A Gr receptor is required for response to the sugar trehalose in taste neurons of Drosophila

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.

Electrophysiological Characterization of Responses from Gustatory Receptor Neurons of sensilla chaetica in the Moth Heliothis virescens

Discrimination of edible and noxious food is crucial for survival in all organisms. We have studied the physiology of the gustatory receptor neurons (GRNs) in contact chemosensilla (insect gustatory organs) located on the antennae of the moth Heliothis virescens, emphasizing putative phagostimulants and deterrents. Sucrose and the 2 bitter substances quinine and sinigrin elicited responses in a larger proportion of GRNs than inositol, KCl, NaCl, and ethanol, and the firing thresholds were lowest for sucrose and quinine. Variations in GRN composition in individual sensilla occurred without any specific patterns to indicate specific sensillum types. Separate neurons showed excitatory responses to sucrose and the 2 bitter substances quinine and sinigrin, implying that the moth might be able to discriminate bitter substances in addition to separating phagostimulants and deterrents. Besides being detected by separate receptors on the moth antennae, the bitter tastants were shown to have an inhibitory effect on phagostimulatory GRNs. Sucrose was highly appetitive in behavioral studies of proboscis extension, whereas quinine had a nonappetitive effect in the moths.

A Drosophila gustatory receptor required for the responses to sucrose, glucose, and maltose identified by mRNA tagging

In Drosophila, detection of tastants is thought to be mediated by members of a family of 68 gustatory receptors (Grs). However, only one receptor, Gr5a, has been associated with a sugar, and it appears to be activated specifically by trehalose. It is unclear whether other sugar receptors are activated by single or multiple sugars. Currently, no Grs are known to colocalize with Gr5a. Such Grs would be candidate sugar receptors because Gr5a-expressing cells function in the responses to attractive tastants. Here we use an "mRNA tagging" approach to identify Gr RNAs that are coexpressed with Gr5a. We found that all seven Grs most related to Gr5a (Gr64a-f and Gr61a) were expressed in Gr5a-expressing cells, whereas none of the other Grs examined were enriched in these Gr neurons (GRNs). We characterized the role of one Gr5a-related receptor, Gr64a, and found that it was required for the behavioral responses to glucose, sucrose, and maltose. Gr64a was required for GRN function because action potentials induced by these sugars were dependent on expression of Gr64a in GRNs. These data demonstrate that multiple Grs are coexpressed with Gr5a and that Drosophila Gr64a is required for the responses to multiple sugars.

Molecular Architecture of Smell and Taste inDrosophila

The chemical senses-smell and taste-allow animals to evaluate and distinguish valuable food resources from dangerous substances in the environment. The central mechanisms by which the brain recognizes and discriminates attractive and repulsive odorants and tastants, and makes behavioral decisions accordingly, are not well understood in any organism. Recent molecular and neuroanatomical advances in Drosophila have produced a nearly complete picture of the peripheral neuroanatomy and function of smell and taste in this insect. Neurophysiological experiments have begun to provide insight into the mechanisms by which these animals process chemosensory cues. Given the considerable anatomical and functional homology in smell and taste pathways in all higher animals, experimental approaches in Drosophila will likely provide broad insights into the problem of sensory coding. Here we provide a critical review of the recent literature in this field and comment on likely future directions.

Architecture of the primary taste center ofDrosophila melanogasterlarvae

A simple nervous system combined with stereotypic behavioral responses to tastants, together with powerful genetic and molecular tools, have turned Drosophila larvae into a very promising model for studying gustatory coding. Using the Gal4/UAS system and confocal microscopy for visualizing gustatory afferents, we provide a description of the primary taste center in the larval central nervous system. Essentially, gustatory receptor neurons target different areas of the subesophageal ganglion (SOG), depending on their segmental and sensory organ origin. We define two major and two smaller subregions in the SOG. One of the major areas is a target of pharyngeal sensilla, the other one receives inputs from both internal and external sensilla. In addition to such spatial organization of the taste center, circumstantial evidence suggests a subtle functional organization: aversive and attractive stimuli might be processed in the anterior and posterior part of the SOG, respectively. Our results also suggest less coexpression of gustatory receptors than proposed in prior studies. Finally, projections of putative second-order taste neurons seem to cover large areas of the SOG. These neurons may thus receive multiple gustatory inputs. This suggests broad sensitivity of secondary taste neurons, reminiscent of the situation in mammals.

Taste and pheromone perception in the fruit fly Drosophila melanogaster

Taste is an essential sense for detection of nutrient-rich food and avoidance of toxic substances. The Drosophila melanogaster gustatory system provides an excellent model to study taste perception and taste-elicited behaviors. "The fly" is unique in the animal kingdom with regard to available experimental tools, which include a wide repertoire of molecular-genetic analyses (i.e., efficient production of transgenics and gene knockouts), elegant behavioral assays, and the possibility to conduct electrophysiological investigations. In addition, fruit flies, like humans, recognize sugars as a food source, but avoid bitter tasting substances that are often toxic to insects and mammals alike. This paper will present recent research progress in the field of taste and contact pheromone perception in the fruit fly. First, we shall describe the anatomical properties of the Drosophila gustatory system and survey the family of taste receptors to provide an appropriate background. We shall then review taste and pheromone perception mainly from a molecular genetic perspective that includes behavioral, electrophysiological and imaging analyses of wild type flies and flies with genetically manipulated taste cells. Finally, we shall provide an outlook of taste research in this elegant model system for the next few years.

