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The Cactus-Microorganism-Drosophila Model System of the Sonoran Desert represents an excellent paradigm of the role of chemistry in plant-animal interactions. In this system, four species of endemic Drosophila feed and reproduce in necrotic tissue of five species of columnar cacti. Studies over the past 35 yr have characterized a myriad of interactions between the three major components of the model system. The cacti contain a variety of allelochemicals which are primarily responsible for the highly specific pattern of host plant utilization exhibited by the desert Drosophila. Plant chemistry, through its effect on the microbially produced volatile patterns, is further involved in host specificity because the flies use the volatile pattern to cue in on necroses in the appropriate species of cactus. The metabolic activities of microorganisms (bacteria and yeasts) living in the necrosis can affect the substrate chemistry in both positive and negative ways (i.e., acting to increase or to decrease the toxicity of the substrate). Finally, cactus chemistry may affect drosophilid mating behavior since larval rearing substrate has been shown to influence adult hydrocarbon epicuticular composition. In D. mojavensis, adult hydrocarbon profile has been implicated as a determinant of mate choice leading to premating isolation between geographically isolated populations that use chemically different cactus substrates. Current research is focused on the evolution and regulation of genes whose products (cytochrome P450 enzymes) are involved in the specific insect-host plant relationships which exist between the Drosophila species and the cactus species.

There are many reasons why investigators choose to focus their research efforts on what are referred to as "model systems." Typically included among these would be the idea that model systems are easier to study because they are less complex than other scientific situations. At the same time, model systems should be representative of more complex, natural systems so that information that is obtained from their study is broadly applicable. For almost a century, the fruit fly, Drosophila melanogaster, has served as a model organism for the study of genetics. As a genetic paradigm, Drosophila is more tractable to scientific investigation than most organisms and has provided important insights into a wide variety of human maladies from alcohol abuse to neurological brain disorders (Bellen, 1998). Similarly, the interrelationships of the columnar cacti and the cactophilic Drosophila species of the Sonoran Desert have, for the past 35 yr, provided an excellent model system with which to study relevant questions in evolution, ecological genetics, and chemical ecology. The intent of this article is to briefly review and characterize the chemical interactions between the plants (cacti) and animals (Drosophila) of this model system, and, in addition, provide some thoughts on possible future directions for integrative approaches in this research area.