Department of BiologyUniversity of Wisconsin Oshkosh

The Mitchell lab explores 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, we 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.

Our research is guided by two major questions:
  1. How do organisms perceive and evaluate chemical signals?

  2. What drives the evolution of chemosensory systems?

We 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. Current lines of research primarily involve the Coloeptera (beetles) and especially the beetle family Cerambycidae (longhorned beetles), which 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 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.

This research is conducted in close collaboration with the Hanks Lab at the University of Illinois and the Millar Lab at UC-Riverside, with the goal of describing pheromones and attractants used by native and invasive cerambycid species. We test the efficacy of these chemicals in field trials across the country, and work to developing broad-spectrum lures to be used in monitoring programs. Most recently, we have been studying how co-occurring species preserve mating isolation despite sharing pheromone components.

Representative Publications:

Odorant Receptors of the Coleoptera

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. In collaboration with the Robertson Lab at the University of Illinois and the Luetje Lab at the University of Miami, we sequenced and functionally characterized the first cerambycid receptors from a transcriptome of the beetle Megacyllene caryae. Three of these receptors that were sensitive to pheromones, providing initial steps toward understanding molecular pheromone reception in the Cerambycidae.

Currently, Mitchell lab members are describing ORs and functionally related proteins from several additional cerambycid species, annotating receptors from the four beetle genome initiatives of the i5K Project, and collaborating with other research groups to identify ORs in diverse beetle families across the Coleoptera.

Representative Publications:

Olfactory neurobiology

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 discrete structures in the AL called glomeruli are associated with groups of neurons that express identical receptors. In fact, recent research suggests that each class of odorant receptor may be correlated to a specific and unique glomerulus, and the size and placement of these glomeruli may even suggest the significance of the associated odor.

Our lab is mapping the diversity of glomeruli in beetle brains, and especially how these structures are correlated to the function of odorant receptors. We are currently using methods developed by the Hildebrand Lab at the University of Arizona to visualize glomeruli across the subfamilies of the Cerambycidae, and connect individual glomeruli to odorant receptor neurons in the antennae. Ultimately, we plan to record neural signals from glomeruli in living beetles as they are activated by odors, thus directly associating individual neural centers with their odor stimuli.

Representative Publications: