The molecular basis of CO<sub>2</sub> reception in <em>Drosophila</em>

Jae Kwon

Anupama Dahanukar

Linnea Weiss

John Carlson

Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, CT 06520-8103

*To whom correspondence should be addressed. E-mail: ude.elay@noslrac.nhoj

Communicated by Sydney Kustu, University of California, Berkeley, CA, January 4, 2007.

Author contributions: J.Y.K., A.D., and L.A.W. designed research; J.Y.K., A.D., and L.A.W. performed research; J.Y.K., A.D., L.A.W., and J.R.C. analyzed data; and J.Y.K., A.D., L.A.W., and J.R.C. wrote the paper.

Received 2006 Dec 15

Abstract

CO₂ elicits a response from many insects, including mosquito vectors of diseases such as malaria and yellow fever, but the molecular basis of CO₂ detection is unknown in insects or other higher eukaryotes. Here we show that Gr21a and Gr63a, members of a large family of Drosophila seven-transmembrane-domain chemoreceptor genes, are coexpressed in chemosensory neurons of both the larva and the adult. The two genes confer CO₂ response when coexpressed in an in vivo expression system, the “empty neuron system.” The response is highly specific for CO₂ and dependent on CO₂ concentration. The response shows an equivalent dependence on the dose of Gr21a and Gr63a. None of 39 other chemosensory receptors confers a comparable response to CO₂. The identification of these receptors may now allow the identification of agents that block or activate them. Such agents could affect the responses of insect pests to the humans they seek.

Keywords: chemoreceptors, insect, Gr genes

Abstract

Ever since the classic experiments of Joseph Priestley in the 18th century, the role of CO₂ in the natural world has been a subject of great interest. However, despite its ubiquity and central role in the metabolism of living organisms, some of the most fundamental questions about how CO₂ interacts with biological systems remain unanswered. One such question is how CO₂ is detected in the animal world.

CO₂ elicits behavioral responses in many insects that seek human hosts, including tsetse flies (1), which carry African sleeping sickness; Aedes mosquitoes (2), which carry dengue and yellow fever; and Anopheles mosquitoes (3), which transmit hundreds of millions of cases of malaria each year. CO₂ also acts as an attractive cue for many insects that seek plants as food sources and oviposition sites (4 –6). In Drosophila, high concentrations of CO₂ evoke an avoidance response (7, 8).

CO₂-sensitive neurons have been identified in many insect species (9) and in most cases are dedicated to the detection of CO₂. In adult Drosophila, odors are detected by olfactory receptor neurons (ORNs) that are housed in sensilla on the antenna and the maxillary palp (10). One class of antennal ORNs, the ab1C class, detects CO₂ (11). Axons of these CO₂-sensitive neurons project to a single glomerulus in the antennal lobe of the brain, the V glomerulus, which has been shown to be responsive to CO₂ (8).

Drosophila contains a family of 60 Odor receptor (Or) genes (12 –14), and a related family of 60 Gustatory receptor (Gr) genes (14, 15), both of which encode seven-transmembrane-domain proteins. In most ORN classes, a single Or gene defines the odorant response profile (16 –18). Typically, the Or gene is coexpressed with the noncanonical receptor Or83b, an atypical family member that is required for efficient localization of the canonical Or receptor to the dendrites (19). CO₂-sensitive neurons are unique in that they do not express an Or gene (20, 21). Instead, a Gr gene, Gr21a, has been shown to be expressed in this class of neurons (8). Genetic ablation of Gr21a-positive neurons results in defects in the behavioral avoidance response to CO₂ in adults (8) as well as in larvae (7). However, there has been no evidence that Gr21a acts in CO₂ detection.

Here we show that another Gr gene, Gr63a, is coexpressed with Gr21a in larvae as well as in the adult. Coexpression of Gr21a and Gr63a in an in vivo expression system confers a CO₂ response. The response depends on the presence of both Gr genes; neither gene alone confers a CO₂ response. The response is highly specific for CO₂ and depends on the concentration of CO₂. Our results suggest that Gr21a and Gr63a form a heterodimeric receptor for the detection of CO₂.

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Acknowledgments

We thank Wynand van der Goes van Naters for help with electrophysiology and suggestions and Scott Kreher for sharing unpublished data. This work was supported by National Institutes of Health Grants GM63364, DC04729, and DC02174.

Acknowledgments

Abbreviation

ORN	olfactory receptor neuron.

Abbreviation

Note Added in Proof.

Similar results were obtained in ref. 39.

Note Added in Proof.

Footnotes

The authors declare no conflict of interest.

This article contains supporting information online at www.pnas.org/cgi/content/full/0700079104/DC1.

Footnotes

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