Integration of complex larval chemosensory organs into the adult nervous system ofDrosophila

A description of the bat star nervous system throughout larval ontogeny

Larvae represent a distinct life history stage in which animal morphology and behavior contrast strongly to adult organisms. This life history stage is a ubiquitous aspect of animal life cycles, particularly in the marine environment. In many species, the structure and function of the nervous system differ significantly between metamorphosed juveniles and larvae. However, the distribution and diversity of neural cell types in larval nervous systems remains incompletely known. Here, the expression of neurotransmitter and neuropeptide synthesis and transport genes in the bat star Patiria miniata is examined throughout larval development. This characterization of nervous system structure reveals three main neural regions with distinct but overlapping territories. These regions include a densely innervated anterior region, an enteric neural plexus, and neurons associated with the ciliary band. In the ciliary band, cholinergic cells are pervasive while dopaminergic, noradrenergic, and GABAergic cells show regional differences in their localization patterns. Furthermore, the distribution of some neural subtypes changes throughout larval development, suggesting that changes in nervous system structure align with shifting ecological priorities during different larval stages, before the development of the adult nervous system. While past work has described aspects of P. miniata larval nervous system structure, largely focusing on early developmental timepoints, this work provides a comprehensive description of neural cell type localization throughout the extensive larval period.

Drosophila melanogaster Chemosensory Pathways as Potential Targets to Curb the Insect Menace

Simple Summary

The perception and processing of chemosensory stimuli are indispensable to the survival of living organisms. In insects, olfaction and gustation play a critical role in seeking food, finding mates and avoiding signs of danger. This review aims to present updated information about olfactory and gustatory signaling in the fruit fly Drosophila melanogaster. We have described the mechanisms involved in olfactory and gustatory perceptions at the molecular level, the receptors along with the allied molecules involved, and their signaling pathways in the fruit fly. Due to the magnifying problems of disease-causing insect vectors and crop pests, the applications of chemosensory signaling in controlling pests and insect vectors are also discussed.

Abstract

From a unicellular bacterium to a more complex human, smell and taste form an integral part of the basic sensory system. In fruit flies Drosophila melanogaster, the behavioral responses to odorants and tastants are simple, though quite sensitive, and robust. They explain the organization and elementary functioning of the chemosensory system. Molecular and functional analyses of the receptors and other critical molecules involved in olfaction and gustation are not yet completely understood. Hence, a better understanding of chemosensory cue-dependent fruit flies, playing a major role in deciphering the host-seeking behavior of pathogen transmitting insect vectors (mosquitoes, sandflies, ticks) and crop pests (Drosophila suzukii, Queensland fruit fly), is needed. Using D. melanogaster as a model organism, the knowledge gained may be implemented to design new means of controlling insects as well as in analyzing current batches of insect and pest repellents. In this review, the complete mechanisms of olfactory and gustatory perception, along with their implementation in controlling the global threat of disease-transmitting insect vectors and crop-damaging pests, are explained in fruit flies.

The chemosensory system of the Drosophila larva: an overview of current understanding

Animals must sense their surroundings and be able to distinguish between relevant and irrelevant cues. An enticing area of research aims to uncover the mechanisms by which animals respond to chemical signals that constitute critical sensory input. In this review, we describe the principles of a model chemosensory system: the Drosophila larva. While distinct in many ways, larval behaviour is reminiscent of the dogmatic goals of life: to reach a stage of reproductive potential. It takes into account a number of distinct and identifiable parameters to ultimately provoke or modulate appropriate behavioural output. In this light, we describe current knowledge of chemosensory anatomy, genetic components, and the processing logic of chemical cues. We outline recent advancements and summarize the hypothesized neural circuits of sensory systems. Furthermore, we note yet-unanswered questions to create a basis for further investigation of molecular and systemic mechanisms of chemosensation in Drosophila and beyond.

Combinatorial Pharyngeal Taste Coding for Feeding Avoidance in Adult Drosophila

Taste drives appropriate food preference and intake. In Drosophila, taste neurons are housed in both external and internal organs, but the latter have been relatively underexplored. Here, we report that Poxn mutants with a minimal taste system of pharyngeal neurons can avoid many aversive tastants, including bitter compounds, acid, and salt, suggesting that pharyngeal taste is sufficient for rejecting intake of aversive compounds. Optogenetic activation of selected pharyngeal bitter neurons during feeding events elicits changes in feeding parameters that can suppress intake. Functional dissection experiments indicate that multiple classes of pharyngeal neurons are involved in achieving behavioral avoidance, by virtue of being inhibited or activated by aversive tastants. Tracing second-order pharyngeal circuits reveals two main relay centers for processing pharyngeal taste inputs. Together, our results suggest that the pharynx can control the ingestion of harmful compounds by integrating taste input from different classes of pharyngeal neurons.

Chen et al. perform functional and behavioral experiments to study the roles of different subsets of pharyngeal neurons in governing food avoidance in flies. They find evidence that rejection of different categories of aversive compounds is dependent on distinct combinations of pharyngeal taste neurons.

Cellular Basis of Bitter-Driven Aversive Behaviors in Drosophila Larva

Feeding, a critical behavior for survival, consists of a complex series of behavioral steps. In Drosophila larvae, the initial steps of feeding are food choice, during which the quality of a potential food source is judged, and ingestion, during which the selected food source is ingested into the digestive tract. It remains unclear whether these steps employ different mechanisms of neural perception. Here, we provide insight into the two initial steps of feeding in Drosophila larva. We find that substrate choice and ingestion are determined by independent circuits at the cellular level. First, we took 22 candidate bitter compounds and examined their influence on choice preference and ingestion behavior. Interestingly, certain bitter tastants caused different responses in choice and ingestion, suggesting distinct mechanisms of perception. We further provide evidence that certain gustatory receptor neurons (GRNs) in the external terminal organ (TO) are involved in determining choice preference, and a pair of larval pharyngeal GRNs is involved in mediating both avoidance and suppression of ingestion. Our results show that feeding behavior is coordinated by a multistep regulatory process employing relatively independent neural elements. These findings are consistent with a model in which distinct sensory pathways act as modulatory circuits controlling distinct subprograms during feeding.

The prandial process in flies

Feeding is fundamental to any heterotroph organism; in its role to quell hunger it overrides most other motivational states. But feeding also literally opens the door to harmful risks, especially for a saprophagous animal like Drosophila; ingestion of poisonous substrate can lead to irreversible damage. Thus feeding incorporates a series of steps with several checkpoints to guarantee that the ingestion remains beneficial and provides a balanced diet, or the feeding process is interrupted. Subsequently, we will summarize and describe the feeding process in Drosophila in a comprehensive manner. We propose eleven distinct steps for feeding, grouped into four categories, to address our current knowledge of prandial regulatory mechanisms in Drosophila.

Identification and functional characterization of D-fructose receptor in an egg parasitoid, Trichogramma chilonis

In insects, the gustatory system has a critical function not only in selecting food and feeding behaviours but also in growth and metabolism. Gustatory receptors play an irreplaceable role in insect gustatory signalling. Trichogramma chilonis is an effective biocontrol agent against agricultural insect pests. However, the molecular mechanism of gustation in T. chilonis remains elusive. In this study, we found that T. chilonis adults had a preference for D-fructose and that D-fructose contributed to prolong longevity and improve fecundity. Then, We also isolated the full-length cDNA encoding candidate gustatory receptor (TchiGR43a) based on the transcriptome data of T. chilonis, and observed that the candidate gustatory receptor gene was expressed from the larval to adult stages. The expression levels of TchiGR43a were similar between female and male. A Xenopus oocyte expression system and two-electrode voltage-clamp recording further verified the function analysis of TchiGR43a. Electrophysiological results showed that TchiGR43a was exclusively tuned to D-fructose. By the studies of behaviour, molecular biology and electrophysiology in T. chilonis, our results lay a basic fundation of further study on the molecular mechanisms of gustatory reception and provide theoretical basis for the nutritional requirement of T. chilonis in biocontrol.