Mechanisms of odor receptor gene choice in Drosophila

A remarkable problem in neurobiology is how olfactory receptor neurons (ORNs) select, from among a large odor receptor repertoire, which receptors to express. We use computational algorithms and mutational analysis to define positive and negative regulatory elements that are required for selection of odor receptor (Or) genes in the proper olfactory organ of Drosophila, and we identify an element that is essential for selection in one ORN class. Two odor receptors are coexpressed by virtue of the alternative splicing of a single gene, and we identify dicistronic mRNAs that each encode two receptors. Systematic analysis reveals no evidence for negative feedback regulation, but provides evidence that the choices made by neighboring ORNs of a sensillum are coordinated via the asymmetric segregation of regulatory factors from a common progenitor. We show that receptor gene choice in Drosophila also depends on a combinatorial code of transcription factors to generate the receptor-to-neuron map.

Insects as chemosensors of humans and crops

Insects transmit disease to hundreds of millions of people a year, and cause enormous losses to the world's agricultural output. Many insects find the human or plant hosts on which they feed, and identify and locate their mates, primarily through olfaction and taste. Major advances have recently been made in understanding insect chemosensation at the molecular and cellular levels. These advances have provided new opportunities to control insects that cause massive damage to health and agriculture across the world.

Olfactory conditioning of proboscis activity in Drosophila melanogaster

Olfactory learning and memory processes in Drosophila have been well investigated with aversive conditioning, but appetitive conditioning has rarely been documented. Here, we report for the first time individual olfactory conditioning of proboscis activity in restrained Drosophila melanogaster. The protocol was adapted from those developed for proboscis extension conditioning in the honeybee Apis mellifera. After establishing a scale of small proboscis movements necessary to characterize responses to olfactory stimulation, we applied Pavlovian conditioning, with five trials consisting of paired presentation of a banana odour and a sucrose reward. Drosophila showed conditioned proboscis activity to the odour, with a twofold increase of percentage of responses after the first trial. No change occurred in flies experiencing unpaired presentations of the stimuli, confirming an associative basis for this form of olfactory learning. The adenylyl cyclase mutant rutabaga did not exhibit learning in this paradigm. This protocol generated at least a short-term memory of 15 min, but no significant associative memory was detected at 1 h. We also showed that learning performance was dependent on food motivation, by comparing flies subjected to different starvation regimes.

A Taste Receptor Required for the Caffeine Response In Vivo

Caffeine is a methylxanthine present in the coffee tree, tea plant, and other naturally occurring sources and is among the most commonly consumed drugs worldwide. Whereas the pharmacological action of caffeine has been studied extensively, relatively little is known concerning the molecular mechanism through which this substance is detected as a bitter compound. Unlike most tastants, which are detected through cell-surface G protein-coupled receptors, it has been proposed that caffeine and related methylxanthines activate taste-receptor cells through inhibition of a cyclic nucleotide phosphodiesterase (PDE) . Here, we show that the gustatory receptor Gr66a is expressed in the dendrites of Drosophila gustatory receptor neurons and is essential for the caffeine response. In a behavioral assay, the aversion to caffeine was specifically disrupted in flies missing Gr66a. Caffeine-induced action potentials were also eliminated, as was the response to theophylline, the methylxanthine in tea. The Gr66a mutant exhibited normal tastant-induced action potentials upon presentation of theobromine, a methylxanthine in cocoa. Given that theobromine and caffeine inhibit PDEs with equal potencies , these data further support the role of Gr66a rather than a PDE in mediating the caffeine response. Gr66a is the first gustatory receptor shown to be essential for caffeine-induced behavior and activity of gustatory receptor cells in vivo.

A novel Takeout-like protein expressed in the taste and olfactory organs of the blowfly, Phormia regina

In insects, the functional molecules responsible for the taste system are still obscure. The gene for a 28.5 kDa protein purified from taste sensilla of the blowfly Phormia regina belongs to a gene family that includes takeout of Drosophila melanogaster. Molecular phylogenetic analysis revealed that the Phormia Takeout-like protein is most similar to the protein encoded by a member of the Drosophila takeout gene family, CG14661, whose expression and function have not been identified yet. Western blot analyses revealed that Phormia Takeout-like protein was exclusively expressed in antennae and labellum of the adult blowfly in both sexes. Immunohistochemical experiments demonstrated that Takeout-like protein was localized around the lamella structure of the auxiliary cells and in the sensillar lymph of the labellar taste sensillum. In antennae, Takeout-like protein was distributed at the base of the olfactory sensilla as well. No significant differences in Takeout-like protein expression were found between the sexes. Our results suggest that Phormia Takeout-like protein is involved in some early events concerned with chemoreception in both the taste and olfactory systems.

