in the resistance to these herbivores and these differences were related to PA variation in the plants. Several PAs were involved in the resistance to insect herbivores. These results support the three hypotheses except Selectively Neutral Theory.