Drosophila Odorant Receptors Research Articles

Stress and odorant receptor feedback during a critical period after hatching regulates olfactory sensory neuron differentiation in Drosophila.

Here, we reveal that the regulation of Drosophila odorant receptor (OR) expression during the pupal stage is permissive and imprecise. We found that directly after hatching an OR feedback mechanism both directs and refines OR expression. We demonstrate that, as in mice, dLsd1 and Su(var)3-9 balance heterochromatin formation to direct OR expression. We show that the expressed OR induces dLsd1 and Su(var)3-9 expression, linking OR level and possibly function to OR expression. OR expression refinement shows a restricted duration, suggesting that a gene regulatory critical period brings olfactory sensory neuron differentiation to an end. Consistent with a change in differentiation, stress during the critical period represses dLsd1 and Su(var)3-9 expression and makes the early permissive OR expression permanent. This induced permissive gene regulatory state makes OR expression resilient to stress later in life. Hence, during a critical period OR feedback, similar to in mouse OR selection, defines adult OR expression in Drosophila.

PKC98E Regulates Odorant Responses in Drosophila melanogaster.

Drosophila odorant receptors (Ors) are ligand gated ion channels composed of a common receptor subunit Or co-receptor (ORCO) and one of 62 "tuning" receptor subunits that confer odorant specificity to olfactory neuron responses. Like other sensory systems studied to date, exposing Drosophila olfactory neurons to activating ligands results in reduced responses to subsequent exposures through a process called desensitization. We recently showed that phosphorylation of serine 289 on the common Or subunit ORCO is required for normal peak olfactory neuron responses. Dephosphorylation of this residue occurs on prolonged odorant exposure, and underlies the slow modulation of olfactory neuron responses we term "slow desensitization." Slow desensitization results in the reduction of peak olfactory neuron responses and flattening of dose-response curves, implicating changes in ORCOS289 phosphorylation state as an important modulator of olfactory neuron responses. Here, we report the identification of the primary kinase responsible for ORCOS289 phosphorylation, PKC98E. Antiserum localizes the kinase to the dendrites of the olfactory neurons. Deletion of the kinase from olfactory neurons in the naive state (the absence of prolonged odor exposure) reduces ORCOS289 phosphorylation and reduces peak odorant responses without altering receptor localization or expression levels. Genetic rescue with a PKC98E predicted to be constitutively active restores ORCO S289 phosphorylation and olfactory neuron sensitivity to the PKC98E mutants in the naive state. However, the dominant kinase is defective for slow desensitization. Together, these findings reveal that PKC98E is an important regulator of ORCO receptors and olfactory neuron function.SIGNIFICANCE STATEMENT We have identified PKC98E as the kinase responsible for phosphorylation of the odorant receptor co-receptor (ORCO) at S289 that is required for normal odorant response kinetics of olfactory neurons. This is a significant step toward revealing the enzymology underlying the regulation of odorant response regulation in insects.

An accelerated miRNA-based screen implicates Atf-3 in Drosophila odorant receptor expression.

The Drosophila olfactory system is highly stereotyped in form and function; olfactory sensory neurons (OSNs) expressing a specific odorant receptor (OR) always appear in the same antennal location and the axons of OSNs expressing the same OR converge on the same antennal lobe glomeruli. Although some transcription factors have been implicated in a combinatorial code specifying OR expression and OSN identity, it is clear other players remain unidentified. In hopes of mitigating the challenges of genome-wide screening, we examined the feasibility of a two-tiered approach comprising a primary “pooling” screen for miRNAs whose tissue-specific over-expression causes a phenotype of interest followed by a focused secondary screen using gene-specific RNAi. Since miRNAs down-regulate their targets, miRNA over-expression phenotypes should be attributable to target loss-of-function. It is the sequence-dependence of miRNA-target pairing that suggests candidates for the secondary screen. Since miRNAs are short, however, miRNA misexpression will likely uncover non-biological miRNA-target relationships. Rather than focusing on miRNA function itself where these non-biological relationships could be misleading, we propose using miRNAs as tools to focus a more traditional RNAi-based screen. Here we describe such a screen that uncovers a role for Atf3 in the expression of the odorant receptor Or47b.

Outside-in signaling--a brief review of GPCR signaling with a focus on the Drosophila GPCR family.