Salt an Essential Nutrient: Advances in Understanding Salt Taste Detection Using Drosophila as a Model System

Taste modalities are conserved in insects and mammals. Sweet gustatory signals evoke attractive behaviors while bitter gustatory information drive aversive behaviors. Salt (NaCl) is an essential nutrient required for various physiological processes, including electrolyte homeostasis, neuronal activity, nutrient absorption, and muscle contraction. Not only mammals, even in Drosophila melanogaster, the detection of NaCl induces two different behaviors: Low concentrations of NaCl act as an attractant, whereas high concentrations act as repellant. The fruit fly is an excellent model system for studying the underlying mechanisms of salt taste due to its relatively simple neuroanatomical organization of the brain and peripheral taste system, the availability of powerful genetic tools and transgenic strains. In this review, we have revisited the literature and the information provided by various laboratories using invertebrate model system Drosophila that has helped us to understand NaCl salt taste so far. We hope that this compiled information from Drosophila will be of general significance and interest for forthcoming studies of the structure, function, and behavioral role of NaCl-sensitive (low and high concentrations) gustatory circuitry for understanding NaCl salt taste in all animals.

Using Pox-Neuro (Poxn) Mutants in Drosophila Gustation Research: A Double-Edged Sword

In Drosophila, Pox-neuro (Poxn) is a member of the Paired box (Pax) gene family that encodes transcription factors with characteristic paired DNA-binding domains. During embryonic development, Poxn is expressed in sensory organ precursor (SOP) cells of poly-innervated external sensory (p-es) organs and is important for specifying p-es organ identity (chemosensory) as opposed to mono-innervated external sensory (m-es) organs (mechanosensory). In Poxn mutants, there is a transformation of chemosensory bristles into mechanosensory bristles. As a result, these mutants have often been considered to be entirely taste-blind, and researchers have used them in this capacity to investigate physiological and behavioral functions that act in a taste-independent manner. However, recent studies show that only external taste bristles are transformed in Poxn mutants whereas all internal pharyngeal taste neurons remain intact, raising concerns about interpretations of experimental results using Poxn mutants as taste-blind flies. In this review, we summarize the value of Poxn mutants in advancing our knowledge of taste-enriched genes and feeding behaviors, and encourage revisiting some of the conclusions about taste-independent nutrient-sensing mechanisms derived from these mutants. Lastly, we highlight that Poxn mutant flies remain a valuable tool for probing the function of the relatively understudied pharyngeal taste neurons in sensing meal properties and regulating feeding behaviors.

Molecular and Cellular Organization of Taste Neurons in Adult Drosophila Pharynx

The Drosophila pharyngeal taste organs are poorly characterized despite their location at important sites for monitoring food quality. Functional analysis of pharyngeal neurons has been hindered by the paucity of molecular tools to manipulate them, as well as their relative inaccessibility for neurophysiological investigations. Here, we generate receptor-to-neuron maps of all three pharyngeal taste organs by performing a comprehensive chemoreceptor-GAL4/LexA expression analysis. The organization of pharyngeal neurons reveals similarities and distinctions in receptor repertoires and neuronal groupings compared to external taste neurons. We validate the mapping results by pinpointing a single pharyngeal neuron required for feeding avoidance of L-canavanine. Inducible activation of pharyngeal taste neurons reveals functional differences between external and internal taste neurons and functional subdivision within pharyngeal sweet neurons. Our results provide roadmaps of pharyngeal taste organs in an insect model system for probing the role of these understudied neurons in controlling feeding behaviors.

Progressive derivation of serially homologous neuroblast lineages in the gnathal CNS of Drosophila

Along the anterior-posterior axis the central nervous system is subdivided into segmental units (neuromeres) the composition of which is adapted to their region-specific functional requirements. In Drosophila melanogaster each neuromere is formed by a specific set of identified neural stem cells (neuroblasts, NBs). In the thoracic and anterior abdominal region of the embryonic ventral nerve cord segmental sets of NBs resemble the ground state (2nd thoracic segment, which does not require input of homeotic genes), and serial (segmental) homologs generate similar types of lineages. The three gnathal head segments form a transitional zone between the brain and the ventral nerve cord. It has been shown recently that although all NBs of this zone are serial homologs of NBs in more posterior segments, they progressively differ from the ground state in anterior direction (labial > maxillary > mandibular segment) with regard to numbers and expression profiles. To study the consequences of their derived characters we traced the embryonic lineages of gnathal NBs using the Flybow and DiI-labelling techniques. For a number of clonal types serial homology is rather clearly reflected by their morphology (location and projection patterns) and cell specific markers, despite of reproducible segment-specific differences. However, many lineages, particularly in the mandibular segment, show a degree of derivation that impedes their assignment to ground state serial homologs. These findings demonstrate that differences in gene expression profiles of gnathal NBs go along with anteriorly directed progressive derivation in the composition of their lineages. Furthermore, lineage sizes decrease from labial to mandibular segments, which in concert with decreasing NB-numbers lead to reduced volumes of gnathal neuromeres, most significantly in the mandibular segment.

Structure and development of the subesophageal zone of the Drosophila brain. I. Segmental architecture, compartmentalization, and lineage anatomy

The subesophageal zone (SEZ) of the Drosophila brain houses the circuitry underlying feeding behavior and is involved in many other aspects of sensory processing and locomotor control. Formed by the merging of four neuromeres, the internal architecture of the SEZ can be best understood by identifying segmentally reiterated landmarks emerging in the embryo and larva, and following the gradual changes by which these landmarks become integrated into the mature SEZ during metamorphosis. In previous works, the system of longitudinal fibers (connectives) and transverse axons (commissures) has been used as a scaffold that provides internal landmarks for the neuromeres of the larval ventral nerve cord. We have extended the analysis of this scaffold to the SEZ and, in addition, reconstructed the tracts formed by lineages and nerves in relationship to the connectives and commissures. As a result, we establish reliable criteria that define boundaries between the four neuromeres (tritocerebrum, mandibular neuromere, maxillary neuromere, labial neuromere) of the SEZ at all stages of development. Fascicles and lineage tracts also demarcate seven columnar neuropil domains (ventromedial, ventro-lateral, centromedial, central, centrolateral, dorsomedial, dorsolateral) identifiable throughout development. These anatomical subdivisions, presented in the form of an atlas including confocal sections and 3D digital models for the larval, pupal and adult stage, allowed us to describe the morphogenetic changes shaping the adult SEZ. Finally, we mapped MARCM-labeled clones of all secondary lineages of the SEZ to the newly established neuropil subdivisions. Our work will facilitate future studies of function and comparative anatomy of the SEZ.

