Dedicated olfactory neurons mediating attraction behavior to ammonia and amines in Drosophila

Acid and Alkali Taste Sensation

Living organisms rely on pH levels for a multitude of crucial biological processes, such as the digestion of food and the facilitation of enzymatic reactions. Among these organisms, animals, including insects, possess specialized taste organs that enable them to discern between acidic and alkaline substances present in their food sources. This ability is vital, as the pH of these compounds directly influences both the nutritional value and the overall health impact of the ingested substances. In response to the various chemical properties of naturally occurring compounds, insects have evolved peripheral taste organs. These sensory structures play a pivotal role in identifying and distinguishing between nourishing and potentially harmful foods. In this concise review, we aim to provide an in-depth examination of the molecular mechanisms governing pH-dependent taste responses, encompassing both acidic and alkaline stimuli, within the peripheral taste organs of the fruit fly, Drosophila melanogaster, drawing insights from a comprehensive analysis of existing research articles.

The proton channel OTOP1 is a sensor for the taste of ammonium chloride

Ammonium (NH4+), a breakdown product of amino acids that can be toxic at high levels, is detected by taste systems of organisms ranging from C. elegans to humans and has been used for decades in vertebrate taste research. Here we report that OTOP1, a proton-selective ion channel expressed in sour (Type III) taste receptor cells (TRCs), functions as sensor for ammonium chloride (NH4Cl). Extracellular NH4Cl evoked large dose-dependent inward currents in HEK-293 cells expressing murine OTOP1 (mOTOP1), human OTOP1 and other species variants of OTOP1, that correlated with its ability to alkalinize the cell cytosol. Mutation of a conserved intracellular arginine residue (R292) in the mOTOP1 tm 6-tm 7 linker specifically decreased responses to NH4Cl relative to acid stimuli. Taste responses to NH4Cl measured from isolated Type III TRCs, or gustatory nerves were strongly attenuated or eliminated in an Otop1-/- mouse strain. Behavioral aversion of mice to NH4Cl, reduced in Skn-1a-/- mice lacking Type II TRCs, was entirely abolished in a double knockout with Otop1. These data together reveal an unexpected role for the proton channel OTOP1 in mediating a major component of the taste of NH4Cl and a previously undescribed channel activation mechanism.

Heterogeneous receptor expression underlies non-uniform peptidergic modulation of olfaction in Drosophila

Sensory systems are dynamically adjusted according to the animal's ongoing needs by neuromodulators, such as neuropeptides. Neuropeptides are often widely-distributed throughout sensory networks, but it is unclear whether such neuropeptides uniformly modulate network activity. Here, we leverage the Drosophila antennal lobe (AL) to resolve whether myoinhibitory peptide (MIP) uniformly modulates AL processing. Despite being uniformly distributed across the AL, MIP decreases olfactory input to some glomeruli, while increasing olfactory input to other glomeruli. We reveal that a heterogeneous ensemble of local interneurons (LNs) are the sole source of AL MIP, and show that differential expression of the inhibitory MIP receptor across glomeruli allows MIP to act on distinct intraglomerular substrates. Our findings demonstrate how even a seemingly simple case of modulation can have complex consequences on network processing by acting non-uniformly within different components of the overall network.

Neural manifolds for odor-driven innate and acquired appetitive preferences

Sensory stimuli evoke spiking neural responses that innately or after learning drive suitable behavioral outputs. How are these spiking activities intrinsically patterned to encode for innate preferences, and could the neural response organization impose constraints on learning? We examined this issue in the locust olfactory system. Using a diverse odor panel, we found that ensemble activities both during ('ON response') and after stimulus presentations ('OFF response') could be linearly mapped onto overall appetitive preference indices. Although diverse, ON and OFF response patterns generated by innately appetitive odorants (higher palp-opening responses) were still limited to a low-dimensional subspace (a 'neural manifold'). Similarly, innately non-appetitive odorants evoked responses that were separable yet confined to another neural manifold. Notably, only odorants that evoked neural response excursions in the appetitive manifold could be associated with gustatory reward. In sum, these results provide insights into how encoding for innate preferences can also impact associative learning.

Taste coding of heavy metal ion-induced avoidance in Drosophila

Increasing pollution of heavy metals poses great risks to animals globally. Their survival likely relies on an ability to detect and avoid harmful heavy metal ions (HMIs). Currently, little is known about the neural mechanisms of HMI detection. Here, we show that Drosophila and related species of Drosophilidae actively avoid toxic HMIs at micromolar concentrations. The high sensitivity to HMIs is biologically relevant. Particularly, their sensitivity to cadmium is as high as that to the most bitter substance, denatonium. Detection of HMIs in food requires Gr66a ⁺ gustatory neurons but is independent of bitter-taste receptors. In these neurons, the ionotropic receptors IR76b, IR25a, and IR7a are required for the perception of heavy metals. Furthermore, IR47a mediates the activation of a distinct group of non-Gr66a ⁺ gustatory neurons elicited by HMIs. Together, our findings reveal a surprising taste quality represented by noxious metal ions.

Characterization of Ionotropic Receptor Gene EonuIR25a in the Tea Green Leafhopper, Empoasca onukii Matsuda

Ionotropic receptors (IRs) play a central role in detecting chemosensory information from the environment and guiding insect behaviors and are potential target genes for pest control. Empoasca onukii Matsuda is a major pest of the tea plant Camellia sinensis (L.) O. Ktze, and seriously influences tea yields and quality. In this study, the ionotropic receptor gene EonuIR25a in E. onukii was cloned, and the expression pattern of EonuIR25a was detected in various tissues. Behavioral responses of E. onukii to volatile compounds emitted by tea plants were determined using olfactometer bioassay and field trials. To further explore the function of EonuIR25a in olfactory recognition of compounds, RNA interference (RNAi) of EonuIR25a was carried out by ingestion of in vitro synthesized dsRNAs. The coding sequence (CDS) length of EonuIR25a was 1266 bp and it encoded a 48.87 kD protein. EonuIR25a was enriched in the antennae of E. onukii. E. onukii was more significantly attracted by 1-phenylethanol at a concentration of 100 µL/mL. Feeding with dsEonuIR25a significantly downregulated the expression level of EonuIR25a, after 3 h of treatment, which disturbed the behavioral responses of E. onukii to 1-phenylethanol at a concentration of 100 µL/mL. The response rate of E. onukii to 1-phenylethanol was significantly decreased after dsEonuIR25a treatment for 12 h. In summary, the ionotropic receptor gene EonuIR25a was highly expressed in the antennae of E. onukii and was involved in olfactory recognition of the tea plant volatile 1-phenylethanol. The present study may help us to use the ionotropic receptor gene as a target for the behavioral manipulation of E. onukii in the future.