G-protein-coupled receptors (GPCRs) are the largest family of receptors in many organisms, including worms, mice and humans. GPCRs are seven-transmembrane pass proteins that are activated by binding a stimulus (or ligand) in the extracellular space and then transduce that information to the inside of the cell through conformational changes. The conformational changes activate heterotrimeric G-proteins, which execute the downstream signaling pathways through the recruitment and activation of cellular enzymes. The highly specific ligand-GPCR interaction prompts an efficient cellular response, which is vital for the health of the cell and organism. In this Commentary, we review general features of GPCR signaling and then focus on the Drosophila GPCRs, which are not as well-characterized as their worm and mammalian counterparts. We discuss findings that the Drosophila odorant and gustatory receptors are not bona fide GPCRs as is the case for their mammalian counterparts. We also present here a phylogenetic analysis of the bona fide Drosophila GPCRs that suggest potential roles for several family members. Finally, we discuss recently discovered roles of GPCRs in Drosophila embryogenesis, a field we expect will uncover many previously unappreciated functions for GPCRs.

Determinants of the Drosophila Odorant Receptor Pattern

In most olfactory systems studied to date, neurons that express the same odorant receptor (Or) gene are scattered across sensory epithelia, intermingled with neurons that express different Or genes. In Drosophila, olfactory sensilla that express the same Or gene are dispersed on the antenna and the maxillary palp. Here we show that Or identity is specified in a spatially stereotyped pattern by the cell-autonomous activity of the transcriptional regulators Engrailed and Dachshund. Olfactory sensilla then become highly motile and disperse beneath the epidermis. Thus, positional information and cell motility underlie the dispersed patterns of Drosophila Or gene expression.

A Deceptive Pollination System Targeting Drosophilids through Olfactory Mimicry of Yeast

In deceptive pollination, insects are bamboozled into performing nonrewarded pollination. A prerequisite for the evolutionary stability in such systems is that the plants manage to generate a perfect sensory impression of a desirable object in the insect nervous system [1]. The study of these plants can provide important insights into sensory preference of their visiting insects. Here, we present the firstdescription of a deceptive pollination system that specifically targets drosophilid flies. We show that the examined plant (Arum palaestinum) accomplishes its deception through olfactory mimicry of fermentation, a strategy that represents a novel pollination syndrome. The lily odor is composed of volatiles characteristic of yeast, and produces in Drosophila melanogaster an antennal detection pattern similar to that elicited by a range of fermentation products. By functional imaging, we show that the lily odors target a specific subset of odorant receptors (ORs), which include the most conserved OR genes in the drosophilid olfactome. Furthermore, seven of eight visiting drosophilid species show a congruent olfactory response pattern to the lily, in spite of comprising species pairs separated by ∼40 million years [2], showing that the lily targets a basal function ofthe fly nose, shared by species with similar ecological preference.

Dissecting the molecular mechanism of drosophila odorant receptors through activity modeling and comparative analysis

To gain insight into the molecular mechanism of odorant receptors (ORs) in Drosophila species, we developed a Quantitative Structure Activity Relationship (QSAR) model that predicts experimentally measured electrophysiological activities between 24 D. melanogaster ORs and 108 odorants. Although the model is limited by the tested odorants,analyzing the model allowed dissection of specific topological and chemical properties necessary for an odorant to elicit excitatory or inhibitory receptor response. Linear odorants with five to eight nonhydrogen atoms at the main chain and hydrogen-bond acceptor and/or hydrogen-bond donor at its ends were found to stimulate strong excitatory response. A comparative sequence analysis of 90 ORs in 15 orthologous groups identified 15 putative specificity-determining residues (SDRs) and 15 globally conserved residues that we postulate as functionally key residues. Mapping to a model of secondary structure resulted in 14 out of 30 key residues locating to the transmembrane (TM) domains. Twelve residues, including six SDRs and six conserved residues, are located at the extracellular halves of the TM domains. Combining the evidence from the QSAR modeling and the comparative sequence analysis, we hypothesize that the Drosophila ORs accept odorants into a binding pocket located on the extracellular halves of its TM domains. The QSAR modeling suggests that the binding pocket is around 15 A in depth and about 6 A in width. Twelve mainly polar or charged key residues, both SDRs and conserved, are located in this pocket and postulated to distinguish docked odorants via primarily geometry fitting and hydrogen-bond interaction.

Scent of a Fly

Sexual courtship is a highly ritualized behavior in many animals. Recent work in the vinegar fly, Drosophila melanogaster, has illuminated how the pheromone cis-vaccenyl acetate modulates sexual behavior in the fly. Chemosensory receptors and a sexually dimorphic circuit activated by this pheromone have been identified. This minireview highlights recent advances in the field of fly courtship.