Structure and development of the subesophageal zone of the Drosophila brain. II. Sensory compartments

The subesophageal zone (SEZ) of the Drosophila brain processes mechanosensory and gustatory sensory input from sensilla located on the head, mouth cavity and trunk. Motor output from the SEZ directly controls the movements involved in feeding behavior. In an accompanying paper (Hartenstein et al., ), we analyzed the systems of fiber tracts and secondary lineages to establish reliable criteria for defining boundaries between the four neuromeres of the SEZ, as well as discrete longitudinal neuropil domains within each SEZ neuromere. Here we use this anatomical framework to systematically map the sensory projections entering the SEZ throughout development. Our findings show continuity between larval and adult sensory neuropils. Gustatory axons from internal and external taste sensilla of the larva and adult form two closely related sensory projections, (a) the anterior central sensory center located deep in the ventromedial neuropil of the tritocerebrum and mandibular neuromere, and (b) the anterior ventral sensory center (AVSC), occupying a superficial layer within the ventromedial tritocerebrum. Additional, presumed mechanosensory terminal axons entering via the labial nerve define the ventromedial sensory center (VMSC) in the maxilla and labium. Mechanosensory afferents of the massive array of chordotonal organs (Johnston's organ) of the adult antenna project into the centrolateral neuropil column of the anterior SEZ, creating the antenno-mechanosensory and motor center (AMMC). Dendritic projections of dye back-filled motor neurons extend throughout a ventral layer of the SEZ, overlapping widely with the AVSC and VMSC. Our findings elucidate fundamental structural aspects of the developing sensory systems in Drosophila.

Involvement of a Gr2a-Expressing Drosophila Pharyngeal Gustatory Receptor Neuron in Regulation of Aversion to High-Salt Foods

Regulation of feeding is essential for animal survival. The pharyngeal sense organs can act as a second checkpoint of food quality, due to their position between external taste organs such as the labellum which initially assess food quality, and the digestive tract. Growing evidence provides support that the pharyngeal sensory neurons regulate feeding, but much is still unknown. We found that a pair of gustatory receptor neurons in the LSO, a Drosophila adult pharyngeal organ which expresses four gustatory receptors, is involved in feeding inhibition in response to high concentrations of sodium ions. RNAi experiments and mutant analysis showed that the gustatory receptor Gr2a is necessary for this process. This feeding preference determined by whether a food source is perceived as appetizing or not is influenced by nutritional conditions, such that when the animal is hungry, the need for energy dominates over how appealing the food source is. Our results provide experimental evidence that factors involved in feeding function in a context-dependent manner.

A receptor and neuron that activate a circuit limiting sucrose consumption

The neural control of sugar consumption is critical for normal metabolism. In contrast to sugar-sensing taste neurons that promote consumption, we identify a taste neuron that limits sucrose consumption in Drosophila. Silencing of the neuron increases sucrose feeding; optogenetic activation decreases it. The feeding inhibition depends on the IR60b receptor, as shown by behavioral analysis and Ca²⁺ imaging of an IR60b mutant. The IR60b phenotype shows a high degree of chemical specificity when tested with a broad panel of tastants. An automated analysis of feeding behavior in freely moving flies shows that IR60b limits the duration of individual feeding bouts. This receptor and neuron provide the molecular and cellular underpinnings of a new element in the circuit logic of feeding regulation. We propose a dynamic model in which sucrose acts via IR60b to activate a circuit that inhibits feeding and prevents overconsumption.

DOI:http://dx.doi.org/10.7554/eLife.24992.001

All animals – from the fruit fly to mammals like humans – must control their dietary intake of nutrients to survive and stay healthy. Taste receptors that sense high-calorie sugars are essential to this process. Typically, when food tastes sweet, it signals that the food contains nutrients and promotes consumption. However, eating too much sugar can be detrimental because the animal wastes time and energy eating food that it does not need, and could eventually lead to obesity and other metabolic diseases. This raised the question: are there any taste receptors that, once they detect sugars, cause animals to eat less?

Joseph et al. worked with the fruit fly Drosophila melanogaster and identified one such taste receptor called IR60b. The experiments showed that this taste receptor responds selectively to sucrose (a high-calorie sugar), and that it activates nerve cells that cause fruit flies to eat less food, rather than more. When the receptor was experimentally inactivated, the fruit flies ate for longer and ate too much sucrose. This indicates that the flies need this receptor to control their sugar intake.

A next step will be to see if mammals similarly use sweet-sensing taste receptors to limit the amount of food they eat. A better insight into how mammals can control what they eat could provide a deeper understanding of how to tackle major health issues, such as obesity, in humans.

DOI:http://dx.doi.org/10.7554/eLife.24992.002

Gene expression profiles uncover individual identities of gnathal neuroblasts and serial homologies in the embryonic CNS of Drosophila

The numbers and types of progeny cells generated by neural stem cells in the developing CNS are adapted to its region-specific functional requirements. In Drosophila, segmental units of the CNS develop from well-defined patterns of neuroblasts. Here we constructed comprehensive neuroblast maps for the three gnathal head segments. Based on the spatiotemporal pattern of neuroblast formation and the expression profiles of 46 marker genes (41 transcription factors), each neuroblast can be uniquely identified. Compared with the thoracic ground state, neuroblast numbers are progressively reduced in labial, maxillary and mandibular segments due to smaller sizes of neuroectodermal anlagen and, partially, to suppression of neuroblast formation and induction of programmed cell death by the Hox gene Deformed. Neuroblast patterns are further influenced by segmental modifications in dorsoventral and proneural gene expression. With the previously published neuroblast maps and those presented here for the gnathal region, all neuroectodermal neuroblasts building the CNS of the fly (ventral nerve cord and brain, except optic lobes) are now individually identified (in total 2×567 neuroblasts). This allows, for the first time, a comparison of the characteristics of segmental populations of stem cells and to screen for serially homologous neuroblasts throughout the CNS. We show that approximately half of the deutocerebral and all of the tritocerebral (posterior brain) and gnathal neuroblasts, but none of the protocerebral (anterior brain) neuroblasts, display serial homology to neuroblasts in thoracic/abdominal neuromeres. Modifications in the molecular signature of serially homologous neuroblasts are likely to determine the segment-specific characteristics of their lineages.

Highlighted article: Characterisation of the neural stem cells in the gnathal head region completes the mapping of all individual neuroblasts that generate the Drosophila CNS.

Caffeine Taste Signaling in Drosophila Larvae

The Drosophila larva has a simple peripheral nervous system with a comparably small number of sensory neurons located externally at the head or internally along the pharynx to assess its chemical environment. It is assumed that larval taste coding occurs mainly via external organs (the dorsal, terminal, and ventral organ). However, the contribution of the internal pharyngeal sensory organs has not been explored. Here we find that larvae require a single pharyngeal gustatory receptor neuron pair called D1, which is located in the dorsal pharyngeal sensilla, in order to avoid caffeine and to associate an odor with caffeine punishment. In contrast, caffeine-driven reduction in feeding in non-choice situations does not require D1. Hence, this work provides data on taste coding via different receptor neurons, depending on the behavioral context. Furthermore, we show that the larval pharyngeal system is involved in bitter tasting. Using ectopic expressions, we show that the caffeine receptor in neuron D1 requires the function of at least four receptor genes: the putative co-receptors Gr33a, Gr66a, the putative caffeine-specific receptor Gr93a, and yet unknown additional molecular component(s). This suggests that larval taste perception is more complex than previously assumed already at the sensory level. Taste information from different sensory organs located outside at the head or inside along the pharynx of the larva is assembled to trigger taste guided behaviors.

