Plant-animal interactions and fruit-trait evolution
Many of the most intricate and fundamental interactions between plants and their herbivores, pathogens, and pollinators are mediated by plant chemistry. Many plants produce a diverse array of chemicals in ripe fruit, but the general importance of this chemistry in mediating interactions between plants and their seed dispersers, seed predators, and fruit pathogens remains largely unexplored. Given the tremendous influence of plant chemistry on plant-herbivore interactions, we are attempting to integrate chemical ecology into general theories of fruit-trait evolution.
The current view of fruit-frugivore interactions generally falls short on two fronts:
- a general failure to acknowledge and integrate the impacts of the diverse array of fruit and seed consumers that affect plant fitness. Fruits are often consumed by a wide range of organisms, including microbes, fungus, invertebrates and vertebrates, and failure to account for these consumer groups, and the interactions between consumers, has led to an over-simplification of the selective pressures, adaptive landscapes, and co-evolutionary dynamics that may characterize the ecology of fruit-frugivore interactions and constrain the evolution of fruit phenotypes.
- a lack of synthetic studies linking ecological benefits and costs of defensive chemical
production with a detailed knowledge of the metabolic pathways and genetic regulation
required for the production of these chemicals. Our understanding of the adaptive significance
of plant chemistry will depend on an understanding of the metabolic, as well as ecological
tradeoffs involved in chemical protection of ripe fruit. .
Why chilies are hot:
an integrative exploration of secondary metabolite
function in ripe fruit: In this project, we use wild chilies (genus Capsicum)
and capsaicinoids (the chemicals responsible for the chili's fiery taste) as a model
system to address both of the shortcomings listed above. This work spans two continents,
as we examine the costs, benefits, and genetic regulation of capsaicinoids in both the
derived C. annuum in Arizona and Mexico and a series of ancestral species that
exhibit stable polymorphisms for the production of capsaicinoids. These species are
found in Bolivia. The agricultural roots, economic importance, and wide variation in
chemical protection provide a strong foundation for exploring tradeoffs involved in chemical
protection of ripe fruit. Much of our current work focuses on C. chacoense, an
ancestral species of chilies exhibiting a natural polymorphism for the production of
capsaicinoids: in some populations, all plants contain capsaicinoids, while in others,
most plants are non-pungent, and completely lack capsaicinoids (see Tewksbury et al. 2006
for details). Over the past five years, we have studied the biogeography and natural history of avian seed dispersers, insect fruit and seed consumers before and after dispersal (Heteropteran insects before dispersal and ants after dispersal) and a fungal fruit and seed consumer in the genus Fusarium that is actively transferred between plants by the foraging of the heteropteran insects (Fig. 1).
Fusarium has
strong negative impacts on seed germination, and capsaicinoids have strong negative
impacts on Fusarium growth. We have developed
and are currently testing a co-evolutionary framework to explain local variation
in fruit phenotypes (capsaicinoid production)and fungal resistance to capsaicinoids
(fig 2). Lab members involved in this work include Noelle Machnicki, Tomas Carlo,
Susan Taylor and Cat Adams. Collaborators include Doug Levey, at the University
of Florida.Concurrent with this work, we are investigating tradeoffs involved in capsaicinoid production, working from ecological costs and benefits through morphological and physiological tradeoffs to molecular mechanisms and genetic control of metabolic end-products. This work focuses on understanding the impacts of capsaicinoid production on the rest of the phenylpropanoid metabolic pathway during developing fruit, and identifying and mapping the genes that control this process and is being spear-headed by David Haak.