Gsalpha is involved in sugar perception in Drosophila melanogaster

In Drosophila melanogaster, gustatory receptor genes (Grs) encode G-protein-coupled receptors (GPCRs) in gustatory receptor neurons (GRNs) and some olfactory receptor neurons. One of the Gr genes, Gr5a, encodes a sugar receptor that is expressed in a subset of GRNs and has been most extensively studied both molecularly and physiologically, but the G-protein alpha subunit (Galpha) that is coupled to this sugar receptor remains unknown. Here, we propose that Gs is the Galpha that is responsible for Gr5a-mediated sugar-taste transduction, based on the following findings: First, immunoreactivities against Gs were detected in a subset of GRNs including all Gr5a-expressing neurons. Second, trehalose-intake is reduced in flies heterozygous for null mutations in DGsalpha, a homolog of mammalian Gs, and trehalose-induced electrical activities in sugar-sensitive GRNs were depressed in those flies. Furthermore, expression of wild-type DGsalpha in sugar-sensitive GRNs in heterozygotic DGsalpha mutant flies rescued those impairments. Third, expression of double-stranded RNA for DGsalpha in sugar-sensitive GRNs depressed both behavioral and electrophysiological responses to trehalose. Together, these findings indicate that DGsalpha is involved in trehalose perception. We suggest that sugar-taste signals are processed through the Gsalpha-mediating signal transduction pathway in sugar-sensitive GRNs in Drosophila.

Pleiotropic fitness effects of the Tre1-Gr5a region in Drosophila melanogaster

The abundance of transposable elements and DNA repeat sequences in mammalian genomes raises the question of whether such insertions represent passive evolutionary baggage or may influence the expression of complex traits. We addressed this question in Drosophila melanogaster, in which the effects of single transposable elements on complex traits can be assessed in genetically identical individuals reared in controlled environments. Here we demonstrate that single P-element insertions in the intergenic region between the gustatory receptor 5a (Gr5a, also known as Tre) and trapped in endoderm 1 (Tre1), which encodes an orphan receptor, exert complex pleiotropic effects on fitness traits, including selective nutrient intake, life span, and resistance to starvation and heat stress. Mutations in this region interact epistatically with downstream components of the insulin signaling pathway. Transposon-induced sex-specific and sex-antagonistic effects further accentuate the complex influences that intergenic transposable elements can contribute to quantitative trait phenotypes.

Molecular and cellular organization of insect chemosensory neurons

Animals use their chemosensory systems to detect and discriminate among chemical cues in the environment. Remarkable progress has recently been made in our knowledge of the molecular and cellular basis of chemosensory perception in insects, based largely on studies in Drosophila. This progress has been possible due to the identification of gene families for olfactory and gustatory receptors, the use of electro-physiological recording techniques on sensory neurons, the multitude of genetic manipulations that are available in this species, and insights from several insect model systems. Recent studies show that the superfamily of chemoreceptor proteins represent the essential elements in chemosensory coding, endowing chemosensory neurons with their abilities to respond to specific sets of odorants, tastants or pheromones. Investigating how insects detect chemicals in their environment can show us how receptor protein structures relate to ligand binding, how nervous systems process complex information, and how chemosensory systems and genes evolve.

Taste recognition: food for thought

The ability to identify food that is nutrient-rich and avoid toxic substances is essential for an animal's survival. Although olfaction and vision contribute to food detection, the gustatory system acts as a final checkpoint control for food acceptance or rejection behavior. Recent studies with model organisms such as mice and Drosophila have identified candidate taste receptors and examined the logic of taste coding in the periphery. Despite differences in terms of gustatory anatomy and taste-receptor families, these gustatory systems share a basic organization that is different from other sensory systems. This review will summarize our current understanding of taste recognition in mammals and Drosophila, highlighting similarities and raising several as yet unanswered questions.

Cellular identification of water gustatory receptor neurons and their central projection pattern in Drosophila

Water perception is important for insects, because they are particularly vulnerable to water loss because their body size is small. In Drosophila, gustatory receptor neurons are located at the base of the taste sensilla on the labellum, tarsi, and wing margins. One of the gustatory receptor neurons in typical sensilla is known to respond to water. To reveal the neural mechanisms of water perception in Drosophila, it is necessary to identify water receptor neurons and their projection patterns. We used a Gal4 enhancer trap strain in which GAL4 is expressed in a single gustatory receptor neuron in each sensillum on the labellum. We investigated the function of these neurons by expressing the upstream activating sequence transgenes, shibire(ts1), tetanus toxin light chain, or diphtheria toxin A chain. Results from the proboscis extension reflex test and electrophysiological recordings indicated that the GAL4-expressing neurons respond to water. We show here that the water receptor neurons project to a specific region in the subesophageal ganglion, thus revealing the water taste sensory map in Drosophila.

Projection patterns of gustatory neurons in the suboesophageal ganglion and tritocerebrum of mosquitoes

Mosquitoes are heavily dependent on gustatory information when feeding. Following the recent elucidation of the molecular basis of gustation in the malaria mosquito, we present a detailed study of primary central projections of gustatory receptor neurons into the brain in the malaria (Anopheles gambiae) and yellow fever (Aedes aegypti) mosquito. In the brain we provide a detailed map of the areas targeted and describe a number of intrinsic neural elements connecting primary taste areas to higher brain levels. The morphological features described are discussed and compared to earlier reports in other insects as, e.g., the fruitfly, Drosophila.