Comparative transcriptomic assessment of the chemosensory receptor repertoire of Drosophila suzukii adult and larval olfactory organs

The spotted wing Drosophila, Drosophila suzukii, has emerged within the past decade as an invasive species on a global scale, and is one of the most economically important pests in fruit and berry production in Europe and North America. Insect ecology, to a strong degree, depends on the chemosensory modalities of smell and taste. Extensive research on the sensory receptors of the olfactory and gustatory systems in Drosophila melanogaster provide an excellent frame of reference to better understand the fundamentals of the chemosensory systems of D. suzukii. This knowledge may enhance the development of semiochemicals for sustainable management of D. suzukii, which is urgently needed. Here, using a transcriptomic approach we report the chemosensory receptor expression profiles in D. suzukii female and male antennae, and for the first time, in larval heads including the dorsal organ that houses larval olfactory sensory neurons. In D. suzukii adults, we generally observed a lack of sexually dimorphic expression levels in male and female antennae. While there was generally conservation of antennal expression of odorant and ionotropic receptor orthologues for D. melanogaster and D. suzukii, gustatory receptors showed more distinct species-specific profiles. In larval head tissues, for all three receptor gene families, there was also a greater degree of species-specific gene expression patterns. Analysis of chemosensory receptor repertoires in the pest species, D. suzukii relative to those of the genetic model D. melanogaster enables comparative studies of the chemosensory, physiology, and ecology of D. suzukii.

Alkaline taste sensation through the alkaliphile chloride channel in Drosophila

The sense of taste is an important sentinel governing what should or should not be ingested by an animal, with high pH sensation playing a critical role in food selection. Here we explore the molecular identities of taste receptors detecting the basic pH of food using Drosophila melanogaster as a model. We identify a chloride channel named alkaliphile (Alka), which is both necessary and sufficient for aversive taste responses to basic food. Alka forms a high-pH-gated chloride channel and is specifically expressed in a subset of gustatory receptor neurons (GRNs). Optogenetic activation of alka-expressing GRNs is sufficient to suppress attractive feeding responses to sucrose. Conversely, inactivation of these GRNs causes severe impairments in the aversion to high pH. Altogether, our discovery of Alka as an alkaline taste receptor lays the groundwork for future research on alkaline taste sensation in other animals.

Experience-dependent plasticity in the olfactory system of Drosophila melanogaster and other insects

It is long known that the nervous system of vertebrates can be shaped by internal and external factors. On the other hand, the nervous system of insects was long assumed to be stereotypic, although evidence for plasticity effects accumulated for several decades. To cover the topic comprehensively, this review recapitulates the establishment of the term "plasticity" in neuroscience and introduces its original meaning. We describe the basic composition of the insect olfactory system using Drosophila melanogaster as a representative example and outline experience-dependent plasticity effects observed in this part of the brain in a variety of insects, including hymenopterans, lepidopterans, locusts, and flies. In particular, we highlight recent advances in the study of experience-dependent plasticity effects in the olfactory system of D. melanogaster, as it is the most accessible olfactory system of all insect species due to the genetic tools available. The partly contradictory results demonstrate that morphological, physiological and behavioral changes in response to long-term olfactory stimulation are more complex than previously thought. Different molecular mechanisms leading to these changes were unveiled in the past and are likely responsible for this complexity. We discuss common problems in the study of experience-dependent plasticity, ways to overcome them, and future directions in this area of research. In addition, we critically examine the transferability of laboratory data to natural systems to address the topic as holistically as possible. As a mechanism that allows organisms to adapt to new environmental conditions, experience-dependent plasticity contributes to an animal's resilience and is therefore a crucial topic for future research, especially in an era of rapid environmental changes.

High-throughput behavioral phenotyping of tiny arthropods: Chemosensory traits in a mesostigmatic hematophagous mite

Pest management using attractive and/or repellent semiochemicals is a key alternative to synthetic insecticides. Its implementation requires a good understanding of the intra- and interspecific chemical interactions of arthropod pests, their interactions with their abiotic environment, as well as their evolutionary dynamics. Although mites include many pest species and biocontrol agents of economic importance in agriculture, their chemical ecology is largely understudied compared to insects. We developed a high-throughput ethomics system to analyze these small arthropods and conducted a study on Dermanyssus gallinae, a problematic poultry parasite in the egg industry. Our purpose was to elucidate the role played by host-derived odorants (synthetic kairomone) and conspecific odorants (mite body odors) in D. gallinae. After validating our nanocomputer controlled olfactometric system with volatile semiochemicals of known biological activity, we characterized response traits to kairomonal and/or pheromonal volatile blends in mites from different populations. We were able to accurately characterize the repulsion or attraction behaviors in >1000 individual specimens in a standardized way. Our results confirm the presence of a volatile aggregation pheromone emitted by D. gallinae and bring new elements to the effect of odor source presentation. Our results also confirm the attractive effect on Dermanyssus gallinae of a blend of volatile compounds contained in hen odor, while highlighting a repellent effect at high concentration. Significant interindividual and interpopulation variation was noted particularly in responses to synthetic kairomone. This information lays a valuable foundation for further exploring the emergence risk of resistance to semiochemicals.

Chemosensory detection of aversive concentrations of ammonia and basic volatile amines in insects

Basic volatiles like ammonia are found in insect environments, and at high concentrations cause an atypical action potential burst, followed by inhibition in multiple classes of olfactory receptor neurons (ORNs) in Drosophila melanogaster. During the period of inhibition, ORNs are unable to fire action potentials to their ligands but continue to display receptor potentials. An increase in calcium is also observed in antennal cells of Drosophila and Aedes aegypti. In the gustatory system, ammonia inhibits sugar and salt responses in a dose-dependent manner. Other amines show similar effects in both gustatory and olfactory neurons, correlated with basicity. The concentrations that inhibit neurons reduce proboscis extension to sucrose in Drosophila. In Aedes, a brief exposure to volatile ammonia abolishes attraction to human skin odor for several minutes. These findings reveal an effect that prevents detection of attractive ligands in the olfactory and gustatory systems and has potential in insect control.

Bacteria-Derived Peptidoglycan Triggers a Noncanonical Nuclear Factor-κB-Dependent Response in Drosophila Gustatory Neurons

Probing the external world is essential for eukaryotes to distinguish beneficial from pathogenic micro-organisms. If it is clear that the main part of this task falls to the immune cells, recent work shows that neurons can also detect microbes, although the molecules and mechanisms involved are less characterized. In Drosophila, detection of bacteria-derived peptidoglycan by pattern recognition receptors of the peptidoglycan recognition protein (PGRP) family expressed in immune cells triggers nuclear factor-κB (NF-κB)/immune deficiency (IMD)-dependent signaling. We show here that one PGRP protein, called PGRP-LB, is expressed in bitter gustatory neurons of proboscises. In vivo calcium imaging in female flies reveals that the PGRP/IMD pathway is cell-autonomously required in these neurons to transduce the peptidoglycan signal. We finally show that NF-κB/IMD pathway activation in bitter-sensing gustatory neurons influences fly behavior. This demonstrates that a major immune response elicitor and signaling module are required in the peripheral nervous system to sense the presence of bacteria in the environment.SIGNIFICANCE STATEMENT In addition to the classical immune response, eukaryotes rely on neuronally controlled mechanisms to detect microbes and engage in adapted behaviors. However, the mechanisms of microbe detection by the nervous system are poorly understood. Using genetic analysis and calcium imaging, we demonstrate here that bacteria-derived peptidoglycan can activate bitter gustatory neurons. We further show that this response is mediated by the PGRP-LC membrane receptor and downstream components of a noncanonical NF-κB signaling cascade. Activation of this signaling cascade triggers behavior changes. These data demonstrate that bitter-sensing neurons and immune cells share a common detection and signaling module to either trigger the production of antibacterial effectors or to modulate the behavior of flies that are in contact with bacteria. Because peptidoglycan detection doesn't mobilize the known gustatory receptors, it also demonstrates that taste perception is much more complex than anticipated.