A Pair of Pharyngeal Gustatory Receptor Neurons Regulates Caffeine-Dependent Ingestion in Drosophila Larvae

The sense of taste is an essential chemosensory modality that enables animals to identify appropriate food sources and control feeding behavior. In particular, the recognition of bitter taste prevents animals from feeding on harmful substances. Feeding is a complex behavior comprised of multiple steps, and food quality is continuously assessed. We here examined the role of pharyngeal gustatory organs in ingestion behavior. As a first step, we constructed a gustatory receptor-to-neuron map of the larval pharyngeal sense organs, and examined corresponding gustatory receptor neuron (GRN) projections in the larval brain. Out of 22 candidate bitter compounds, we found 14 bitter compounds that elicit inhibition of ingestion in a dose-dependent manner. We provide evidence that certain pharyngeal GRNs are necessary and sufficient for the ingestion response of larvae to caffeine. Additionally, we show that a specific pair of pharyngeal GRNs, DP1, responds to caffeine by calcium imaging. In this study we show that a specific pair of GRNs in the pharyngeal sense organs coordinates caffeine sensing with regulation of behavioral responses such as ingestion. Our results indicate that in Drosophila larvae, the pharyngeal GRNs have a major role in sensing food palatability to regulate ingestion behavior. The pharyngeal sense organs are prime candidates to influence ingestion due to their position in the pharynx, and they may act as first level sensors of ingested food.

Behavioral Analysis of Bitter Taste Perception in Drosophila Larvae

Insect larvae, which recognize food sources through chemosensory cues, are a major source of global agricultural loss. Gustation is an important factor that determines feeding behavior, and the gustatory receptors (Grs) act as molecular receptors that recognize diverse chemicals in gustatory receptor neurons (GRNs). The behavior of Drosophila larvae is relatively simpler than the adult fly, and a gustatory receptor-to-neuron map was established in a previous study of the major external larval head sensory organs. Here, we extensively study the bitter taste responses of larvae using 2-choice behavioral assays. First, we tested a panel of 23 candidate bitter compounds to compare the behavioral responses of larvae and adults. We define 9 bitter compounds which elicit aversive behavior in a dose-dependent manner. A functional map of the larval GRNs was constructed with the use of Gr-GAL4 lines that drive expression of UAS-tetanus toxin and UAS-VR1 in specific gustatory neurons to identify bitter tastants-GRN combinations by suppressing and activating discrete subsets of taste neurons, respectively. Our results suggest that many gustatory neurons act cooperatively in larval bitter sensing, and that these neurons have different degrees of responsiveness to different bitter compounds.

Drosophila Chemoreceptors: A Molecular Interface Between the Chemical World and the Brain

Chemoreception is essential for survival. Feeding, mating, and avoidance of predators depend on detection of sensory cues. Drosophila contains diverse families of chemoreceptors that detect odors, tastants, pheromones, and noxious stimuli, including receptors of the odor receptor (Or), gustatory receptor (Gr), ionotropic receptor (IR), Pickpocket (Ppk), and Trp families. We consider recent progress in understanding chemoreception in the fly, including the identification of new receptors, the discovery of novel biological functions for receptors, and the localization of receptors in unexpected places. We discuss major unsolved problems and suggest areas that may be particularly ripe for future discoveries, including the roles of these receptors in driving the circuits and behaviors that are essential to the survival and reproduction of the animal.

Taste processing in Drosophila larvae

The sense of taste allows animals to detect chemical substances in their environment to initiate appropriate behaviors: to find food or a mate, to avoid hostile environments and predators. Drosophila larvae are a promising model organism to study gustation. Their simple nervous system triggers stereotypic behavioral responses, and the coding of taste can be studied by genetic tools at the single cell level. This review briefly summarizes recent progress on how taste information is sensed and processed by larval cephalic and pharyngeal sense organs. The focus lies on several studies, which revealed cellular and molecular mechanisms required to process sugar, salt, and bitter substances.

Is the Schwabe Organ a Retained Larval Eye? Anatomical and Behavioural Studies of a Novel Sense Organ in Adult Leptochiton asellus (Mollusca, Polyplacophora) Indicate Links to Larval Photoreceptors

The discovery of a sensory organ, the Schwabe organ, was recently reported as a unifying feature of chitons in the order Lepidopleurida. It is a patch of pigmented tissue located on the roof of the pallial cavity, beneath the velum on either side of the mouth. The epithelium is densely innervated and contains two types of potential sensory cells. As the function of the Schwabe organ remains unknown, we have taken a cross-disciplinary approach, using anatomical, histological and behavioural techniques to understand it. In general, the pigmentation that characterises this sensory structure gradually fades after death; however, one particular concentrated pigment dot persists. This dot is positionally homologous to the larval eye in chiton trochophores, found in the same neuroanatomical location, and furthermore the metamorphic migration of the larval eye is ventral in species known to possess Schwabe organs. Here we report the presence of a discrete subsurface epithelial structure in the region of the Schwabe organ in Leptochiton asellus that histologically resembles the chiton larval eye. Behavioural experiments demonstrate that Leptochiton asellus with intact Schwabe organs actively avoid an upwelling light source, while Leptochiton asellus with surgically ablated Schwabe organs and a control species lacking the organ (members of the other extant order, Chitonida) do not (Kruskal-Wallis, H = 24.82, df = 3, p < 0.0001). We propose that the Schwabe organ represents the adult expression of the chiton larval eye, being retained and elaborated in adult lepidopleurans.

Rhomboid Enhancer Activity Defines a Subset of Drosophila Neural Precursors Required for Proper Feeding, Growth and Viability

Organismal growth regulation requires the interaction of multiple metabolic, hormonal and neuronal pathways. While the molecular basis for many of these are well characterized, less is known about the developmental origins of growth regulatory structures and the mechanisms governing control of feeding and satiety. For these reasons, new tools and approaches are needed to link the specification and maturation of discrete cell populations with their subsequent regulatory roles. In this study, we characterize a rhomboid enhancer element that selectively labels four Drosophila embryonic neural precursors. These precursors give rise to the hypopharyngeal sensory organ of the peripheral nervous system and a subset of neurons in the deutocerebral region of the embryonic central nervous system. Post embryogenesis, the rhomboid enhancer is active in a subset of cells within the larval pharyngeal epithelium. Enhancer-targeted toxin expression alters the morphology of the sense organ and results in impaired larval growth, developmental delay, defective anterior spiracle eversion and lethality. Limiting the duration of toxin expression reveals differences in the critical periods for these effects. Embryonic expression causes developmental defects and partially penetrant pre-pupal lethality. Survivors of embryonic expression, however, ultimately become viable adults. In contrast, post-embryonic toxin expression results in fully penetrant lethality. To better define the larval growth defect, we used a variety of assays to demonstrate that toxin-targeted larvae are capable of locating, ingesting and clearing food and they exhibit normal food search behaviors. Strikingly, however, following food exposure these larvae show a rapid decrease in consumption suggesting a satiety-like phenomenon that correlates with the period of impaired larval growth. Together, these data suggest a critical role for these enhancer-defined lineages in regulating feeding, growth and viability.