Insect odor and taste receptors

Insect odor and taste receptors are highly sensitive detectors of food, mates, and oviposition sites. Following the identification of the first insect odor and taste receptors in Drosophila melanogaster, these receptors were identified in a number of other insects, including the malaria vector mosquito Anopheles gambiae; the silk moth, Bombyx mori; and the tobacco budworm, Heliothis virescens. The chemical specificities of many of the D. melanogaster receptors, as well as a few of the A. gambiae and B. mori receptors, have now been determined either by analysis of deletion mutants or by ectopic expression in in vivo or heterologous expression systems. Here we discuss recent advances in our understanding of the molecular and cellular basis of odor and taste coding in insects.

Insect chemoreception

Insect chemoreception is mediated by a large and diverse superfamily of seven-transmembrane domain receptors. These receptors were first identified in Drosophila, but have since been found in other insects, including mosquitoes and moths. Expression and functional analysis of these receptors have been used to identify receptor ligands and to map receptors to functional classes of neurons. Many receptors detect general odorants or tastants, whereas some detect pheromones. The non-canonical receptor Or83b, which is highly conserved across insect orders, dimerizes with odorant and pheromone receptors and is required for efficient localization of these proteins to dendrites of sensory neurons. These studies provide a foundation for understanding the molecular and cellular basis of olfactory and gustatory coding.

Antennal expressed genes of the yellow fever mosquito (Aedes aegypti L.); characterization of odorant-binding protein 10 and takeout

A small cDNA library was constructed from antennae of 100 adult male Aedes aegypti yellow fever mosquitoes. Sequencing of 80 clones identified 49 unique gene products, including a member of the Odorant Binding Protein family (Aaeg-OBP10), a homologue of Takeout (Aaeg-TO), and transposable elements of the LINE, SINE and MITE classes. Aaeg-OBP10 encodes a 140 amino acid protein including a predicted 25 amino acid signal peptide. Aaeg-OBP10 expression was adult male enriched, increased with adult age, and greatest in antennae and wings but also present in maxillary palps, proboscis and leg. Aaeg-OBP10 is a likely orthologue of Agam-OBP10 of the malaria mosquito Anopheles gambiae and shares significant similarity with members of the OBP56 gene cluster of Drosophila melanogaster. These OBP genes may represent a unified class of OBPs with unique roles in chemodetection; the expression pattern of Aaeg-OBP10 suggests it may play a role in adult male chemosensory behavior. Aaeg-TO encodes a 248 amino acid protein including a predicted 22 amino acid signal peptide. Aaeg-TO is homologous with the circadian/feeding regulated D. melanogaster Takeout protein (Dmel-TO) and a subclass of Juvenile Hormone Binding Proteins (JHBP) characterized by Moling from Manduca sexta; both Dmel-TO and Moling are sensitive to feeding, suggesting Aaeg-TO might regulate the antennal response to food, host or pheromonal odors in a JH sensitive manner. Aaeg-TO was used to identify 25 D. melanogaster and 13 A. gambiae homologues by Blast analysis suggesting these may comprise a relatively large class of protein involved in the hormonal regulation of behavior.

Gustatory Perception and Behavior in Drosophila melanogaster

The sense of taste is essential for the survival of virtually all animals. Considered a 'primitive sense' and present in the form of chemotaxis in many bacteria, taste is also a sense of sophistication in humans. Regardless, taste behavior is a crucial activity for the world's most abundant (insects) and most successful (mammals) inhabitants, providing a means of discrimination between nutrient-rich substrates, such as sugars and amino acids, from harmful, mostly bitter-tasting chemicals present in many plants. In this review, we present an update on progress in understanding taste perception in the model fruit fly Drosophila melanogaster. An introduction to the fly's taste system will be presented first, followed by a description of relevant behavioral assays developed to quantify taste perception at the organismal level and a short overview of electrophysiological studies performed on taste cells. The focal point will be the recent molecular-genetic investigations of the gustatory receptor (Gr) genes, which is complemented by a comparison between Drosophila and mammalian taste perception and transduction. Finally, we provide a perspective on the future of Drosophila taste research, including three specific proposals that seem uniquely applicable to this exquisite model system and cannot, at least currently, be pursued elsewhere.

The molecular basis of odor coding in the Drosophila larva

We have analyzed the molecular basis of odor coding in the Drosophila larva. A subset of Or genes is found to be expressed in larval olfactory receptor neurons (ORNs). Using an in vivo expression system and electrophysiology, we demonstrate that these genes encode functional odor receptors and determine their response spectra with 27 odors. The receptors vary in their breadth of tuning, exhibit both excitation and inhibition, and show different onset and termination kinetics. An individual receptor appears to transmit signals via a single ORN to a single glomerulus in the larval antennal lobe. We provide a spatial map of odor information in the larval brain and find that ORNs with related functional specificity map to related spatial positions. The results show how one family of receptors underlies odor coding in two markedly different olfactory systems; they also provide a molecular mechanism to explain longstanding observations of larval odor discrimination.

