Intraspecific variation in plant chemistry and implications for ecological interactions

Abstract

Plants produce a wide range of secondary metabolites (PSMs) that are involved in defence against herbivores and other natural enemies as well as influencing with higher trophic levels. Variation of PSMs within species can be both quantitative and qualitative. In this research thesis I focused on intraspecific qualitative variation in PSMs and its consequences on ecological interactions, particularly between plants and herbivores. I used a range of experimental approaches and three different plant species to answer fundamental questions about how plant intraspecific variation in chemistry arises and its effects on herbivores and pathogens, at different scales. First, I explored how chemical polymorphisms or chemotypes in terpene composition of Australian tea tree Melaleuca alternifolia affect specialist herbivores. Although there was little evidence of effects on herbivore performance, the leaf beetle Faex sp. preferred to lay eggs on some chemotypes depending on the larval diet. Similarly, the severity of infection of the pathogen myrtle rust (Puccinia psidii) depended on the chemotype. I then showed that feeding by herbivores increases the volatile emissions from M. alternifolia and changes terpene composition. However, I found no evidence that these emissions affected the behaviour of specialized beetles or their fly parasitoid. Following this, I studied how the expression of terpene synthase genes and genome-wide differential expression of terpene pathway genes varies amongst chemotypes of M. alternifolia. I also used plants of the wild cabbage Brassica oleracea to test whether spatial variation in a different group of PSMs (glucosinolates) among neighbouring plants affected the properties of the herbivore community and the individual traits of the plants. I discovered that a higher diversity in glucosinolates among neighbouring plants correlated with higher herbivore abundance and diversity. It also led to increased plant size and reduced damage by herbivores, probably due to the effects of herbivore interference competition. Lastly, I described the terpene variation in the widest distributed eucalypt species Eucalyptus camaldulensis across the Australian continent and showed that increases in CO2 concentration can reduce the concentration of terpenes. In general, I found that the effects of plant chemical variation depend on the plant system. I showed that terpene chemotypes in M. alternifolia originate through differences in expression of terpene synthase genes. Although variation amongst the terpene chemotypes of M. alternifolia has few effects on natural enemies, it might be more important for interactions with invasive pathogens like the myrtle rust. Intraspecific chemical variation can have spatial effects that scale up to the community level and benefit whole plant populations as demonstrated with the B. oleracea experiments. In addition, changes in CO2 can influence the expression of terpenes in a widely distributed species such as E. camaldulensis which may have potential effects on ecological interactions under global climate change. With this body of work I explored the importance of intraspecific variation in PSM in an ecological context from different perspectives and in different biological systems. Each set of experiments opened new questions but contribute to increasing our understanding of the origins and effects of intraspecific variation in PSMs.