Year of Award

2015

Document Type

Dissertation - Campus Access Only

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology and Ecology

Department or School/College

Division of Biological Sciences

Committee Chair

Bret Tobalske

Commitee Members

H. Arthur Woods, Andre Kessler, Anna Sala, John Maron

Abstract

We have all experienced sensory overload. Imagine a crowded night-market in a foreign country, filled with visuals and sounds. The shouts of vendors mix with the sizzle of fried food, music blaring from overdriven homemade sound-systems on the back of trucks, all cast in a glow of yellow incandescence and garish, flashing neon. The combination can be overwhelming, even disorienting. Scents blend together too, a mixture of garbage, sweet perfume drifting out of a nearby shop, roasting meat and day-old fish. Though odors can be as evocative as sights and sounds, they rarely inhibit our ability to communicate with each other, or =ind our way through the world around us. Moreover, we don’t perceive even a fraction of their variation and potency - we don’t have the biological systems required. As humans, we live in a world that is dominated by our sensory experience formed by light and sound.

This bias has had profound impacts on research in the last century. Our technological and engineering feats in communication systems use light and sound, not chemistry, and have helped generate fundamental concepts of information, how it operates, and the best ways to move it around. Though there is an abundance of research in biology on olfaction and chemoreception, theories of how communication and signaling evolves have focused almost exclusively on systems of sight and hearing. Endler’s light-dappled forests (1993) and Morton’s acoustically complex forests (1975) shape the way we think about the evolution and ecology of animal communication because they use concepts from engineered human systems to inform similar systems of biology. One thing is clear from these theories: signals are not sent and received in a vacuum, and the environment can impose noise on biological communication systems, just as it distracts us in the chaos of a market. We know that noise can affect the evolution and ecology of visual and auditory communication, but what about odors?

First, we know that many organisms have extraordinary abilities to detect chemicals in the environment, to the point where some insects can detect a single molecule of an airborne pheromone, a feat unmatched by modern chemical analyses. Second, we have learned in the past 30 years that chemical communication is not relegated to animals, and that the Tolkienesque fantasy of talking-trees is perhaps more true than we thought; we simply didn’t recognize the language. Plants frequently release chemicals into the surrounding atmosphere when attacked by herbivores. These compounds appear to be used for a variety of functions including within-plant signaling, amongplant signaling, and even signaling between plants and insects. We’ve begun to decode the language of plants, and the mechanisms by which signals are generated and received, but the evolutionary history and ecological dynamics of these systems are still in question. Researchers are just beginning to apply concepts from information theory and communication to better understand olfactory communication, and work is starting to move from controlled settings with agricultural species in laboratories to natural populations in the field, where things are undoubtedly noisier. The work presented here builds on the few studies that have examined noise in infochemical systems by 1) examining a previously un-described use of infochemicals by two groups of parasitoid insects, 2) determining what the consequences of parasitization are for plants and insects and 3) identifying what sources of environmental variation contribute to variation in signal content.

Chapter 1 outlines a conceptual framework by which information theory can inform systems of infochemical communication, focusing on communication between plants and insects. Chapter 2 focuses on the use of infochemical cues by an insect parasitoid (Drino rhoeo) and assesses the effectiveness of D. rhoeo as a defender for D. wrightii plants. I show that tachinid flies use olfactory cues generated from plants, that performance and feeding rate of caterpillars is affected by parasitism, and that this has complex effects on damage to plants. Chapter 3 focuses on the use of infochemical signals by another insect parasitoid that attacks M. sexta on D. wrightii plants. Here, I show that a population of wasp egg parasitoids use olfactory cues generated from D. wrightii, and are capable of learning these cues during exposure to stimuli as adults. Chapter 4 attempts to quantify signal noise in VOC release by D. wrightii plants in the field and laboratory. Interestingly, even though insects are using VOCs as host-finding cues, signals from plants in the field appear to be incredibly noisy, with the majority of variation in the quantity and quality of signals driven by environmental variation, not by herbivory.

Together, these chapters provide evidence that applying ideas from information theory can be beneficial in understanding patterns of chemical signal release and reception. Just like lightdappled, acoustically dynamic forests, and busy foreign night-markets, the olfactory world around us is a noisy place. To more fully understand the ecology and evolution of chemical signaling, we must understand what forces generate noise in the environment and how organisms cope with noise on short-term and evolutionary scales.

References

Endler, J. A. 1993. The color of light in forests and its implications. Ecological monographs, 2-27.

Morton, E. S. 1975. Ecological sources of selection on avian sounds. American Naturalist, 17-34.

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© Copyright 2015 Jerome Keaton Wilson