Neural circuitry that governs Drosophila male courtship behavior

Male-specific fruitless (fru) products (Fru(M)) are both necessary and sufficient to "hardwire" the potential for male courtship behavior into the Drosophila nervous system. Fru(M) is expressed in approximately 2% of neurons in the male nervous system, but not in the female. We have targeted the insertion of GAL4 into the fru locus, allowing us to visualize and manipulate the Fru(M)-expressing neurons in the male as well as their counterparts in the female. We present evidence that these neurons are directly and specifically involved in male courtship behavior and that at least some of them are interconnected in a circuit. This circuit includes olfactory neurons required for the behavioral response to sex pheromones. Anatomical differences in this circuit that might account for the dramatic differences in male and female sexual behavior are not apparent.

Pseudogenization of a sweet-receptor gene accounts for cats' indifference toward sugar

Although domestic cats (Felis silvestris catus) possess an otherwise functional sense of taste, they, unlike most mammals, do not prefer and may be unable to detect the sweetness of sugars. One possible explanation for this behavior is that cats lack the sensory system to taste sugars and therefore are indifferent to them. Drawing on work in mice, demonstrating that alleles of sweet-receptor genes predict low sugar intake, we examined the possibility that genes involved in the initial transduction of sweet perception might account for the indifference to sweet-tasting foods by cats. We characterized the sweet-receptor genes of domestic cats as well as those of other members of the Felidae family of obligate carnivores, tiger and cheetah. Because the mammalian sweet-taste receptor is formed by the dimerization of two proteins (T1R2 and T1R3; gene symbols Tas1r2 and Tas1r3), we identified and sequenced both genes in the cat by screening a feline genomic BAC library and by performing PCR with degenerate primers on cat genomic DNA. Gene expression was assessed by RT-PCR of taste tissue, in situ hybridization, and immunohistochemistry. The cat Tas1r3 gene shows high sequence similarity with functional Tas1r3 genes of other species. Message from Tas1r3 was detected by RT-PCR of taste tissue. In situ hybridization and immunohistochemical studies demonstrate that Tas1r3 is expressed, as expected, in taste buds. However, the cat Tas1r2 gene shows a 247-base pair microdeletion in exon 3 and stop codons in exons 4 and 6. There was no evidence of detectable mRNA from cat Tas1r2 by RT-PCR or in situ hybridization, and no evidence of protein expression by immunohistochemistry. Tas1r2 in tiger and cheetah and in six healthy adult domestic cats all show the similar deletion and stop codons. We conclude that cat Tas1r3 is an apparently functional and expressed receptor but that cat Tas1r2 is an unexpressed pseudogene. A functional sweet-taste receptor heteromer cannot form, and thus the cat lacks the receptor likely necessary for detection of sweet stimuli. This molecular change was very likely an important event in the evolution of the cat's carnivorous behavior.

A single-amino-acid change of the gustatory receptor gene, Gr5a, has a major effect on trehalose sensitivity in a natural population of Drosophila melanogaster

Variation in trehalose sensitivity and nucleotide sequence polymorphism of the Gr5a gene encoding the gustatory receptor to sugar trehalose were investigated in 152 male lines of Drosophila melanogaster collected from a natural population. Among the observed 59 segregating sites, some pairs of sites showed significant linkage disequilibrium. A single SNP, which results in the Ala218Thr amino acid change, was significantly associated with trehalose sensitivity, as previously suggested. Threonine at amino acid position 218 was found to be the ancestral form in D. melanogaster, suggesting that low trehalose sensitivity was an ancestral form with respect to the receptor function. There was large genetic variation in trehalose sensitivity. It was continuously distributed, indicating that trehalose sensitivity measured by the behavioral assay is a quantitative trait. These results suggest that apart from the Gr5a gene, other genetic factors contribute to variation in trehalose sensitivity. Nucleotide diversity (pi) and nucleotide variation (theta) per site were 0.00874 and 0.00590, respectively. Fu and Li's test and the MK test showed no significant departure from the expectation of selective neutrality in the Gr5a gene. However, we rejected selective neutrality by Tajima's test and Fay and Wu's test with the observed level of recombination. We discuss possible causes of the observed pattern of nucleotide variation in the gustatory receptor Gr5a gene.

Taste perception and coding in Drosophila

BACKGROUND

Discrimination between edible and contaminated foods is crucial for the survival of animals. In Drosophila, a family of gustatory receptors (GRs) expressed in taste neurons is thought to mediate the recognition of sugars and bitter compounds, thereby controlling feeding behavior.

RESULTS

We have characterized in detail the expression of eight Gr genes in the labial palps, the fly's main taste organ. These genes fall into two distinct groups: seven of them, including Gr66a, are expressed in 22 or fewer taste neurons in each labial palp. Additional experiments show that many of these genes are coexpressed in partially overlapping sets of neurons. In contrast, Gr5a, which encodes a receptor for trehalose, is expressed in a distinct and larger set of taste neurons associated with most chemosensory sensilla, including taste pegs. Mapping the axonal targets of cells expressing Gr66a and Gr5a reveals distinct projection patterns for these two groups of neurons in the brain. Moreover, tetanus toxin-mediated inactivation of Gr66a- or Gr5a-expressing cells shows that these two sets of neurons mediate distinct taste modalities-the perception of bitter (caffeine) and sweet (trehalose) taste, respectively.