Drosophila odorant receptors are novel seven transmembrane domain proteins that can signal independently of heterotrimeric G proteins

Olfaction in Drosophila is mediated by a large family of membrane-bound odorant receptor proteins (Ors). In heterologous cells, we investigated whether the structural features and signalling mechanisms of ligand-binding Drosophila Ors are consistent with them being G protein-coupled receptors (GPCRs). The detailed membrane topology of Or22a was determined by inserting epitope tags into the termini and predicted loop regions. Immunocytochemistry experiments in Drosophila S2 cells imply that Or22a has seven transmembrane domains but that its membrane topology is opposite to that of GPCRs, with a cytoplasmic N-terminus and extracellular C-terminus. To investigate Or signalling mechanisms, we expressed Or43b in Sf9 and HEK293 cells, and show that inhibitors of heterotrimeric G proteins (GDP- β-S), adenylate cyclase (SQ22536), guanylyl cyclase (ODQ), cyclic nucleotide phosphodiesterases (IBMX) and phospholipase C (U73122) have negligible impact on Or43b responses. Whole cell patching of Or43b/Or83b-transfected HEK293 cells revealed the opening of plasma membrane cation channels on addition of ligand. The response was blocked by lanthanum and by 2-APB, but not by Ruthenium red or SKF96365. Based on these data, we conclude that Drosophila Ors comprise a novel family of seven transmembrane receptors that in HEK293 cells signal by opening cation channels, through a mechanism that is largely independent of G proteins.

How Do Bugs Smell?

Two papers published this week report both clarifying and somewhat contradictory findings regarding signaling by insect odorant receptors. Odorant receptors (ORs) in mammals are canonical heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) and are often coupled to generation of the second messenger adenosine 3′,5′-monophosphate (cAMP), which can modulate conductance of ion channels. Insect ORs have presented a puzzle, however, as they only vaguely resemble GPCRs and assemble as heterodimers of one variable OR and a common co-receptor called Or83b. Wicher et al . expressed Drosophila receptors Or22a and Or83b in human embryonic kidney cells and used patch clamping to characterize odorant-induced ion currents in the transfected cells. They report that expression of Or83b alone conferred a nonselective cation channel whose conductance was increased by cAMP or guanosine 3′,5′-monophosphate (cGMP). When the authors transfected cells with the Or22a-Or83b pair and also with known mammalian cyclic nucleotide-gated channels, odorant-induced activation of the mammalian channel indicated that the insect OR activated cAMP signaling. Thus, they conclude that the insect ORs themselves form ligand-gated ion channels that sense odorant and amplify signals by G-protein signaling. Sato et al . used similar strategies to analyze various insect ORs. Human cells expressing receptor proteins from fruit fly, mosquito, or silk moth showed a response to odorant that included Ca 2+ influx and increased nonselective cation conductance. Outside-out patch clamp studies from membranes of oocytes or human cells expressing the receptor proteins confirmed that the insect ORs themselves conferred the observed currents. However, Sato et al .’s experiments did not detect odorant-induced generation of cAMP, and pharmacological inhibition of G-protein signaling failed to block odorant-induced currents. As noted in commentary from Chesler and Firestein, resolution of the discrepancies will require further work but may have to do with Sato et al .’s focus on early events (within seconds) and Wicher et al .’s characterization of later events—30 seconds to a minute after receptor stimulation. Insect ORs do not resemble known ion channels at all, so the work suggests there is more to learn about such proteins and raises the question of whether orphan GPCRs in other organisms might also function as ligand-gated ion channels. K. Sato, M. Pellegrino, T. Nakagawa, T. Nakagawa, L. B. Vosshall, K. Touhara, Insect olfactory receptors are heteromeric ligand-gated ion channels. Nature 452 , 1002-1006 (2008). [PubMed] D. Wicher, R. Schäfer, R. Bauernfeind, M. C. Stensmyr, R. Heller, S. H. Heinemann, B. S. Hannson, Drosophila odorant receptors are both ligand-gated and cyclic-nucleotide-activated cation channels. Nature 452 , 1007-1011 (2008). [PubMed] A. Chesler, S. Firestein, Current views on odour receptors. Nature 452 , 944 (2008). [PubMed]

Drosophila odorant receptors are both ligand-gated and cyclic-nucleotide-activated cation channels

