Abstract
The diversity of secondary metabolites (SMs) has been poorly understood from both a mechanistic and a functional perspective. Hybridization is suggested to contribute to the evolution of diversity of SMs. In this paper we discuss the effects of hybridization on SMs and herbivore resistance by evaluating the literature and with special reference to our own research results from the hybrids between Jacobaea vulgaris (syn. Senecio jacobaea) and Jacobaea aquatica (syn. Senecio aquaticus). We also review the possible genetic mechanism which causes the variation of SMs and herbivore resistance in hybrids. Most SMs in hybrids are present in the parents as well. But hybrids may miss some parental SMs or have novel SMs. The concentration of parental SMs in hybrids generally is constrained by that in parental plants, but transgressive expression was present in some hybrids. Hybrids may be as susceptible (resistant) as the parents or more susceptible than the parents, but rarely more resistant than the parents. However, different hybrid classes (F1, F2, backcrossing and mixed genotypes) show different patterns in relation to herbivore resistance. The variation in SMs and herbivore resistance occurring in hybrids could be explained by complicated genetic mechanisms rather than a simple one-gene model. Most previous work in this field only reported mean trait values for hybrid classes and few studies focused on genotype differences within hybrid classes. Our study in Jacobaea hybrids showed transgressive segregation in most SMs and herbivore resistance. To summarize, our article shows that hybridization may increase the variation of SMs and affect herbivore resistance, which may partially explain the evolution of chemical diversity in plants.