CONCLUSION

Discrimination between two taste modalities-sweet and bitter-requires specific sets of gustatory receptor neurons that express different Gr genes. Unlike the Drosophila olfactory system, where each neuron expresses a single olfactory receptor gene, taste neurons can express multiple receptors and do so in a complex Gr gene code that is unique for small sets of neurons.

The molecular basis of odor coding in the Drosophila antenna

We have undertaken a functional analysis of the odorant receptor repertoire in the Drosophila antenna. Each receptor was expressed in a mutant olfactory receptor neuron (ORN) used as a "decoder," and the odor response spectrum conferred by the receptor was determined in vivo by electrophysiological recordings. The spectra of these receptors were then matched to those of defined ORNs to establish a receptor-to-neuron map. In addition to the odor response spectrum, the receptors dictate the signaling mode, i.e., excitation or inhibition, and the response dynamics of the neuron. An individual receptor can mediate both excitatory and inhibitory responses to different odorants in the same cell, suggesting a model of odorant receptor transduction. Receptors vary widely in their breadth of tuning, and odorants vary widely in the number of receptors they activate. Together, these properties provide a molecular basis for odor coding by the receptor repertoire of an olfactory organ.

Drosophila gustatory receptors: from gene identification to functional expression

Recent years have seen long-awaited progress in understanding of the molecular mechanisms of taste perception in insects. The breakthrough came in the early 2000 with the identification of a novel family of candidate gustatory receptor (Gr) genes in the first release of the Drosophila melanogaster genome sequence. The 60 Gr genes are expressed in the subsets of gustatory neurons in the fly's taste organs and, without exception, encode heptahelical G protein-coupled receptors (GPCRs). Here I review our current knowledge about Gr genes and their products focusing on the newly emerging information regarding the function of the Gr-encoded proteins.

Taste Representations in the Drosophila Brain

Drosophila taste compounds with gustatory neurons on many parts of the body, suggesting that a fly detects both the location and quality of a food source. For example, activation of taste neurons on the legs causes proboscis extension or retraction, whereas activation of proboscis taste neurons causes food ingestion or rejection. We examined whether the features of taste location and taste quality are mapped in the fly brain using molecular, genetic, and behavioral approaches. We find that projections are segregated by the category of tastes that they recognize: neurons that recognize sugars project to a region different from those recognizing noxious substances. Transgenic axon labeling experiments also demonstrate that gustatory projections are segregated based on their location in the periphery. These studies reveal the gustatory map in the first relay of the fly brain and demonstrate that taste quality and position are represented in anatomical projection patterns.

A highly conserved candidate chemoreceptor expressed in both olfactory and gustatory tissues in the malaria vector Anopheles gambiae

Anopheles gambiae is a highly anthropophilic mosquito responsible for the majority of malaria transmission in Africa. The biting and host preference behavior of this disease vector is largely influenced by its sense of smell, which is presumably facilitated by G protein-coupled receptor signaling [Takken, W. & Knols, B. (1999) Annu. Rev. Entomol. 44, 131-157]. Because of the importance of host preference to the mosquitoes' ability to transmit disease, we have initiated studies intended to elucidate the molecular mechanisms underlying olfaction in An. gambiae. In the course of these studies, we have identified a number of genes potentially involved in signal transduction, including a family of candidate odorant receptors. One of these receptors, encoded by GPRor7 (hereafter referred to as AgOr7), is remarkably similar to an odorant receptor that is expressed broadly in olfactory tissues and has been identified in Drosophila melanogaster and other insects [Krieger, J., Klink, O., Mohl, C., Raming, K. & Breer, H. (2003) J. Comp. Physiol. A 189, 519-526; Vosshall, L. B., Amrein, H., Morozov, P. S., Rzhetsky, A. & Axel, R. (1999) Cell 96, 725-736]. We have observed AgOr7 expression in olfactory and gustatory tissues in adult An. gambiae and during several stages of the mosquitoes' development. Within the female adult peripheral chemosensory system, antiserum against the AgOR7 polypeptide labels most sensilla of the antenna and maxillary palp as well as a subset of proboscis sensilla. Furthermore, AgOR7 antiserum labeling is observed within the larval antenna and maxillary palpus. These results are consistent with a role for AgOr7 in both olfaction and gustation in An. gambiae and raise the possibility that AgOr7 orthologs may also be of general importance to both modalities of chemosensation in other insects.