Candidate ionotropic taste receptors in the Drosophila larva

We examine in Drosophila a group of ∼35 ionotropic receptors (IRs), the IR20a clade, about which remarkably little is known. Of 28 genes analyzed, GAL4 drivers representing 11 showed expression in the larva. Eight drivers labeled neurons of the pharynx, a taste organ, and three labeled neurons of the body wall that may be chemosensory. Expression was not observed in neurons of one taste organ, the terminal organ, although these neurons express many drivers of the Gr (Gustatory receptor) family. For most drivers of the IR20a clade, we observed expression in a single pair of cells in the animal, with limited coexpression, and only a fraction of pharyngeal neurons are labeled. The organization of IR20a clade expression thus appears different from the organization of the Gr family or the Odor receptor (Or) family in the larva. A remarkable feature of the larval pharynx is that some of its organs are incorporated into the adult pharynx, and several drivers of this clade are expressed in the pharynx of both larvae and adults. Different IR drivers show different developmental dynamics across the larval stages, either increasing or decreasing. Among neurons expressing drivers in the pharynx, two projection patterns can be distinguished in the CNS. Neurons exhibiting these two kinds of projection patterns may activate different circuits, possibly signaling the presence of cues with different valence. Taken together, the simplest interpretation of our results is that the IR20a clade encodes a class of larval taste receptors.

Pharyngeal sense organs drive robust sugar consumption in Drosophila

The fly pharyngeal sense organs lie at the transition between external and internal nutrient sensing mechanisms. Here, we investigate the function of pharyngeal sweet gustatory receptor neurons (GRNs), demonstrating that they express a subset of the nine previously identified sweet receptors and respond to stimulation with a panel of sweet compounds. We show that pox-neuro (poxn) mutants lacking taste function in the legs and labial palps have intact pharyngeal sweet taste, which is both necessary and sufficient to drive preferred consumption of sweet compounds by prolonging ingestion. Moreover, flies putatively lacking all sweet taste show little preference for nutritive or non-nutritive sugars in a short-term feeding assay. Together, our data demonstrate that pharyngeal sense organs play an important role in directing sustained consumption of sweet compounds, and suggest that post-ingestive sugar sensing does not effectively drive food choice in a simple short-term feeding paradigm.

Metamorphic labral axis patterning in the beetle Tribolium castaneum requires multiple upstream, but few downstream, genes in the appendage patterning network

The arthropod labrum is an anterior appendage-like structure that forms the dorsal side of the preoral cavity. Conflicting interpretations of fossil, nervous system, and developmental data have led to a proliferation of scenarios for labral evolution. The best supported hypothesis is that the labrum is a novel structure that shares development with appendages as a result of co-option. Here, we use RNA interference in the red flour beetle Tribolium castaneum to compare metamorphic patterning of the labrum to previously published data on ventral appendage patterning. As expected under the co-option hypothesis, depletion of several genes resulted in similar defects in the labrum and ventral appendages. These include proximal deletions and proximal-to-distal transformations resulting from depletion of the leg gap genes homothorax and extradenticle, large-scale deletions resulting from depletion of the leg gap gene Distal-less, and smaller distal deletions resulting from knockdown of the EGF ligand Keren. However, depletion of dachshund and many of the genes that function downstream of the leg gap genes in the ventral appendages had either subtle or no effects on labral axis patterning. This pattern of partial similarity suggests that upstream genes act through different downstream targets in the labrum. We also discovered that many appendage axis patterning genes have roles in patterning the epipharyngeal sensillum array, suggesting that they have become integrated into a novel regulatory network. These genes include Notch, Delta, and decapentaplegic, and the transcription factors abrupt, bric à brac, homothorax, extradenticle and the paralogs apterous a and apterous b.

The Drosophila IR20a clade of ionotropic receptors are candidate taste and pheromone receptors

Insects use taste to evaluate food, hosts, and mates. Drosophila has many "orphan" taste neurons that express no known taste receptors. The Ionotropic Receptor (IR) superfamily is best known for its role in olfaction, but virtually nothing is known about a clade of ∼35 members, the IR20a clade. Here, a comprehensive analysis of this clade reveals expression in all taste organs of the fly. Some members are expressed in orphan taste neurons, whereas others are coexpressed with bitter- or sugar-sensing Gustatory receptor (Gr) genes. Analysis of the closely related IR52c and IR52d genes reveals signatures of adaptive evolution, roles in male mating behavior, and sexually dimorphic expression in neurons of the male foreleg, which contacts females during courtship. These neurons are activated by conspecific females and contact a neural circuit for sexual behavior. Together, these results greatly expand the repertoire of candidate taste and pheromone receptors in the fly.

Bitter-sweet processing in larval Drosophila

"Sweet-" and "bitter-" tasting substances distinctively support attractive and aversive choice behavior, respectively, and therefore are thought to be processed by distinct pathways. Interestingly, electrophysiological recordings in adult Drosophila suggest that bitter and salty tastants, in addition to activating bitter, salt, or bitter/salt sensory neurons, can also inhibit sweet-sensory neurons. However, the behavioral significance of such a potential for combinatorial coding is little understood. Using larval Drosophila as a study case, we find that the preference towards fructose is inhibited when assayed in the background of the bitter tastant quinine. When testing the influence of quinine on the preference to other, equally preferred sweet tastants, we find that these sweet tastants differ in their susceptibility to be inhibited by quinine. Such stimulus specificity argues that the inhibitory effect of quinine is not due to general effects on locomotion or nausea. In turn, not all bitter tastants have the same potency to inhibit sweet preference; notably, their inhibitory potency is not determined by the strength of the avoidance of them. Likewise, equally avoided concentrations of sodium chloride differ in their potency to inhibit sugar preference. Furthermore, Gr33a-Gal4-positive neurons, while being necessary for bitter avoidance, are dispensable for inhibition of the sweet pathway. Thus, interactions across taste modalities are behaviorally significant and, as we discuss, arguably diverse in mechanism. These results suggest that the coding of tastants and the organization of gustatory behavior may be more combinatorial than is generally acknowledged.

The neuronal and molecular basis of quinine-dependent bitter taste signaling in Drosophila larvae

The sensation of bitter substances can alert an animal that a specific type of food is harmful and should not be consumed. However, not all bitter compounds are equally toxic and some may even be beneficial in certain contexts. Thus, taste systems in general may have a broader range of functions than just in alerting the animal. In this study we investigate bitter sensing and processing in Drosophila larvae using quinine, a substance perceived by humans as bitter. We show that behavioral choice, feeding, survival, and associative olfactory learning are all directly affected by quinine. On the cellular level, we show that 12 gustatory sensory receptor neurons that express both GR66a and GR33a are required for quinine-dependent choice and feeding behavior. Interestingly, these neurons are not necessary for quinine-dependent survival or associative learning. On the molecular receptor gene level, the GR33a receptor, but not GR66a, is required for quinine-dependent choice behavior. A screen for gustatory sensory receptor neurons that trigger quinine-dependent choice behavior revealed that a single GR97a receptor gene expressing neuron located in the peripheral terminal sense organ is partially necessary and sufficient. For the first time, we show that the elementary chemosensory system of the Drosophila larva can serve as a simple model to understand the neuronal basis of taste information processing on the single cell level with respect to different behavioral outputs.