Mutagenesis of the odorant receptor co-receptor (Orco) reveals severe olfactory defects in the crop pest moth Helicoverpa armigera

BACKGROUND

Odorant receptors (ORs) as odorant-gated ion channels play a crucial role in insect olfaction. They are formed by a heteromultimeric complex of the odorant receptor co-receptor (Orco) and a ligand-selective Or. Other types of olfactory receptor proteins, such as ionotropic receptors (IRs) and some gustatory receptors (GRs), are also involved in the olfactory system of insects. Orco as an obligatory subunit of ORs is highly conserved, providing an opportunity to systematically evaluate OR-dependent olfactory responses.

RESULTS

Herein, we successfully established a homozygous mutant (Orco^-/-) of Helicoverpa armigera, a notorious crop pest, using the CRISPR/Cas9 gene editing technique. We then compared the olfactory response characteristics of wild type (WT) and Orco^-/- adults and larvae. Orco^-/- males were infertile, while Orco^-/- females were fertile. The lifespan of Orco^-/- females was longer than that of WT females. The expressions of most Ors, Irs, and other olfaction-related genes in adult antennae of Orco^-/- moths were not obviously affected, but some of them were up- or down-regulated. In addition, there was no change in the neuroanatomical phenotype of Orco^-/- moths at the level of the antennal lobe (including the macroglomerular complex region of the male). Using EAG and SSR techniques, we discovered that electrophysiological responses of Orco^-/- moths to sex pheromone components and many host plant odorants were absent. The upwind flight behaviors toward sex pheromones of Orco^-/- males were severely reduced in a wind tunnel experiment. The oviposition selectivity of Orco^-/- females to the host plant (green pepper) has completely disappeared, and the chemotaxis toward green pepper was also lost in Orco^-/- larvae.

CONCLUSIONS

Our study indicates that OR-mediated olfaction is essential for pheromone communication, oviposition selection, and larval chemotaxis of H. armigera, suggesting a strategy in which mate searching and host-seeking behaviors of moth pests could be disrupted by inhibiting or silencing Orco expression.

Foreleg Transcriptomic Analysis of the Chemosensory Gene Families in Plagiodera versicolora (Coleoptera: Chrysomelidae)

Plagiodera versicolora (Coleoptera: Chrysomelidae) is a worldwide leaf-eating forest pest in salicaceous trees. The forelegs play important roles in the chemoreception of insects. In this study, we conducted a transcriptome analysis of adult forelegs in P. versicolora and identified a total of 53 candidate chemosensory genes encoding 4 chemosensory proteins (CSPs), 19 odorant binding proteins (OBPs), 10 odorant receptors (ORs), 10 gustatory receptors (GRs), 6 ionotropic receptors (IRs), and 4 sensory neuron membrane proteins (SNMPs). Compared with the previous antennae transcriptome data, 1 CSP, 4 OBPs, 1 OR, 3 IRs, and 4 GRs were newly identified in the forelegs. Subsequently, the tissue expression profiles of 10 P. versicolora chemosensory genes were performed by real-time quantitative PCR. The results showed that PverOBP25, PverOBP27, and PverCSP6 were highly expressed in the antennae of both sexes. PverCSP11 and PverIR9 are predominately expressed in the forelegs than in the antennae. In addition, the expression levels of PverGR15 in female antennae and forelegs were significantly higher than those in the male antennae, implying that it may be involved in some female-specific behaviors such as oviposition site seeking. This work would greatly further the understanding of the chemoreception mechanism in P. versicolora.

Non-canonical odor coding in the mosquito

Aedes aegypti mosquitoes are a persistent human foe, transmitting arboviruses including dengue when they feed on human blood. Mosquitoes are intensely attracted to body odor and carbon dioxide, which they detect using ionotropic chemosensory receptors encoded by three large multi-gene families. Genetic mutations that disrupt the olfactory system have modest effects on human attraction, suggesting redundancy in odor coding. The canonical view is that olfactory sensory neurons each express a single chemosensory receptor that defines its ligand selectivity. We discovered that Ae. aegypti uses a different organizational principle, with many neurons co-expressing multiple chemosensory receptor genes. In vivo electrophysiology demonstrates that the broad ligand-sensitivity of mosquito olfactory neurons depends on this non-canonical co-expression. The redundancy afforded by an olfactory system in which neurons co-express multiple chemosensory receptors may increase the robustness of the mosquito olfactory system and explain our long-standing inability to disrupt the detection of humans by mosquitoes.

Humans produce a complex blend of odor cues that attract female mosquitoes, and these cues are typically detected by olfactory neurons expressing a single receptor. In female Aedes aegypti mosquitos, however, many of these neurons co-express multiple chemosensory receptors directly affecting mosquito behavior and challenging the canonical one-receptor-to-one-neuron organization.

Discrete roles of Ir76b ionotropic coreceptor impact olfaction, blood feeding, and mating in the malaria vector mosquito Anopheles coluzzii

Anopheline mosquitoes rely on their highly sensitive chemosensory apparatus to detect diverse chemical stimuli that drive the host-seeking and blood-feeding behaviors required to vector pathogens for malaria and other diseases. This process incorporates a variety of chemosensory receptors and transduction pathways. We used advanced in vivo gene-editing and -labeling approaches to localize and functionally characterize the ionotropic coreceptor AcIr76b in the malaria mosquito Anopheles coluzzii, where it impacts both olfactory and gustatory systems. AcIr76b has a broad expression pattern in female adult antennal grooved pegs, coeloconic sensilla, and T1 and T2 sensilla on the labellum, stylets, and tarsi, as well as the larval sensory peg. AcIr76b is colocalized with the Orco odorant receptor (OR) coreceptor in a subset of cells across the female antennae and labella. In contrast to Orco and Ir8a, chemosensory coreceptors that appear essential for the activity of their respective sets of chemosensory neurons in mosquitoes, AcIr76b−/− mutants maintain wild-type peripheral responses to volatile amines on the adult palps, labellum, and larval sensory cone. Interestingly, AcIr76b−/− mutants display significantly increased responses to amines in antennal grooved peg sensilla, while coeloconic sensilla reveal significant deficits in responses to several acids and amines. Behaviorally, AcIr76b mutants manifest significantly female-specific insemination deficits, and although AcIr76b−/− mutant females can locate, alight on, and probe artificial blood hosts, they are incapable of blood feeding successfully. Taken together, our findings reveal a multidimensional functionality of Ir76b in anopheline olfactory and gustatory pathways that directly impacts the vectorial capacity of these mosquitoes.