Tre1, a G protein-coupled receptor, directs transepithelial migration of Drosophila germ cells

In most organisms, germ cells are formed distant from the somatic part of the gonad and thus have to migrate along and through a variety of tissues to reach the gonad. Transepithelial migration through the posterior midgut (PMG) is the first active step during Drosophila germ cell migration. Here we report the identification of a novel G protein-coupled receptor (GPCR), Tre1, that is essential for this migration step. Maternal tre1 RNA is localized to germ cells, and tre1 is required cell autonomously in germ cells. In tre1 mutant embryos, most germ cells do not exit the PMG. The few germ cells that do leave the midgut early migrate normally to the gonad, suggesting that this gene is specifically required for transepithelial migration and that mutant germ cells are still able to recognize other guidance cues. Additionally, inhibiting small Rho GTPases in germ cells affects transepithelial migration, suggesting that Tre1 signals through Rho1. We propose that Tre1 acts in a manner similar to chemokine receptors required during transepithelial migration of leukocytes, implying an evolutionarily conserved mechanism of transepithelial migration. Recently, the chemokine receptor CXCR4 was shown to direct migration in vertebrate germ cells. Thus, germ cells may more generally use GPCR signaling to navigate the embryo toward their target.

A novel G protein-coupled receptor (GPCR) is shown to be essential for transepithelial migration of Drosophila germ cells. Leukocyte transepithelial migration also requires GPCR signaling, suggesting a conserved mechanism.

Drosophila Gr5a encodes a taste receptor tuned to trehalose

Recent studies have suggested that Drosophila taste receptors are encoded by a family of G protein-coupled receptor genes comprising at least 56 members. One of these genes, Gr5a, has been shown by genetic analysis to be required by the fly for behavioral and sensory responses to a sugar, trehalose. Here, we show that Gr5a is expressed in neurons of taste sensilla located on the labellum and legs. Expression is observed in most if not all labellar sensilla and suggests that many taste neurons express more than one receptor. We demonstrate by heterologous expression in a Drosophila S2 cell line that Gr5a encodes a receptor tuned to trehalose. This is the first functional expression of an invertebrate taste receptor.

Molecular evolution of the insect chemoreceptor gene superfamily in Drosophila melanogaster

The insect chemoreceptor superfamily in Drosophila melanogaster is predicted to consist of 62 odorant receptor (Or) and 68 gustatory receptor (Gr) proteins, encoded by families of 60 Or and 60 Gr genes through alternative splicing. We include two previously undescribed Or genes and two previously undescribed Gr genes; two previously predicted Or genes are shown to be alternative splice forms. Three polymorphic pseudogenes and one highly defective pseudogene are recognized. Phylogenetic analysis reveals deep branches connecting multiple highly divergent clades within the Gr family, and the Or family appears to be a single highly expanded lineage within the superfamily. The genes are spread throughout the Drosophila genome, with some relatively recently diverged genes still clustered in the genome. The Gr5a gene on the X chromosome, which encodes a receptor for the sugar trehalose, has transposed from one such tandem cluster of six genes at cytological location 64, as has Gr61a, and all eight of these receptors might bind sugars. Analysis of intron evolution suggests that the common ancestor consisted of a long N-terminal exon encoding transmembrane domains 1-5 followed by three exons encoding transmembrane domains 6-7. As many as 57 additional introns have been acquired idiosyncratically during the evolution of the superfamily, whereas the ancestral introns and some of the older idiosyncratic introns have been lost at least 48 times independently. Altogether, these patterns of molecular evolution suggest that this is an ancient superfamily of chemoreceptors, probably dating back at least to the origin of the arthropods.

A putative Drosophila pheromone receptor expressed in male-specific taste neurons is required for efficient courtship

Propagation in higher animals requires the efficient and accurate display of innate mating behaviors. In Drosophila melanogaster, male courtship consists of a stereotypic sequence of behaviors involving multiple sensory modalities, such as vision, audition, and chemosensation. For example, taste bristles located in the male forelegs and the labial palps are thought to recognize nonvolatile pheromones secreted by the female. Here, we report the identification of the putative pheromone receptor GR68a, which is expressed in chemosensory neurons of about 20 male-specific gustatory bristles in the forelegs. Gr68a expression is dependent on the sex determination gene doublesex, which controls many aspects of sexual differentiation and is necessary for normal courtship behavior. Tetanus toxin-mediated inactivation of Gr68a-expressing neurons or transgene-mediated RNA interference of Gr68a RNA leads to a significant reduction in male courtship performance, suggesting that GR68a protein is an essential component of pheromone-driven courtship behavior in Drosophila.

Experimental studies of adult Drosophila chemosensory behaviour

Drosophila melanogaster is a powerful animal model to study the processes underlying behavioural responses to chemical cues. This paper provides a review of the important literature to present recent advances in our understanding of how gustatory and olfactory stimuli are perceived. An overview is given of the experimental procedures currently used to characterize the fly chemosensory behaviour. Since this species provides extremely useful genetic tools, a focus is made on those allowing to manipulate behaviour, and hence to understand its molecular and cellular bases. Such tools include single-gene mutants and the Gal4/UAS system. They can be combined with studies of the natural polymorphism of behavioural responses. Recent data obtained with these various approaches unravel some important aspects of taste and olfaction. These appear as rather complex processes, as revealed by results showing dose-dependence, plasticity and sexual dimorphism. Taken together, these results and the available tools open interesting perspectives for the years to come, in our attempts to make the link between genes and behaviour.