From worm to man, many odorant signals are perceived by the binding of volatile ligands to odorant receptors that belong to the G-protein-coupled receptor (GPCR) family. They couple to heterotrimeric G-proteins, most of which induce cAMP production. This second messenger then activates cyclic-nucleotide-gated ion channels to depolarize the olfactory receptor neuron, thus providing a signal for further neuronal processing. Recent findings, however, have challenged this concept of odorant signal transduction in insects, because their odorant receptors, which lack any sequence similarity to other GPCRs, are composed of conventional odorant receptors (for example, Or22a), dimerized with a ubiquitously expressed chaperone protein, such as Or83b in Drosophila. Or83b has a structure akin to GPCRs, but has an inverted orientation in the plasma membrane. However, G proteins are expressed in insect olfactory receptor neurons, and olfactory perception is modified by mutations affecting the cAMP transduction pathway. Here we show that application of odorants to mammalian cells co-expressing Or22a and Or83b results in non-selective cation currents activated by means of an ionotropic and a metabotropic pathway, and a subsequent increase in the intracellular Ca(2+) concentration. Expression of Or83b alone leads to functional ion channels not directly responding to odorants, but being directly activated by intracellular cAMP or cGMP. Insect odorant receptors thus form ligand-gated channels as well as complexes of odorant-sensing units and cyclic-nucleotide-activated non-selective cation channels. Thereby, they provide rapid and transient as well as sensitive and prolonged odorant signalling.

Mining phenotypes for gene function prediction

BackgroundHealth and disease of organisms are reflected in their phenotypes. Often, a genetic component to a disease is discovered only after clearly defining its phenotype. In the past years, many technologies to systematically generate phenotypes in a high-throughput manner, such as RNA interference or gene knock-out, have been developed and used to decipher functions for genes. However, there have been relatively few efforts to make use of phenotype data beyond the single genotype-phenotype relationships.ResultsWe present results on a study where we use a large set of phenotype data – in textual form – to predict gene annotation. To this end, we use text clustering to group genes based on their phenotype descriptions. We show that these clusters correlate well with several indicators for biological coherence in gene groups, such as functional annotations from the Gene Ontology (GO) and protein-protein interactions. We exploit these clusters for predicting gene function by carrying over annotations from well-annotated genes to other, less-characterized genes in the same cluster. For a subset of groups selected by applying objective criteria, we can predict GO-term annotations from the biological process sub-ontology with up to 72.6% precision and 16.7% recall, as evaluated by cross-validation. We manually verified some of these clusters and found them to exhibit high biological coherence, e.g. a group containing all available antennal Drosophila odorant receptors despite inconsistent GO-annotations.ConclusionThe intrinsic nature of phenotypes to visibly reflect genetic activity underlines their usefulness in inferring new gene functions. Thus, systematically analyzing these data on a large scale offers many possibilities for inferring functional annotation of genes. We show that text clustering can play an important role in this process.

Evolution of Gene Expression in the Drosophila Olfactory System

Host plant shifts by phytophagous insects play a key role in insect evolution and plant ecology. Such shifts often involve major behavioral changes as the insects must acquire an attraction and/or lose the repulsion to the new host plant's odor and taste. The evolution of chemotactic behavior may be due, in part, to gene expression changes in the peripheral sensory system. To test this hypothesis, we compared gene expression in the olfactory organs of Drosophila sechellia, a narrow ecological specialist that feeds on the fruit of Morinda citrifolia, with its close relatives Drosophila simulans and Drosophila melanogaster, which feed on a wide variety of decaying plant matter. Using whole-genome microarrays and quantitative polymerase chain reaction, we surveyed the entire repertoire of Drosophila odorant receptors (ORs) and odorant-binding proteins (OBPs) expressed in the antennae. We found that the evolution of OR and OBP expression was accelerated in D. sechellia compared both with the genome average in that species and with the rate of OR and OBP evolution in the other species. However, some of the gene expression changes that correlate with D. sechellia's increased sensitivity to Morinda odorants may predate its divergence from D. simulans. Interspecific divergence of olfactory gene expression cannot be fully explained by changes in the relative abundance of different sensilla as some ORs and OBPs have evolved independently of other genes expressed in the same sensilla. A number of OR and OBP genes are upregulated in D. sechellia compared with its generalist relatives. These genes include Or22a, which likely responds to a key odorant of M. citrifolia, and several genes that are yet to be characterized in detail. Increased expression of these genes in D. sechellia may have contributed to the evolution of its unique chemotactic behavior.