Diverse roles for the Drosophila fructose sensor Gr43a

The detection of nutrients, both in food and within the body, is crucial for the regulation of feeding behavior, growth, and metabolism. While the molecular basis for sensing food chemicals by the taste system has been firmly linked to specific taste receptors, relatively little is known about the molecular nature of the sensors that monitor nutrients internally. Recent reports of taste receptors expressed in other organ systems, foremost in the gastrointestinal tract of mammals and insects, has led to the proposition that some taste receptors may also be used as sensors of internal nutrients. Indeed, we provided direct evidence that the Drosophila gustatory receptor 43a (Gr43a) plays a critical role in sensing internal fructose levels in the fly brain. In addition to the brain and the taste system, Gr43a is also expressed in neurons of the proventricular ganglion and the uterus. Here, we discuss the multiple potential roles of Gr43a in the fly. We also provide evidence that its activation in the brain is likely mediated by the neuropeptide Corazonin. Finally, we posit that Gr43a may represent only a precedent for other taste receptors that sense internal nutrients, not only in flies but, quite possibly, in other animals, including mammals.

Tissue-specific activation of a single gustatory receptor produces opposing behavioral responses in Drosophila

Understanding sensory systems that perceive environmental inputs and neural circuits that select appropriate motor outputs is essential for studying how organisms modulate behavior and make decisions necessary for survival. Drosophila melanogaster oviposition is one such important behavior, in which females evaluate their environment and choose to lay eggs on substrates they may find aversive in other contexts. We employed neurogenetic techniques to characterize neurons that influence the choice between repulsive positional and attractive egg-laying responses toward the bitter-tasting compound lobeline. Surprisingly, we found that neurons expressing Gr66a, a gustatory receptor normally involved in avoidance behaviors, receive input for both attractive and aversive preferences. We hypothesized that these opposing responses may result from activation of distinct Gr66a-expressing neurons. Using tissue-specific rescue experiments, we found that Gr66a-expressing neurons on the legs mediate positional aversion. In contrast, pharyngeal taste cells mediate the egg-laying attraction to lobeline, as determined by analysis of mosaic flies in which subsets of Gr66a neurons were silenced. Finally, inactivating mushroom body neurons disrupted both aversive and attractive responses, suggesting that this brain structure is a candidate integration center for decision-making during Drosophila oviposition. We thus define sensory and central neurons critical to the process by which flies decide where to lay an egg. Furthermore, our findings provide insights into the complex nature of gustatory perception in Drosophila. We show that tissue-specific activation of bitter-sensing Gr66a neurons provides one mechanism by which the gustatory system differentially encodes aversive and attractive responses, allowing the female fly to modulate her behavior in a context-dependent manner.

Nutritional value-dependent and nutritional value-independent effects on Drosophila melanogaster larval behavior

Gustatory stimuli allow an organism not only to orient in its environment toward energy-rich food sources to maintain nutrition but also to avoid unpleasant or even poisonous substrates. For both mammals and insects, sugars-perceived as "sweet"-potentially predict nutritional benefit. Interestingly, even Drosophila adult flies are attracted to most high-potency sweeteners preferred by humans. However, the gustatory information of a sugar may be misleading as some sugars, although perceived as "sweet," cannot be metabolized. Accordingly, in adult Drosophila, a postingestive system that additionally evaluates the nutritional benefit of an ingested sugar has been shown to exist. By using a set of seven different sugars, which either offer (fructose, sucrose, glucose, maltodextrin, and sorbitol) or lack (xylose and arabinose) nutritional benefit, we show that Drosophila, at the larval stage, can perceive and evaluate sugars based on both nutrition-dependent and -independent qualities. In detail, we find that larval survival and feeding mainly depend on the nutritional value of a particular sugar. In contrast, larval choice behavior and learning are regulated in a more complex way by nutrition value-dependent and nutrition value-independent information. The simplicity of the larval neuronal circuits and their accessibility to genetic manipulation may ultimately allow one to identify the neuronal and molecular basis of the larval sugar perception systems described here behaviorally.

Molecular and cellular organization of the taste system in the Drosophila larva

We examine the molecular and cellular basis of taste perception in the Drosophila larva through a comprehensive analysis of the expression patterns of all 68 Gustatory receptors (Grs). Gr-GAL4 lines representing each Gr are examined, and 39 show expression in taste organs of the larval head, including the terminal organ (TO), the dorsal organ (DO), and the pharyngeal organs. A receptor-to-neuron map is constructed. The map defines 10 neurons of the TO and DO, and it identifies 28 receptors that map to them. Each of these neurons expresses a unique subset of Gr-GAL4 drivers, except for two neurons that express the same complement. All of these neurons express at least two drivers, and one neuron expresses 17. Many of the receptors map to only one of these cells, but some map to as many as six. Conspicuously absent from the roster of Gr-GAL4 drivers expressed in larvae are those of the sugar receptor subfamily. Coexpression analysis suggests that most larval Grs act in bitter response and that there are distinct bitter-sensing neurons. A comprehensive analysis of central projections confirms that sensory information collected from different regions (e.g., the tip of the head vs the pharynx) is processed in different regions of the suboesophageal ganglion, the primary taste center of the CNS. Together, the results provide an extensive view of the molecular and cellular organization of the larval taste system.

A behavior-based circuit model of how outcome expectations organize learned behavior in larval Drosophila

Drosophila larvae combine a numerically simple brain, a correspondingly moderate behavioral complexity, and the availability of a rich toolbox for transgenic manipulation. This makes them attractive as a study case when trying to achieve a circuit-level understanding of behavior organization. From a series of behavioral experiments, we suggest a circuitry of chemosensory processing, odor-tastant memory trace formation, and the "decision" process to behaviorally express these memory traces--or not. The model incorporates statements about the neuronal organization of innate vs. conditioned chemosensory behavior, and the types of interaction between olfactory and gustatory pathways during the establishment as well as the behavioral expression of odor-tastant memory traces. It in particular suggests that innate olfactory behavior is responsive in nature, whereas conditioned olfactory behavior is captured better when seen as an action in pursuit of its outcome. It incorporates the available neuroanatomical and behavioral data and thus should be useful as scaffold for the ongoing investigations of the chemo-behavioral system in larval Drosophila.

Gustatory feedback affects feeding related motor pattern generation in starved 3rd instar larvae of Calliphora vicina

Gustatory feedback allows animals to distinguish between edible and noxious food and adapts centrally generated feeding motor patterns to environmental demands. In reduced preparations obtained from starved Calliphora larvae, putatively appetitive (ethanol), aversive (sodium acetate) and neutral (glucose) gustatory stimuli were applied to the anterior sense organs. The resulting sensory response was recorded from the maxillary and antennal nerves. All three stimuli increased the neural activity in both nerves. Recordings obtained from the antennal nerve to monitor the activation pattern of the cibarial dilator muscles, demonstrated an effect of gustatory input on the central pattern generator for feeding. Ethanol consistently enhanced the rhythmic activity of the CDM motor neurons either by speeding up the rhythm or by increasing the burst duration. Ethanol also had an enhancing effect on the motor patterns of a protractor muscle which moves the cephalopharyngeal skeleton relative to the body. Sodium acetate showed a state dependent effect: in preparations without spontaneous CDM activity it initiated rhythmic motor patterns, while an ongoing CDM rhythm was inhibited. Surprisingly glucose had an enhancing effect which was less pronounced than that of ethanol. Gustatory feedback therefore can modify and adapt the motor output of the multifunctional central pattern generator for feeding.