Higher-order olfactory neurons in the lateral horn support odor valence and odor identity coding in Drosophila

Understanding neuronal representations of odor-evoked activities and their progressive transformation from the sensory level to higher brain centers features one of the major aims in olfactory neuroscience. Here, we investigated how odor information is transformed and represented in higher-order neurons of the lateral horn, one of the higher olfactory centers implicated in determining innate behavior, using Drosophila melanogaster. We focused on a subset of third-order glutamatergic lateral horn neurons (LHNs) and characterized their odor coding properties in relation to their presynaptic partner neurons, the projection neurons (PNs) by two-photon functional imaging. We show that odors evoke reproducible, stereotypic, and odor-specific response patterns in LHNs. Notably, odor-evoked responses in these neurons are valence-specific in a way that their response amplitude is positively correlated with innate odor preferences. We postulate that this valence-specific activity is the result of integrating inputs from multiple olfactory channels through second-order neurons. GRASP and micro-lesioning experiments provide evidence that glutamatergic LHNs obtain their major excitatory input from uniglomerular PNs, while they receive an odor-specific inhibition through inhibitory multiglomerular PNs. In summary, our study indicates that odor representations in glutamatergic LHNs encode hedonic valence and odor identity and primarily retain the odor coding properties of second-order neurons.

Chemoreceptor co-expression in Drosophila melanogaster olfactory neurons

Drosophila melanogaster olfactory neurons have long been thought to express only one chemosensory receptor gene family. There are two main olfactory receptor gene families in Drosophila, the odorant receptors (ORs) and the ionotropic receptors (IRs). The dozens of odorant-binding receptors in each family require at least one co-receptor gene in order to function: Orco for ORs, and Ir25a, Ir8a, and Ir76b for IRs. Using a new genetic knock-in strategy, we targeted the four co-receptors representing the main chemosensory families in D. melanogaster (Orco, Ir8a, Ir76b, Ir25a). Co-receptor knock-in expression patterns were verified as accurate representations of endogenous expression. We find extensive overlap in expression among the different co-receptors. As defined by innervation into antennal lobe glomeruli, Ir25a is broadly expressed in 88% of all olfactory sensory neuron classes and is co-expressed in 82% of Orco+ neuron classes, including all neuron classes in the maxillary palp. Orco, Ir8a, and Ir76b expression patterns are also more expansive than previously assumed. Single sensillum recordings from Orco-expressing Ir25a mutant antennal and palpal neurons identify changes in olfactory responses. We also find co-expression of Orco and Ir25a in Drosophila sechellia and Anopheles coluzzii olfactory neurons. These results suggest that co-expression of chemosensory receptors is common in insect olfactory neurons. Together, our data present the first comprehensive map of chemosensory co-receptor expression and reveal their unexpected widespread co-expression in the fly olfactory system.

Optogenetic and thermogenetic manipulation of defined neural circuits and behaviors in Drosophila

Neural network dynamics underlying flexible animal behaviors remain elusive. The fruit fly Drosophila melanogaster is considered an excellent model in behavioral neuroscience because of its simple neuroanatomical architecture and the availability of various genetic methods. Moreover, Drosophila larvae's transparent body allows investigators to use optical methods on freely moving animals, broadening research directions. Activating or inhibiting well-defined events in excitable cells with a fine temporal resolution using optogenetics and thermogenetics led to the association of functions of defined neural populations with specific behavioral outputs such as the induction of associative memory. Furthermore, combining optogenetics and thermogenetics with state-of-the-art approaches, including connectome mapping and machine learning-based behavioral quantification, might provide a complete view of the experience- and time-dependent variations of behavioral responses. These methodologies allow further understanding of the functional connections between neural circuits and behaviors such as chemosensory, motivational, courtship, and feeding behaviors and sleep, learning, and memory.

Valence opponency in peripheral olfactory processing

Are olfactory receptor neurons (ORNs) arranged in a functionally meaningful manner to facilitate information processing? Here, we address this long-standing question by uncovering a valence map in the olfactory periphery of Drosophila. Within sensory hairs, we find that neighboring ORNs antagonistically regulate behaviors: stereotypically compartmentalized large- and small-spike ORNs, recognized by their characteristic spike amplitudes, either promote or inhibit the same type of behavior, respectively. Systematic optogenetic and thermogenetic assays—covering the majority of antennal sensilla—highlight a valence-opponent organization. Critically, odor-mixture behavioral experiments show that lateral inhibition between antagonistic ORNs mediates robust behavioral decisions in response to countervailing cues. Computational modeling predicts that the robustness of behavioral output depends on odor mixture ratios.

A hallmark of complex sensory systems is the organization of neurons into functionally meaningful maps, which allow for comparison and contrast of parallel inputs via lateral inhibition. However, it is unclear whether such a map exists in olfaction. Here, we address this question by determining the organizing principle underlying the stereotyped pairing of olfactory receptor neurons (ORNs) in Drosophila sensory hairs, wherein compartmentalized neurons inhibit each other via ephaptic coupling. Systematic behavioral assays reveal that most paired ORNs antagonistically regulate the same type of behavior. Such valence opponency is relevant in critical behavioral contexts including place preference, egg laying, and courtship. Odor-mixture experiments show that ephaptic inhibition provides a peripheral means for evaluating and shaping countervailing cues relayed to higher brain centers. Furthermore, computational modeling suggests that this organization likely contributes to processing ratio information in odor mixtures. This olfactory valence map may have evolved to swiftly process ethologically meaningful odor blends without involving costly synaptic computation.

Evolving the olfactory system with machine learning

The convergent evolution of the fly and mouse olfactory system led us to ask whether the anatomic connectivity and functional logic of olfactory circuits would evolve in artificial neural networks trained to perform olfactory tasks. Artificial networks trained to classify odor identity recapitulate the connectivity inherent in the olfactory system. Input units are driven by a single receptor type, and units driven by the same receptor converge to form a glomerulus. Glomeruli exhibit sparse, unstructured connectivity onto a larger expansion layer of Kenyon cells. When trained to both classify odor identity and to impart innate valence onto odors, the network develops independent pathways for identity and valence classification. Thus, the defining features of fly and mouse olfactory systems also evolved in artificial neural networks trained to perform olfactory tasks. This implies that convergent evolution reflects an underlying logic rather than shared developmental principles.

Ir76b is a Co-receptor for Amine Responses in Drosophila Olfactory Neurons

Two large families of olfactory receptors, the Odorant Receptors (ORs) and Ionotropic Receptors (IRs), mediate responses to most odors in the insect olfactory system. Individual odorant binding "tuning" OrX receptors are expressed by olfactory neurons in basiconic and trichoid sensilla and require the co-receptor Orco. The situation for IRs is more complex. Different tuning IrX receptors are expressed by olfactory neurons in coeloconic sensilla and rely on either the Ir25a or Ir8a co-receptors; some evidence suggests that Ir76b may also act as a co-receptor, but its function has not been systematically examined. Surprisingly, recent data indicate that nearly all coeloconic olfactory neurons co-express Ir25a, Ir8a, and Ir76b. Here, we demonstrate that Ir76b and Ir25a function together in all amine-sensing olfactory receptor neurons. In most neurons, loss of either co-receptor abolishes amine responses. In contrast, amine responses persist in the absence of Ir76b or Ir25a in ac1 sensilla but are lost in a double mutant. We show that responses mediated by acid-sensing neurons do not require Ir76b, despite their expression of this co-receptor. Our study also demonstrates that one population of coeloconic olfactory neurons exhibits Ir76b/Ir25a-dependent and Orco-dependent responses to distinct odorants. Together, our data establish the role of Ir76b as a bona fide co-receptor, which acts in partnership with Ir25a. Given that these co-receptors are among the most highly conserved olfactory receptors and are often co-expressed in chemosensory neurons, our data suggest Ir76b and Ir25a also work in tandem in other insects.