Membrane topology of the Drosophila OR83b odorant receptor

By analogy to mammals, odorant receptors (ORs) in insects, such as Drosophila melanogaster, have long been thought to belong to the G-protein coupled receptor (GPCR) superfamily. However, recent work has cast doubt on this assumption and has tentatively suggested an inverted topology compared to the canonical N out − C in 7 transmembrane (TM) GPCR topology, at least for some Drosophila ORs. Here, we report a detailed topology mapping of the Drosophila OR83b receptor using engineered glycosylation sites as topology markers. Our results are inconsistent with a classical GPCR topology and show that OR83b has an intracellular N-terminus, an extracellular C-terminus, and 7TM helices.

Sugar Receptors in Drosophila

The detection and discrimination of chemical compounds in potential foods are essential sensory processes when animals feed. The fruit fly Drosophila melanogaster employs 68 different gustatory receptors (GRs) for the detection of mostly nonvolatile chemicals that include sugars, a diverse group of toxic compounds present in many inedible plants and spoiled foods, and pheromones [1-6]. With the exception of a trehalose (GR5a) and a caffeine (GR66a) receptor [7-9], the functions of GRs involved in feeding are unknown. Here, we show that the Gr64 genes encode receptors for numerous sugars. We generated a fly strain that contained a deletion for all six Gr64 genes (DeltaGr64) and showed that these flies exhibit no or a significantly diminished proboscis extension reflex (PER) response when stimulated with glucose, maltose, sucrose, and several other sugars. The only considerable response was detected when Gr64 mutant flies were stimulated with fructose. Interestingly, response to trehalose is also abolished in these flies, even though they contain a functional Gr5a gene, which has been previously shown to encode a receptor for this sugar [8, 9]. This observation indicates that two or more Gr genes are necessary for trehalose detection, suggesting that GRs function as multimeric receptor complexes. Finally, we present evidence that some members of the Gr64 gene family are transcribed as a polycistronic mRNA, providing a mechanism for the coexpression of multiple sugar receptors in the same taste neurons.

Selective Pressures on Drosophila Chemosensory Receptor Genes

The evolution and patterns of selection of genes encoding 10 Drosophila odorant receptors (Or) and the sex pheromone receptor Gr68a were investigated by comparing orthologous sequences across five to eight ecologically diverse species of Drosophila. Using maximum likelihood estimates of dN/dS ratios we show that all 11 genes sampled are under purifying selection, indicating functional constraint. Four of these genes (Or33c, Or42a, Or85e, and Gr68a) may be under positive selection, and if so, there is good evidence that 12 specific amino acid sites may be under positive selection. All of these sites are predicted to be located either in loop regions or just inside membrane spanning regions, and interestingly one of the two sites in Gr68a is in a similar position to a previously described polymorphism in Gr5a that causes a shift in sensitivity to its ligand trehalose. For three Ors, possible evidence for positive selection was detected along a lineage. These include Or22a in the lineage leading to D. mauritiana and Or22b in the lineage leading to D. simulans. This is of interest in light of previous data showing a change in ligand response profile for these species in the sensory neuron (ab3A) which expresses both Or22a and Or22b in D. melanogaster. In summary, while the main chemosensory function and/or structural integrity of these 10 Or genes and Gr68a are evolutionarily preserved, positive selection appears to be acting on some of these genes, at specific sites and along certain lineages, and provides testable hypotheses for further functional experimentation.

Odor and pheromone detection in Drosophila melanogaster

Drosophila melanogaster has proven to be a useful model system to probe the mechanisms underlying the detection, discrimination, and perception of volatile odorants. The relatively small receptor repertoire of 62 odorant receptors makes the goal of understanding odor responses from the total receptor repertoire approachable in this system, and recent work has been directed toward this goal. In addition, new work not only sheds light but also raises more questions about the initial steps in odor perception in this system. Odorant receptor genes in Drosophila are predicted to encode seven transmembrane receptors, but surprising data suggest that these receptors may be inverted in the plasma membrane compared to classical G-protein coupled receptors. Finally, although some Drosophila odorant receptors are activated directly by odorant molecules, detection of a volatile pheromone, 11-cis vaccenyl acetate requires an extracellular adapter protein called LUSH for activation of pheromone sensitive neurons. Because pheromones are used by insects to trigger mating and other behaviors, these insights may herald new approaches to control behavior in pathogenic and agricultural pest insects.