The cephalic and pharyngeal sense organs of Calliphora vicina 3rd instar larvae are mechanosensitive but have no profound effect on ongoing feeding related motor patterns

The anterior segments of cyclorraphous Diptera larvae bear various sense organs: the dorsal- and terminal organ located on the cephalic lobes, the ventral- and labial organs associated with the mouthplate and the internal labral organ which lies on the dorsal surface of the esophagus. The sense organs are connected to the brain via the antennal nerve (dorsal- and labral organ) or the maxillary nerve (terminal-, ventral-, labial organ). Although their ultrastructure suggests also a mechanosensory function only their response to olfactory and gustatory stimuli has been investigated electrophysiologically. Here we stimulated the individual organs with step-, ramp-, and sinusoidal stimuli of different amplitude while extracellulary recording their afferents from the respective nerves. The external organs show a threshold of approximately 2 microm. All organs responded phasically and did not habituate to repetitive stimuli. The low threshold of the external organs combined with their rhythmically exposure to the substrate suggested a putative role in the temporal coordination of feeding. We therefore repetitively stimulated individual organs while simultaneously monitoring the centrally generated motor pattern for food ingestion. Neither the dorsal-, terminal- or ventral organ afferents had an obvious effect on the ongoing motor rhythm. Various reasons explaining these results are discussed.

Neurite elongation from Drosophila neural BG2-c6 cells stimulated by 20-hydroxyecdysone

Neurite elongation is a critical process in the formation of nerve systems from neural cells. During metamorphosis, the holometabolous insect Drosophila melanogaster reorganizes its central nervous system (CNS) under the influence of the steroid molting hormone 20-hydroxyecdysone (20E). A neural cell line that responds to 20E treatment is therefore desired in order to analyze its signal transduction process. Here, we show that cells of the Drosophila neural cell line BG2-c6 extended long projections of over 30 microm in length after being stimulated with 20E. Most of these projections contained both actin filaments and microtubules. Since microtubules are structural markers of neurites, the projections were considered to be neurites. Live imaging of cells expressing GFP tagged alpha-tubulin showed that the neurites did not have a lamellipodial structure at their tips. Under an electron microscope, microtubules were found to run alongside the actin filaments in the neurite shaft but did not reach the tip, where the actin filaments were loosely bundled rather than being arranged into a meshwork as in lamellipodia. These results indicate that BG2-c6 cells project neurites without the typical growth-corn structure at their tips after 20E stimulation.

The thoracic muscular system and its innervation in third instar Calliphora vicina Larvae. I. Muscles of the pro- and mesothorax and the pharyngeal complex

An anatomical description is given by the muscles in the pro- and mesothorax, and those associated with the feeding apparatus (cephalopharyngeal skeleton, CPS) that participate in feeding behavior in third instar Calliphora larvae. The body wall muscles in the pro- and mesothoracic segments are organized in three layers: internal, intermedial, and external. The muscles were labeled with roman numerals according to the nomenclature in use for the abdominal segments. Muscles associated with the CPS are labeled according to their function. The prothorax bears five pairs of lateral symmetrically longitudinal segmental body wall muscles and lacks the transversal muscle group present in the mesothorax and abdominal segments. Additionally, four pairs of intersegmental muscles project from the prothorax to the second, fourth, and fifth segment. The mesothorax bears 15 pairs of segmental longitudinal and 18 pairs of transversal muscles. The accessory pharyngeal muscles span the CPS and the cuticle. Three pairs of protractors and retractors and two pairs of mouth hook accessors (MH(AC)) exist, which move the CPS relative to the body. The pharyngeal muscles are exclusively attached to the structures of the CPS. The mouth hook elevators and depressors, which mediate the hooks rotation are attached to the ventral arm of the CPS and project to a dorsal (elevators) or ventral (depressors) protuberance of the mouth hooks. The cibarial dilator muscles (CDM) span the dorsal arms of the CPS and the dorsal surface of the esophagus and mediate food ingestion. The labial retractors (LRs) lack antagonists and project from the ventral surface of the CPS to the unpaired labium. Contractions of these muscles open the mouth cavity.

The brain can eat: establishing the existence of a central pattern generator for feeding in third instar larvae of Drosophila virilis and Drosophila melanogaster

To establish the existence of a central pattern generator for feeding in the larval central nervous system of two Drosophila species, the gross anatomy of feeding related muscles and their innervation is described, the motor units of the muscles identified and rhythmic motor output recorded from the isolated CNS. The cibarial dilator muscles that mediate food ingestion are innervated by the frontal nerve. Their motor pathway projects from the brain through the antennal nerves, the frontal connectives and the frontal nerve junction. The mouth hook elevator and depressor system is innervated by side branches of the maxillary nerve. The motor units of the two muscle groups differ in amplitude: the elevator is always activated by a small unit, the depressor by a large one. The dorsal protractors span the cephalopharyngeal skeleton and the body wall hence mediating an extension of the CPS. These muscles are innervated by the prothoracic accessory nerve. Rhythmic motor output produced by the isolated central nervous system can simultaneously be recorded from all three nerves. The temporal pattern of the identified motor units resembles the sequence of muscle contractions deduced from natural feeding behavior and is therefore considered as fictive feeding. Phase diagrams show an almost identical fictive feeding pattern is in both species.

Molecular and cellular designs of insect taste receptor system

The insect gustatory receptors (GRs) are members of a large G-protein coupled receptor family distantly related to the insect olfactory receptors. They are phylogenetically different from taste receptors of most other animals. GRs are often coexpressed with other GRs in single receptor neurons. Taste receptors other than GRs are also expressed in some neurons. Recent molecular studies in the fruitfly Drosophila revealed that the insect taste receptor system not only covers a wide ligand spectrum of sugars, bitter substances or salts that are common to mammals but also includes reception of pheromone and somatosensory stimulants. However, the central mechanism to perceive and discriminate taste information is not yet elucidated. Analysis of the primary projection of taste neurons to the brain shows that the projection profiles depend basically on the peripheral locations of the neurons as well as the GRs that they express. These results suggest that both peripheral and central design principles of insect taste perception are different from those of olfactory perception.

Analysis of Drosophila TRPA1 reveals an ancient origin for human chemical nociception

Chemical nociception, the detection of tissue-damaging chemicals, is important for animal survival and causes human pain and inflammation, but its evolutionary origins are largely unknown. Reactive electrophiles are a class of noxious compounds humans find pungent and irritating, like allyl isothiocyanate (in wasabi) and acrolein (in cigarette smoke)1–3. Insects to humans find reactive electrophiles aversive1–3, but whether this reflects conservation of an ancient sensory modality has been unclear. Here we identify the molecular basis of reactive electrophile detection in flies. We demonstrate that dTRPA1, the Drosophila melanogaster ortholog of the human irritant sensor, acts in gustatory chemosensors to inhibit reactive electrophile ingestion. We show that fly and mosquito TRPA1 orthologs are molecular sensors of electrophiles, using a mechanism conserved with vertebrate TRPA1s. Phylogenetic analyses indicate invertebrate and vertebrate TRPA1s share a common ancestor that possessed critical characteristics required for electrophile detection. These findings support emergence of TRPA1-based electrophile detection in a common bilaterian ancestor, with widespread conservation throughout vertebrate and invertebrate evolution. Such conservation contrasts with the evolutionary divergence of canonical olfactory and gustatory receptors and may relate to electrophile toxicity. We propose human pain perception relies on an ancient chemical sensor conserved across ~500 million years of animal evolution.