Ionotropic receptors mediate nitrogenous waste avoidance in Drosophila melanogaster

Ammonia and its amine-containing derivatives are widely found in natural decomposition byproducts. Here, we conducted biased chemoreceptor screening to investigate the mechanisms by which different concentrations of ammonium salt, urea, and putrescine in rotten fruits affect feeding and oviposition behavior. We identified three ionotropic receptors, including the two broadly required IR25a and IR76b receptors, as well as the narrowly tuned IR51b receptor. These three IRs were fundamental in eliciting avoidance against nitrogenous waste products, which is mediated by bitter-sensing gustatory receptor neurons (GRNs). The aversion of nitrogenous wastes was evaluated by the cellular requirement by expressing Kir2.1 and behavioral recoveries of the mutants in bitter-sensing GRNs. Furthermore, by conducting electrophysiology assays, we confirmed that ammonia compounds are aversive in taste as they directly activated bitter-sensing GRNs. Therefore, our findings provide insights into the ecological roles of IRs as a means to detect and avoid toxic nitrogenous waste products in nature.

Drosophila sensory receptors-a set of molecular Swiss Army Knives

Genetic approaches in the fruit fly, Drosophila melanogaster, have led to a major triumph in the field of sensory biology-the discovery of multiple large families of sensory receptors and channels. Some of these families, such as transient receptor potential channels, are conserved from animals ranging from worms to humans, while others, such as "gustatory receptors," "olfactory receptors," and "ionotropic receptors," are restricted to invertebrates. Prior to the identification of sensory receptors in flies, it was widely assumed that these proteins function in just one modality such as vision, smell, taste, hearing, and somatosensation, which includes thermosensation, light, and noxious mechanical touch. By employing a vast combination of genetic, behavioral, electrophysiological, and other approaches in flies, a major concept to emerge is that many sensory receptors are multitaskers. The earliest example of this idea was the discovery that individual transient receptor potential channels function in multiple senses. It is now clear that multitasking is exhibited by other large receptor families including gustatory receptors, ionotropic receptors, epithelial Na+ channels (also referred to as Pickpockets), and even opsins, which were formerly thought to function exclusively as light sensors. Genetic characterizations of these Drosophila receptors and the neurons that express them also reveal the mechanisms through which flies can accurately differentiate between different stimuli even when they activate the same receptor, as well as mechanisms of adaptation, amplification, and sensory integration. The insights gleaned from studies in flies have been highly influential in directing investigations in many other animal models.

An ammonium transporter is a non-canonical olfactory receptor for ammonia

Numerous hematophagous insects are attracted to ammonia, a volatile released in human sweat and breath.1-3 Low levels of ammonia also attract non-biting insects such as the genetic model organism Drosophila melanogaster and several species of agricultural pests.4,5 Two families of ligand-gated ion channels function as olfactory receptors in insects,6-10 and studies have linked ammonia sensitivity to a particular olfactory receptor in Drosophila.5,11,12 Given the widespread importance of ammonia to insect behavior, it is surprising that the genomes of most insects lack an ortholog of this gene.6 Here, we show that canonical olfactory receptors are not necessary for responses to ammonia in Drosophila. Instead, we demonstrate that a member of the ancient electrogenic ammonium transporter family, Amt, is likely a new type of olfactory receptor. We report two hitherto unidentified olfactory neuron populations that mediate neuronal and behavioral responses to ammonia in Drosophila. Their endogenous ammonia responses are lost in Amt mutant flies, and ectopic expression of either Drosophila or Anopheles Amt confers ammonia sensitivity. These results suggest that Amt is the first transporter known to function as an olfactory receptor in animals and that its function may be conserved across insect species.

Ammonium transporter AcAmt mutagenesis uncovers reproductive and physiological defects without impacting olfactory responses to ammonia in the malaria vector mosquito Anopheles coluzzii

Anopheline mosquitoes are the sole vectors of malaria and rely on olfactory cues for host seeking in which ammonia derived from human sweat plays an essential role. To investigate the function of the Anopheles coluzzii ammonium transporter (AcAmt) in the mosquito olfactory system, we generated an AcAmt null mutant line using CRISPR/Cas9. AcAmt^-/- mutants displayed a series of novel phenotypes compared with wild-type mosquitoes including significantly lower insemination rates during mating and increased mortality during eclosion. Furthermore, AcAmt^-/- males showed significantly lower sugar consumption while AcAmt^-/- females and pupae displayed significantly higher ammonia levels than their wild-type counterparts. Surprisingly, in contrast to previous studies in Drosophila that revealed that the mutation of the ammonium transporter (DmAmt) induces a dramatic reduction of ammonia responses in antennal coeloconic sensilla, no significant differences were observed across a range of peripheral sensory neuron responses to ammonia and other odorants between wild-type and AcAmt^-/- females. These data support the existence in mosquitoes of novel compensatory ammonia-sensing mechanisms that are likely to have evolved as a result of the importance of ammonia in host-seeking and other behaviors.

Unraveling the Genetic Basis for the Rapid Diversification of Male Genitalia between Drosophila Species

In the last 240,000 years, males of the Drosophila simulans species clade have evolved striking differences in the morphology of their epandrial posterior lobes and claspers (surstyli). These appendages are used for grasping the female during mating and so their divergence is most likely driven by sexual selection. Mapping studies indicate a highly polygenic and generally additive genetic basis for these morphological differences. However, we have limited understanding of the gene regulatory networks that control the development of genital structures and how they evolved to result in this rapid phenotypic diversification. Here, we used new D. simulans/D. mauritiana introgression lines on chromosome arm 3L to generate higher resolution maps of posterior lobe and clasper differences between these species. We then carried out RNA-seq on the developing genitalia of both species to identify the expressed genes and those that are differentially expressed between the two species. This allowed us to test the function of expressed positional candidates during genital development in D. melanogaster. We identified several new genes involved in the development and possibly the evolution of these genital structures, including the transcription factors Hairy and Grunge. Furthermore, we discovered that during clasper development Hairy negatively regulates tartan (trn), a gene known to contribute to divergence in clasper morphology. Taken together, our results provide new insights into the regulation of genital development and how this has evolved between species.

Information flow, cell types and stereotypy in a full olfactory connectome

The hemibrain connectome provides large-scale connectivity and morphology information for the majority of the central brain of Drosophila melanogaster. Using this data set, we provide a complete description of the Drosophila olfactory system, covering all first, second and lateral horn-associated third-order neurons. We develop a generally applicable strategy to extract information flow and layered organisation from connectome graphs, mapping olfactory input to descending interneurons. This identifies a range of motifs including highly lateralised circuits in the antennal lobe and patterns of convergence downstream of the mushroom body and lateral horn. Leveraging a second data set we provide a first quantitative assessment of inter- versus intra-individual stereotypy. Comparing neurons across two brains (three hemispheres) reveals striking similarity in neuronal morphology across brains. Connectivity correlates with morphology and neurons of the same morphological type show similar connection variability within the same brain as across two brains.