Atypical Membrane Topology and Heteromeric Function of Drosophila Odorant Receptors In Vivo

Drosophila olfactory sensory neurons (OSNs) each express two odorant receptors (ORs): a divergent member of the OR family and the highly conserved, broadly expressed receptor OR83b. OR83b is essential for olfaction in vivo and enhances OR function in vitro, but the molecular mechanism by which it acts is unknown. Here we demonstrate that OR83b heterodimerizes with conventional ORs early in the endomembrane system in OSNs, couples these complexes to the conserved ciliary trafficking pathway, and is essential to maintain the OR/OR83b complex within the sensory cilia, where odor signal transduction occurs. The OR/OR83b complex is necessary and sufficient to promote functional reconstitution of odor-evoked signaling in sensory neurons that normally respond only to carbon dioxide. Unexpectedly, unlike all known vertebrate and nematode chemosensory receptors, we find that Drosophila ORs and OR83b adopt a novel membrane topology with their N-termini and the most conserved loops in the cytoplasm. These loops mediate direct association of ORs with OR83b. Our results reveal that OR83b is a universal and integral part of the functional OR in Drosophila. This atypical heteromeric and topological design appears to be an insect-specific solution for odor recognition, making the OR/OR83b complex an attractive target for the development of highly selective insect repellents to disrupt olfactory-mediated host-seeking behaviors of insect disease vectors.

Length analyses of Drosophila odorant receptors

Odorant receptors comprise a unique family of G-protein-coupled seven-transmembrane receptors both in mammals and insects. In the fruit fly Drosophila melanogaster, all 61 candidate odorant receptor genes have been identified based on the complete genome sequence, and their expression patterns have been examined. A given odorant receptor is expressed in the antenna or maxillary palp, or not expressed at all. Here we have applied a set of statistical analyses to the length of the extra- and intracellular loops and terminals (LTs) of Drosophila odorant receptors to examine possible inter- and intramolecular relations at the population level. We have first provided some useful statistical information such as mean length values and length histograms to depict a general nature of Drosophila odorant receptors at the population level, after focusing on discrepancy on assigning transmembrane domains between researchers. In a preferable transmembrane assignment, all extracellular LTs, especially the second extracellular loops, were relatively large in length, suggesting their functional significance. Somewhat surprisingly, principle component analysis (PCA) indicated that the maxillary palp receptors were almost as diverse as the antenna receptors despite their much smaller population size. PCA together with histograms also revealed that receptors with an abnormal length configuration tended not to be expressed, suggesting that LT length deviations are related to transcriptional silencing of odorant receptor genes. Rank transformation tests pointed out possible LTs that could have different length between differently expressed receptors at the population level. Taken together, length analyses provide us with a general picture, i.e. “length configuration,” of Drosophila odorant receptors at the population level that could point out putatively important functional sites for experimental studies.

The molecular logic of olfaction in Drosophila.

Drosophila fruit flies display robust olfactory-driven behaviors with an olfactory system far simpler than that of vertebrates. Endowed with 1300 olfactory receptor neurons, these insects are able to recognize and discriminate between a large number of distinct odorants. Candidate odorant receptor molecules were identified by complimentary approaches of differential cloning and genome analysis. The Drosophila odorant receptor (DOR) genes encode a novel family of proteins with seven predicted membrane-spanning domains, unrelated to vertebrate or nematode chemosensory receptors. There are on the order of 60 or more members of this gene family in the Drosophila genome, far fewer than the hundreds to thousands of receptors found in vertebrates or nematodes. DOR genes are selectively expressed in small subsets of olfactory neurons, in expression domains that are spatially conserved between individuals, bilaterally symmetric and not sexually dimorphic. Double in situ RNA hybridization with a number of pairwise combinations of DOR genes fails to reveal any overlap in gene expression, suggesting that each olfactory neuron expresses one or a small number of receptor genes and is therefore functionally distinct. How is activation of such a subpopulation of olfactory receptor neurons in the periphery sensed by the brain? In the mouse, all neurons expressing a given receptor project with precision to two of 1800 olfactory bulb glomeruli, creating a spatial map of odor quality in the brain. We have employed DOR promoter transgenes that recapitulate expression of endogenous receptor to visualize the projections of individual populations of receptor neurons to subsets of the 43 glomeruli in the Drosophila antennal lobe. The results suggest functional conservation in the logic of olfactory discrimination from insects to mammals.