My research is in the field of chemical ecology, a broad discipline that lies at the union of behavior, ecology, physiology, chemistry, and molecular biology. Briefly stated, it is a field that addresses how organisms produce and use chemical signals to exchange information, locate food and mates, and generally navigate their environment. Specifically, I study the chemical ecology of insects, many species of which rely heavily on olfaction (smell) and gustation (taste) as sensory inputs. Research in chemical ecology is therefore critical to understanding how insects interact with each other and their environment, and how we might manage species that are pests.My research is guided by two major questions:
I address these through several lines of research, including the ecology and behavior of insects in the field, the odorant receptors that detect chemicals at the molecular level, and the neural pathways that encode and process olfactory information. Currently, my work includes two different insect groups: the longhorned beetles (Coleoptera: Cerambycidae) and the sphinx moths (Lepidoptera: Sphingidae). Both families are excellent models for olfactory biology because of their strong reliance on chemical signals for mating and oviposition.
Chemical Ecology of Longhorned Beetles
The longhorned beetles (Cerambycidae) are among the most diverse and spectacular groups of insects. The trademark antennae are often longer than the body, and the large, colorful adults are an important aesthetic contribution to any insect collection. Though larvae of most species in this family feed in the wood of recently dead or decaying trees, others feed in living trees and can be terribly destructive forest pests. Thus, chemical signals and cues used by the beetles are of critical importance, as they can become powerful tools for monitoring pests and manipulating their behavior.
Pheromones - chemical signals used for communication within a species - play a prominent role in cerambycid ecology. Males of many species produce long-range pheromones that aggregate both sexes, and females of a few species produce pheromones that are only attractive to males. In either case, these pheromones are strongly conserved, meaning that a few types of pheromone are produced by many species, even when these species overlap both geographically and seasonally. This is interesting from an ecological standpoint (how can they tell one species from another?), and also hints at a broad-spectrum method for controlling species of pests.
I conducted much of my dissertation research in the Hanks Lab at the University of Illinois and in conjunction with Jocelyn Millar at UC-Riverside, during which I described pheromones and attractants used by numerous cerambycid species. I tested the efficacy of these chemicals in field trials across the country, and aided in developing broad-spectrum lures to be used in monitoring programs. Most recently, I have been studying how co-occurring species preserve mating isolation despite sharing pheromone components, and how volatiles of host plants might interact with pheromones and contribute to attraction (see Neurobiology below).
- Mitchell, RF, JG Millar, and LM Hanks. 2013. Blends of (R)-3-hydroxyhexan-2-one and alkan-2-ones identified as potential pheromones produced by multiple species of cerambycid beetles. Chemoecol. 23: 121-127.
- Wong, JCH, RF Mitchell, BL Striman, JG Millar, and LM Hanks. 2012. Blending synthetic pheromones of cerambycid beetles to develop trap lures that simultaneously attract multiple species. J. Econ. Entomol. 105: 906-915.
- Mitchell, RF, EE Graham, JCH Wong, PF Reagel, BL Striman, GP Hughes, MA Paschen, MD Ginzel, JG Millar, and LM Hanks. 2011. Fuscumol and fuscumol acetate are general attractants for many species of cerambycid beetles in the subfamily Lamiinae. Entomol. Exp. Appl. 141: 71-77.
Odorant Receptors of Longhorned Beetles
The odorant receptors ("Ors") are a large radiation of chemoreceptor proteins and one of the primary mechanisms used by insects to detect volatile chemicals. The family is unique to insects and structurally distinct from the olfactory receptors found in vertebrates, but they are part of a neural architecture in the brain that is surprisingly similar. The Ors are among the most basic units of insect olfaction, and thus a first step in understanding the repertoire of chemicals that can affect insect behavior. Once isolated, Ors can be assayed against panels of chemicals to find new attractants ("reverse chemical ecology"), developed into genetic markers for olfactory sensitivity, and reveal evolutionary histories.
However, the Ors of cerambycids - and of the Coleoptera as a whole - remain almost completely unexplored. During my doctoral research in the Robertson Lab at the University of Illinois, I sequenced the first cerambycid receptors from a transcriptome of the beetle Megacyllene caryae. I further collaborated with Charles Luetje and his lab at the University of Miami to functionally characterize three of these receptors that were sensitive to pheromones, providing initial steps toward understanding molecular pheromone reception in the Cerambycidae.
Currently, I am describing Ors from genomic sequences of two additional cerambycids, and I am annotating olfactory genes from the recently sequenced Asian longhorned beetle genome (led by Duane McKenna at the University of Memphis). As the set of known cerambycid Ors continues to expand, I plan to identify additional pheromone receptors and map the evolution of these genes throughout the cerambycid subfamilies.
- Mitchell, RF, DT Hughes, CW Luetje, JG Millar, F Soriano-Agatón, LM Hanks, and HM Robertson. 2012. Sequencing and characterizing odorant receptors of the cerambycid beetle Megacyllene caryae. Insect Biochem. Mol. Biol. 42: 499-505.
Neurobiology of Manduca sexta
Odorant receptors may be the key initial step in olfaction, but the interpretation of odors occurs further downstream in the deuto- and protocerebra of the insect brain. The deutocerebrum in particular is the primary hub for olfactory input from the antennae: odorant receptor neurons directly innervate the antennal lobes (AL) of the deutocerebrum, and recent research suggests the projection neurons that exit the AL may already be integrating olfactory signals. In other words, information in the AL may include both the raw input and early-stage encoding of a chemical signal.
In 2012, I received a postdoctoral fellowship through the NIH-funded PERT program at the University of Arizona to study how odor blends are processed in the antennal lobe. I am collaborating on this project with John Hildebrand, whose lab specializes in recording from neurons in the brain of the sphinx moth Manduca sexta. Female moths choose among several host plant species of varying quality when laying eggs, and they perceive and compare these plants through complex blends of chemicals released from leaves and flowers. I am studying how these different blends affect (and effect) neural activity in the AL of the moth, and especially how differences in the patterns of activity correlate to choices made by the female.
I measure neural activity through a technique called multichannel recording, which simultaneously records electrical activity from many different sites in the brain while an antenna is exposed to an odor source. This technique not only reveals the activity or firing rate of individual neurons, but synchronous firing across many regions of the brain, creating a unique "neural fingerprint" in the AL that arises from each blend of odors. I can also identify specific chemicals in each blend that trigger activity in the brain ("active components") by coupling multichannel recording with gas chromatography, thus separating chemical blends into single components and recording how the AL responds to each. For more details of multichannel recording, click here to view a PBS documentary where I discuss this research.
Ultimately, I hope to apply these techniques of neurobiology to other groups of insects, especially beetles, and I am currently mapping the structure of the deutocerebra in several cerambycid species in preparation for future projects.