Peripheral coding of bitter taste in Drosophila

Taste receptors play a crucial role in detecting the presence of bitter compounds such as alkaloids, and help to prevent the ingestion of toxic food. In Drosophila, we show for the first time that several taste sensilla on the prothoracic legs detect bitter compounds both through the activation of specific taste neurons but also through inhibition of taste neurons activated by sugars and water. Each sensillum usually houses a cluster of four taste neurons classified according to their best stimulus (S for sugar, W for Water, L1 and L2 for salts). Using a new statistical approach based on the analysis of interspike intervals, we show that bitter compounds activate the L2 cell. Bitter-activated L2 cells were excited with a latency of at least 50 ms. Their sensitivity to bitter compounds was different between sensilla, suggesting that specific receptors to bitter compounds are differentially expressed among L2 cells. When presented in mixtures, bitter compounds inhibited the responses of S and W, but not the L1 cell. The inhibition was effective even in sensilla where bitter compounds did not activate the L2 cell, indicating that bitter compounds directly interact with the S and W cells. Interestingly, this inhibition occurred with latencies similar to the excitation of bitter-activated L2 cells. It suggests that the inhibition in the W and S cells shares similar transduction pathways with the excitation in the L2 cells. Combined with molecular approaches, the results presented here should provide a physiological basis to understand how bitter compounds are detected and discriminated.

Integrating the molecular and cellular basis of odor coding in the Drosophila antenna

We investigate how the molecular and cellular maps of the Drosophila olfactory system are integrated. A correspondence is established between individual odor receptors, neurons, and odors. We describe the expression of the Or22a and Or22b receptor genes, show localization to dendritic membranes, and find sexual dimorphism. Or22a maps to the ab3A neuron, which responds to ethyl butyrate. Analysis of a deletion mutant lacking Or22a, along with transgenic rescue experiments, confirms the mapping and demonstrates that an Or gene is required for olfactory function in vivo. Ectopic expression of Or47a in a mutant cell identifies the neuron from which it derives and its odor ligands. Ectopic expression in a wild-type cell shows that two receptors can function in a single cell. The ab3A neuron does not depend on normal odor receptor gene expression to navigate to its target in the CNS.

Taste and pheromone perception in mammals and flies

The olfactory systems of insects and mammals have analogous anatomical features and use similar molecular logic for olfactory coding. The molecular underpinnings of the chemosensory systems that detect taste and pheromone cues have only recently been characterized. Comparison of these systems in Drosophila and mouse uncovers clear differences and a few surprising similarities.

G protein-coupled receptors in Anopheles gambiae

We used bioinformatic approaches to identify a total of 276 G protein-coupled receptors (GPCRs) from the Anopheles gambiae genome. These include GPCRs that are likely to play roles in pathways affecting almost every aspect of the mosquito's life cycle. Seventy-nine candidate odorant receptors were characterized for tissue expression and, along with 76 putative gustatory receptors, for their molecular evolution relative to Drosophila melanogaster. Examples of lineage-specific gene expansions were observed as well as a single instance of unusually high sequence conservation.

Contribution of different taste cells and signaling pathways to the discrimination of "bitter" taste stimuli by an insect

Animals can discriminate among many different types of foods. This discrimination process involves multiple sensory systems, but the sense of taste is known to play a central role. We asked how the taste system contributes to the discrimination of different "bitter" taste stimuli in Manduca sexta caterpillars. This insect has approximately eight bilateral pairs of taste cells that respond selectively to bitter taste stimuli. Each bilateral pair of bitter-sensitive taste cells has a different molecular receptive range (MRR); some of these taste cells also contain two signaling pathways with distinctive MRRs and temporal patterns of spiking. To test for discrimination, we habituated the caterpillar's taste-mediated aversive response to one bitter taste stimulus (salicin) and then asked whether this habituation phenomenon generalized to four other bitter taste stimuli (caffeine, aristolochic acid, Grindelia extract, and Canna extract). We inferred that the two compounds were discriminable if the habituation phenomenon failed to generalize (e.g., from salicin to aristolochic acid). We found that M. sexta could discriminate between salicin and those bitter taste stimuli that activate (1) different populations of bitter-sensitive taste cells (Grindelia extract and Canna extract) or (2) different signaling pathways within the same bitter-sensitive taste cell (aristolochic acid). M. sexta could not discriminate between salicin and a bitter taste stimulus that activates the same signaling pathway within the same bitter-sensitive taste cell (caffeine). We propose that the heterogeneous population of bitter-sensitive taste cells and signaling pathways within this insect facilitates the discrimination of bitter taste stimuli.

Gene discovery by e-genetics: Drosophila odor and taste receptors

A new algorithm that examines DNA databases for proteins that have a particular structure, as opposed to a particular sequence, represents a novel`e-genetics' approach to gene discovery. The algorithm has successfully identified new G-protein-coupled receptors, which have a characteristic seven-transmembrane-domain structure, from the Drosophila genome database. In particular, it has revealed novel families of odor receptors and taste receptors, which had long eluded identification by other means. The two new gene families, the Or and Gr genes, are expressed in neurons of olfactory and taste sensilla and are highly divergent from all other known G-protein-coupled receptor genes. Modification of the algorithm should allow identification of other classes of multitransmembrane-domain protein.