Detailed analysis of leucokinin-expressing neurons and their candidate functions in the Drosophila nervous system

The distribution of leucokinin (LK) neurons in the central nervous system (CNS) of Drosophila melanogaster was described by immunolabelling many years ago. However, no detailed underlying information of the input or output connections of their neurites was then available. Here, we provide a more accurate morphological description by employing a novel LK-specific GAL4 line that recapitulates LK expression. In order to analyse the possible afferent and efferent neural candidates of LK neurons, we used this lk-GAL4 line together with other CNS-Gal4 lines, combined with antisera against various neuropeptides or neurotransmitters. We found four kinds of LK neurons in the brain. (1) The lateral horn neurons connect the antennal glomerula to the mushroom bodies. (2) The suboesophageal neurons connect the gustatory receptors to the suboesophageal ganglia and ventral nerve cord. (3) The anterior neurons innervate the corpus cardiacum of the ring gland but LK expression is surprisingly not detectable from the third instar onwards in these neurons. (4) A set of abdominal ganglion neurons connect to the dorsal median tract in larvae and send their axons to a segmental muscle 8. Thus, the methods employed in our study can be used to identify individual neuropeptidergic neurons and thereby characterize functional cues or developmental transformations in their differentiation.

A taste of the Drosophila gustatory receptors

Insects such as the fruit fly, Drosophila melanogaster, rely on contact chemosensation to detect nutrient-rich foods, to avoid consuming toxic chemicals, and to select mates and hospitable zones to deposit eggs. Flies sense tastants and nonvolatile pheromones through gustatory bristles and pegs distributed on multiple body parts including the proboscis, wing margins, legs, and ovipositor. The sensilla house gustatory receptor neurons, which express members of the family of 68 gustatory receptors (GRs). In contrast to mammalian chemosensation or Drosophila olfaction, which are initiated by receptors composed of dimers of one or two receptor types, the functional Drosophila GRs may include three or more subunits. Several GRs appear to be expressed in multiple cell types that are not associated with contact chemosensation raising the possibility that these proteins may have roles that extend beyond the detection of tastants and pheromones.

Plant insecticide L-canavanine repels Drosophila via the insect orphan GPCR DmX

An orphan G-protein-coupled gustatory receptor mediates detection of the plant poison L-canavanine in fruit flies.

For all animals, the taste sense is crucial to detect and avoid ingesting toxic molecules. Many toxins are synthesized by plants as a defense mechanism against insect predation. One example of such a natural toxic molecule is l-canavanine, a nonprotein amino acid found in the seeds of many legumes. Whether and how insects are informed that some plants contain l-canavanine remains to be elucidated. In insects, the taste sense relies on gustatory receptors forming the gustatory receptor (Gr) family. Gr proteins display highly divergent sequences, suggesting that they could cover the entire range of tastants. However, one cannot exclude the possibility of evolutionarily independent taste receptors. Here, we show that l-canavanine is not only toxic, but is also a repellent for Drosophila. Using a pharmacogenetic approach, we find that flies sense food containing this poison by the DmX receptor. DmXR is an insect orphan G-protein–coupled receptor that has partially diverged in its ligand binding pocket from the metabotropic glutamate receptor family. Blockade of DmXR function with an antagonist lowers the repulsive effect of l-canavanine. In addition, disruption of the DmXR encoding gene, called mangetout (mtt), suppresses the l-canavanine repellent effect. To avoid the ingestion of l-canavanine, DmXR expression is required in bitter-sensitive gustatory receptor neurons, where it triggers the premature retraction of the proboscis, thus leading to the end of food searching. These findings show that the DmX receptor, which does not belong to the Gr family, fulfills a gustatory function necessary to avoid eating a natural toxin.

Plants evolve to fend off the insects that attack them, often by synthesizing compounds toxic to insects. In turn, insects develop strategies to avoid these plants or resist their toxins. Some plant toxins are nonprotein amino acids. For example, seeds from numerous legumes contain high amounts of l-canavanine, a nonprotein amino acid that is structurally related to l-arginine and is highly toxic to most insects. How insects can detect l-canavanine remains to be elucidated. Using pharmacology, genetics, and behavioral approaches, we show that flies sense l-canavanine using the receptor DmX, an orphan G-protein–coupled receptor that has diverged in its ligand binding pocket from metabotropic glutamate receptors. Disruption of the DmXR gene, called mangetout (mtt), suppresses the l-canavanine repellent effect. DmXR is expressed and required in aversive gustatory receptor neurons, where it triggers the premature retraction of the proboscis, thus leading to the end of food searching. Our results indicate a mechanism by which some insects may detect and avoid a plant toxin.

The role of dopamine in Drosophila larval classical olfactory conditioning

Learning and memory is not an attribute of higher animals. Even Drosophila larvae are able to form and recall an association of a given odor with an aversive or appetitive gustatory reinforcer. As the Drosophila larva has turned into a particularly simple model for studying odor processing, a detailed neuronal and functional map of the olfactory pathway is available up to the third order neurons in the mushroom bodies. At this point, a convergence of olfactory processing and gustatory reinforcement is suggested to underlie associative memory formation. The dopaminergic system was shown to be involved in mammalian and insect olfactory conditioning. To analyze the anatomy and function of the larval dopaminergic system, we first characterize dopaminergic neurons immunohistochemically up to the single cell level and subsequent test for the effects of distortions in the dopamine system upon aversive (odor-salt) as well as appetitive (odor-sugar) associative learning. Single cell analysis suggests that dopaminergic neurons do not directly connect gustatory input in the larval suboesophageal ganglion to olfactory information in the mushroom bodies. However, a number of dopaminergic neurons innervate different regions of the brain, including protocerebra, mushroom bodies and suboesophageal ganglion. We found that dopamine receptors are highly enriched in the mushroom bodies and that aversive and appetitive olfactory learning is strongly impaired in dopamine receptor mutants. Genetically interfering with dopaminergic signaling supports this finding, although our data do not exclude on naïve odor and sugar preferences of the larvae. Our data suggest that dopaminergic neurons provide input to different brain regions including protocerebra, suboesophageal ganglion and mushroom bodies by more than one route. We therefore propose that different types of dopaminergic neurons might be involved in different types of signaling necessary for aversive and appetitive olfactory memory formation respectively, or for the retrieval of these memory traces. Future studies of the dopaminergic system need to take into account such cellular dissociations in function in order to be meaningful.

Water taste transduction pathway is calcium dependent in Drosophila

In mammals, detection of osmolarity by the gustatory system was overlooked until recently. In insects, specific taste receptor neurons detect hypoosmotic stimuli and are commonly called "W" (water) cells. W cells are easy to access in vivo and represent a good model to study the transduction of hypoosmotic stimuli. Using pharmacological and genetic approaches in Drosophila, we show that tarsal W cell firing activity depends on the concentration of external calcium bathing the dendrite. This dependence was confirmed by the strong inhibition of W cell responses to hypoosmotic stimuli by lanthanum (IC(50) = 8 nM), an ion known to inhibit calcium-permeable channels. Downstream, the transduction pathway likely involves calmodulin because calmodulin antagonists such as W-7 (IC(50) = 2 microM) and fluphenazine (IC(50) = 30 microM) prevented the activation of the W cell by hypoosmotic stimuli. A protein kinase C (PKC) may also be involved as W cell responses were blocked by PKC inhibitors, chelerythrine (IC(50) = 20 microM) and staurosporine (IC(50) = 30 microM). It was also reduced when expressing an inhibitory pseudosubstrate of PKC in gustatory receptor neurons. In the rat, the transduction pathway underlying low osmolarity detection involves aquaporin and swelling-activated ion channels. Our study suggests that the transduction pathway of hypoosmotic stimuli in insects differs from mammals.