Distinct protocerebral neuropils associated with attractive and aversive female-produced odorants in the male moth brain

The pheromone system of heliothine moths is an optimal model for studying principles underlying higher-order olfactory processing. In Helicoverpa armigera, three male-specific glomeruli receive input about three female-produced signals, the primary pheromone component, serving as an attractant, and two minor constituents, serving a dual function, that is, attraction versus inhibition of attraction. From the antennal-lobe glomeruli, the information is conveyed to higher olfactory centers, including the lateral protocerebrum, via three main paths – of which the medial tract is the most prominent. In this study, we traced physiologically identified medial-tract projection neurons from each of the three male-specific glomeruli with the aim of mapping their terminal branches in the lateral protocerebrum. Our data suggest that the neurons’ widespread projections are organized according to behavioral significance, including a spatial separation of signals representing attraction versus inhibition – however, with a unique capacity of switching behavioral consequence based on the amount of the minor components.

Manipulations of the olfactory circuit highlight the role of sensory stimulation in regulating sleep amount

STUDY OBJECTIVES

While wake duration is a major sleep driver, an important question is if wake quality also contributes to controlling sleep. In particular, we sought to determine whether changes in sensory stimulation affect sleep in Drosophila. As Drosophila rely heavily on their sense of smell, we focused on manipulating olfactory input and the olfactory sensory pathway.

METHODS

Sensory deprivation was first performed by removing antennae or applying glue to antennae. We then measured sleep in response to neural activation, via expression of the thermally gated cation channel TRPA1, or inhibition, via expression of the inward rectifying potassium channel KIR2.1, of subpopulations of neurons in the olfactory pathway. Genetically restricting manipulations to adult animals prevented developmental effects.

RESULTS

We find that olfactory deprivation reduces sleep, largely independently of mushroom bodies that integrate olfactory signals for memory consolidation and have previously been implicated in sleep. However, specific neurons in the lateral horn, the other third-order target of olfactory input, affect sleep. Also, activation of inhibitory second-order projection neurons increases sleep. No single neuronal population in the olfactory processing pathway was found to bidirectionally regulate sleep, and reduced sleep in response to olfactory deprivation may be masked by temperature changes.

CONCLUSIONS

These findings demonstrate that Drosophila sleep is sensitive to sensory stimulation, and identify novel sleep-regulating neurons in the olfactory circuit. Scaling of signals across the circuit may explain the lack of bidirectional effects when neuronal activity is manipulated. We propose that olfactory inputs act through specific circuit components to modulate sleep in flies.

The Structure and Function of Ionotropic Receptors in Drosophila

Ionotropic receptors (IRs) are a highly divergent subfamily of ionotropic glutamate receptors (iGluR) and are conserved across Protostomia, a major branch of the animal kingdom that encompasses both Ecdysozoa and Lophothrochozoa. They are broadly expressed in peripheral sensory systems, concentrated in sensory dendrites, and function in chemosensation, thermosensation, and hygrosensation. As iGluRs, four IR subunits form a functional ion channel to detect environmental stimuli. Most IR receptors comprise individual stimulus-specific tuning receptors and one or two broadly expressed coreceptors. This review summarizes the discoveries of the structure of IR complexes and the expression and function of each IR, as well as discusses the future direction for IR studies.

Functional properties of insect olfactory receptors: ionotropic receptors and odorant receptors

The majority of insect olfactory receptors belong to two distinct protein families, the ionotropic receptors (IRs), which are related to the ionotropic glutamate receptor family, and the odorant receptors (ORs), which evolved from the gustatory receptor family. Both receptor types assemble to heteromeric ligand-gated cation channels composed of odor-specific receptor proteins and co-receptor proteins. We here present in short the current view on evolution, function, and regulation of IRs and ORs. Special attention is given on how their functional properties can meet the environmental and ecological challenges an insect has to face.

Olfactory processing in the lateral horn of Drosophila

Sensing olfactory signals in the environment represents a crucial and significant task of sensory systems in almost all organisms to facilitate survival and reproduction. Notably, the olfactory system of diverse animal phyla shares astonishingly many fundamental principles with regard to anatomical and functional properties. Binding of odor ligands by chemosensory receptors present in the olfactory peripheral organs leads to a neuronal activity that is conveyed to first and higher-order brain centers leading to a subsequent odor-guided behavioral decision. One of the key centers for integrating and processing innate olfactory behavior is the lateral horn (LH) of the protocerebrum in insects. In recent years the LH of Drosophila has garnered increasing attention and many studies have been dedicated to elucidate its circuitry. In this review we will summarize the recent advances in mapping and characterizing LH-specific cell types, their functional properties with respect to odor tuning, their neurotransmitter profiles, their connectivity to pre-synaptic and post-synaptic partner neurons as well as their impact for olfactory behavior as known so far.

The ionotropic receptor gene family in Lepidoptera and Trichoptera: Annotation, evolutionary and functional perspectives

Lepidoptera (moths and butterflies) and Trichoptera (caddisflies), belonging to the superorder Amphiesmenoptera, are the most diverse insect orders as representatives of the terrestrial and aquatic insects, respectively. The insects of the two orders possess different biological and behavioral characteristics, especially their larvae, presumably resulting in the differences of the ionotropic receptor (IR) genes in numbers, sequence characteristics or gene structure. Here, we employed genomics, transcriptomics, bioinformatics, phylogenetics and molecular biology strategies to characterize the IR gene repertoire in Lepidoptera and Trichoptera. Genome and transcriptome analyses with exhaustive homology-based searches and manual efforts, in 32 lepidopterans and five trichopterans, led to the identification of 1449 genes encoding IRs with 1170 full-length sequences, representing the most comprehensive set of chemoreceptor superfamilies across the Amphiesmenoptera. Analysis of gene gains and losses in orthologous groups implied that some IRs were lost in related species, and multiple gene copies occurred mainly in divergent IRs (D-IRs) by gene duplications. Phylogenetic analysis of 2442 IR proteins from 67 species revealed that Lepidoptera and Trichoptera IRs could be classified into three subfamilies, i.e., 14 antennal IRs (A-IRs), five Lepidoptera-specific IRs (LS-IRs) and four D-IRs. Of the three subfamilies, A-IRs and LS-IRs members within orthologous groups exhibited high conservation of gene structure, but D-IRs shared extremely low amino acid identities (below 30%). Expression profiles revealed functional diversities of IRs from Bombyx mori and Papilio xuthus involving smell, taste or reproduction, in which some genes displayed sex-biased expression in antennae associated with specific chemosensory behaviors of female or male adults. Our current study has provided insights into the evolution, conservation and divergence of IRs between/within Lepidoptera and Trichoptera, and allows for further experiments to investigate IR functions.

Identification and Expression Profiling of Chemosensory Genes in Hermetia illucens via a Transcriptomic Analysis

The black soldier fly, Hermetia illucens, is a cosmopolitan insect of the family Stratiomyidae (Diptera). Chemosensory genes encode proteins involved directly in the detection of odorants. In this study, we sequenced the antennal transcriptome of H. illucens adults to identify chemosensory genes. Putative unigenes encoding 27 odorant binding proteins (OBPs), five chemosensory proteins (CSPs), 70 odorant receptors (ORs), 25 ionotropic receptors (IRs), 10 gustatory receptors (GRs) and two sensory neuron membrane proteins (SNMPs) were identified. Tissue-specific expression profiles of the identified OBPs, CSPs and SNMPs were investigated using RT-PCR. Eight OBPs (HillOBP1-2, 9, 11-14, and 17), one CSP (HillCSP5) and one SNMP (HillSNMP1) were predominantly expressed in antennae. Further real-time quantitative PCR analyses revealed that the antennae-enriched unigenes also exhibited significant differences in expression between males and females. Among the sex-biased unigenes, six ORs showed female-biased expression, suggesting that these genes might participate in female-specific behaviors such as oviposition site searching. Sixteen ORs and two OBPs showed male-biased expression, indicating that they may play key roles in the detection of female sex pheromones. Our study is the first attempt to delineate the molecular basis of chemoreception in H. illucens. Our data provide useful information for comparative studies on the differentiation and evolution of Dipteran chemosensory gene families.

