Bustos Segura, Carlos Eduardo
Description
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...[Show more] 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.
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