Heterogeneous expression of the ammonium transporter AgAmt in chemosensory appendages of the malaria vector, Anopheles gambiae

Ammonia is one of the principal kairomones originating from human and other animal emanations and in that context, plays an essential role in the host-seeking behaviors of the malaria vector mosquito Anopheles gambiae. Nevertheless, despite its importance in directing host-seeking, the mechanisms underlying ammonia detection in the mosquito olfactory system remains largely unknown. In addition to ongoing efforts to identify and characterize the molecular receptors that underlie ammonia sensitivity, previous studies have revealed a prominent role for ammonium transporters (Amt) in modulating antennal and behavioral responses in Drosophila melanogaster and An. gambiae. In the former, localization of DmAmt in antennal sensilla to auxiliary cells surrounding the ammonia sensory neurons led to the hypothesis that its role was to clear excess ammonium ions in the sensillar lymph. In the latter, RT-PCR and heterologous expression have been used to examine the expression and functional characteristics of the An. gambiae ammonium transporter, AgAmt. We now employ advanced transgenic tools to comprehensively examine AgAmt spatial localization across the peripheral chemosensory appendages in larvae and adult female An. gambiae. In the larval antennae, AgAmt appears localized in both neuronal and auxiliary cells. In contrast to D. melanogaster, in the adult antennae, AgAmt-derived signals are observed in both non-neuronal auxiliary cells and in sensory neurons in ammonia-responsive basiconic and coeloconic sensilla. In the maxillary palps, labella, and tarsi, AgAmt appears restricted to sensory neurons. We have also characterized the responses to ammonia of adult antennal coeloconic sensilla and maxillary palp capitate pegs revealing a correlation between sensillar AgAmt expression and ammonia sensitivity. Taken together, these data suggest that AgAmt may play heterogeneous roles in the adult and larval chemosensory apparatus and potentially broad utility as a supra-receptor target in mosquito control.

Behavioral and Transcriptional Response to Selection for Olfactory Behavior in Drosophila

The detection, discrimination, and behavioral responses to chemical cues in the environment can have marked effects on organismal survival and reproduction, eliciting attractive or aversive behavior. To gain insight into mechanisms mediating this hedonic valence, we applied thirty generations of divergent artificial selection for Drosophila melanogaster olfactory behavior. We independently selected for positive and negative behavioral responses to two ecologically relevant chemical compounds: 2,3-butanedione and cyclohexanone. We also tested the correlated responses to selection by testing behavioral responses to other odorants and life history traits. Measurements of behavioral responses of the selected lines and unselected controls to additional odorants showed that the mechanisms underlying responses to these odorants are, in some cases, differentially affected by selection regime and generalization of the response to other odorants was only detected in the 2,3-butanedione selection lines. Food consumption and lifespan varied with selection regime and, at times, sex. An analysis of gene expression of both selection regimes identified multiple differentially expressed genes. New genes and genes previously identified in mediating olfactory behavior were identified. In particular, we found functional enrichment of several gene ontology terms, including cell-cell adhesion and sulfur compound metabolic process, the latter including genes belonging to the glutathione S-transferase family. These findings highlight a potential role for glutathione S-transferases in the evolution of hedonic valence to ecologically relevant volatile compounds and set the stage for a detailed investigation into mechanisms by which these genes mediate attraction and aversion.

Sexual-biased gene expression of olfactory-related genes in the antennae of Conogethes pinicolalis (Lepidoptera: Crambidae)

BACKGROUND

Conogethes pinicolalis (Lepidoptera: Crambidae), is similar to Conogethes punctiferalis (yellow peach moth) and its host plant is gymnosperms, especially for masson pine. So far, less literature was reported on this pest. In the present study, we sequenced and characterized the antennal transcriptomes of male and female C. pinicolalis for the first time.

RESULTS

Totally, 26 odorant-binding protein (OBP) genes, 19 chemosensory protein (CSP) genes, 55 odorant receptor (OR) genes and 20 ionotropic receptor (IR) genes were identified from the C. pinicolalis antennae transcriptome and amino sequences were annotated against homologs of C. punctiferalis. The neighbor-joining tree indicated that the amino acid sequence of olfactory related genes is highly homologous with C. punctiferalis. Furthermore, the reference genes were selected, and we recommended the phosphate dehydrogenase gene (GAPDH) or ribosomal protein 49 gene (RP49) to verify the target gene expression during larval development stages and RP49 or ribosomal protein L13 gene (RPL13) for adult tissues.

CONCLUSIONS

Our study provides a starting point on the molecular level characterization between C. pinicolalis and C. punctiferalis, which might be supportive for pest management studies in future.

Bidirectional Regulation of Sleep and Synapse Pruning after Neural Injury

Following acute neural injury, severed axons undergo programmed Wallerian degeneration over several following days. While sleep has been linked with synaptic reorganization under other conditions, the role of sleep in responses to neural injuries remains poorly understood. To study the relationship between sleep and neural injury responses, we examined Drosophila melanogaster following the removal of antennae or other sensory tissues. Daytime sleep is elevated after antennal or wing injury, but sleep returns to baseline levels within 24 h after injury. Similar increases in sleep are not observed when olfactory receptor neurons are silenced or when other sensory organs are severed, suggesting that increased sleep after injury is not attributed to sensory deprivation, nociception, or generalized inflammatory responses. Neuroprotective disruptions of the E3 ubiquitin ligase highwire and c-Jun N-terminal kinase basket in olfactory receptor neurons weaken the sleep-promoting effects of antennal injury, suggesting that post-injury sleep may be influenced by the clearance of damaged neurons. Finally, we show that pre-synaptic active zones are preferentially removed from severed axons within hours after injury and that depriving recently injured flies of sleep slows the removal of both active zones and damaged axons. These data support a bidirectional interaction between sleep and synapse pruning after antennal injury: locally increasing the need to clear neural debris is associated with increased sleep, which is required for efficient active zone removal after injury.

Recent advances in the genetic basis of taste detection in Drosophila

The insect gustatory system senses taste information from environmental food substrates and processes it to control feeding behaviors. Drosophila melanogaster has been a powerful genetic model for investigating how various chemical cues are detected at the molecular and cellular levels. In addition to an understanding of how tastants belonging to five historically described taste modalities (sweet, bitter, acid, salt, and amino acid) are sensed, recent findings have identified taste neurons and receptors that recognize tastants of non-canonical modalities, including fatty acids, carbonated water, polyamines, H2O2, bacterial lipopolysaccharide (LPS), ammonia, and calcium. Analyses of response profiles of taste neurons expressing different suites of chemosensory receptors have allowed exploration of taste coding mechanisms in primary sensory neurons. In this review, we present the current knowledge of the molecular and cellular basis of taste detection of various categories of tastants. We also summarize evidence for organotopic and multimodal functions of the taste system. Functional characterization of peripheral taste neurons in different organs has greatly increased our understanding of how insect behavior is regulated by the gustatory system, which may inform development of novel insect pest control strategies.

The diversity and evolution of odorant receptors in beetles (Coleoptera)

The insect odorant receptors (ORs) are amongst the largest gene families in insect genomes and the primary means by which insects recognize volatile compounds. The evolution of ORs is thus instrumental in explaining the chemical ecology of insects and as a model of evolutionary biology. However, although ORs have been described from numerous insect species, their analysis within and amongst the insect orders has been hindered by a combination of limited genomic information and a tendency of the OR family toward rapid divergence, gain, and loss. We addressed these issues in the insect order Coleoptera through a targeted genomic annotation effort that included 1181 ORs from one species of the sister order Strepsiptera and 10 species representing the four coleopteran suborders. The numbers of ORs in each species varied from hundreds to fewer than 10, but coleopteran ORs could nevertheless be represented within a scheme of nine monophyletic subfamilies. We observed many radiations and losses of genes amongst OR subfamilies, and the diversity of ORs appeared to parallel the host breadth of the study species. However, some small lineages of ORs persisted amongst many coleopteran families, suggesting receptors of key function that underlie the olfactory ecology of beetles.

Chemosensation and Evolution of Drosophila Host Plant Selection

The ability to respond to chemosensory cues is critical for survival of most organisms. Among insects, Drosophila melanogaster has the best characterized olfactory system, and the availability of genome sequences of 30 Drosophila species provides an ideal scenario for studies on evolution of chemosensation. Gene duplications of chemoreceptor genes allow for functional diversification of the rapidly evolving chemoreceptor repertoire. Although some species of the genus Drosophila are generalists for host plant selection, rapid evolution of olfactory receptors, gustatory receptors, odorant-binding proteins, and cytochrome P450s has enabled diverse host specializations of different members of the genus. Here, I review diversification of the chemoreceptor repertoire among members of the genus Drosophila along with co-evolution of detoxification mechanisms that may have enabled occupation of diverse host plant ecological niches.

Presynaptic MAST kinase controls opposing postsynaptic responses to convey stimulus valence in Caenorhabditis elegans

Animals need to quickly extract the valence information of sensory stimulus and assess whether the stimulus is attractive or aversive. Deciphering the molecular and circuit mechanisms that determine the stimulus valence is fundamental to understand how the nervous system generates the animal behaviors. Here we report that the AFD thermosensory neurons of C. elegans evoke in its postsynaptic AIY interneurons opposing neuronal responses that correlate with the valence of thermal stimuli. The C. elegans homologs of MAST kinase, Stomatin, and Diacylglycerol kinase function in AFD and regulate the opposing AIY responses. Our results further suggest that the alteration between excitatory and inhibitory AIY responses is mediated by controlling the balance of two opposing signals released from the AFD neurons.

Presynaptic plasticity is known to modulate the strength of synaptic transmission. However, it remains unknown whether regulation in presynaptic neurons can evoke excitatory and inhibitory postsynaptic responses. We report here that the Caenorhabditis elegans homologs of MAST kinase, Stomatin, and Diacylglycerol kinase act in a thermosensory neuron to elicit in its postsynaptic neuron an excitatory or inhibitory response that correlates with the valence of thermal stimuli. By monitoring neural activity of the valence-coding interneuron in freely behaving animals, we show that the alteration between excitatory and inhibitory responses of the interneuron is mediated by controlling the balance of two opposing signals released from the presynaptic neuron. These alternative transmissions further generate opposing behavioral outputs necessary for the navigation on thermal gradients. Our findings suggest that valence-encoding interneuronal activity is determined by a presynaptic mechanism whereby MAST kinase, Stomatin, and Diacylglycerol kinase influence presynaptic outputs.

Sensorimotor Transformations in the Zebrafish Auditory System

Organisms use their sensory systems to acquire information from their environment and integrate this information to produce relevant behaviors. Nevertheless, how sensory information is converted into adequate motor patterns in the brain remains an open question. Here, we addressed this question using two-photon and light-sheet calcium imaging in intact, behaving zebrafish larvae. We monitored neural activity elicited by auditory stimuli while simultaneously recording tail movements. We observed a spatial organization of neural activity according to four different response profiles (frequency tuning curves), suggesting a low-dimensional representation of frequency information, maintained throughout the development of the larvae. Low frequencies (150-450 Hz) were locally processed in the hindbrain and elicited motor behaviors. In contrast, higher frequencies (900-1,000 Hz) rarely induced motor behaviors and were also represented in the midbrain. Finally, we found that the sensorimotor transformations in the zebrafish auditory system are a continuous and gradual process that involves the temporal integration of the sensory response in order to generate a motor behavior.

Scaling the interactive effects of attractive and repellent odours for insect search behaviour

Insects searching for resources are exposed to a complexity of mixed odours, often involving both attractant and repellent substances. Understanding how insects respond to this complexity of cues is crucial for understanding consumer-resource interactions, but also to develop novel tools to control harmful pests. To advance our understanding of insect responses to combinations of attractive and repellent odours, we formulated three qualitative hypotheses; the response-ratio hypothesis, the repellent-threshold hypothesis and the odour-modulation hypothesis. The hypotheses were tested by exposing Drosophila melanogaster in a wind tunnel to combinations of vinegar as attractant and four known repellents; benzaldehyde, 1-octen-3-ol, geosmin and phenol. The responses to benzaldehyde, 1-octen-3-ol and geosmin provided support for the response-ratio hypothesis, which assumes that the behavioural response depends on the ratio between attractants and repellents. The response to phenol, rather supported the repellent-threshold hypothesis, where aversion only occurs above a threshold concentration of the repellent due to overshadowing of the attractant. We hypothesize that the different responses may be connected to the localization of receptors, as receptors detecting phenol are located on the maxillary palps whereas receptors detecting the other odorants are located on the antennae.

Identification and Expression Analyses of Olfactory Gene Families in the Rice Grasshopper, Oxya chinensis, From Antennal Transcriptomes

The rice grasshopper Oxya chinensis is an important agricultural pest of rice and other gramineous plants. Chemosensory genes are crucial factors in direct interactions with odorants in the olfactory process. Here we identified genes encoding 18 odorant-binding proteins (OBPs), 13 chemosensory proteins (CSPs), 94 olfactory receptors (ORs), 12 ionotropic receptors (IRs), and two sensory neuron membrane proteins (SNMPs) from O. chinensis using an transcriptomic approach. Semi-quantitative RT-PCR assays revealed that six OBP-encoding genes (OchiOBP4, 5, 8, 9, 10, and 14), one CSP gene (OchiOBP10) and two IR genes (OchiIR28 and 29) were exclusively expressed in antennae, suggesting their roles in olfaction. Real-time quantitative PCR analyses revealed that genes expressed exclusively or predominantly in antennae also displayed significant differences in expression levels between males and females. Among the differentially expressed genes, 17 OR-encoding genes, one CSP- and one SNMP-gene showed female-biased expression, suggesting that they may be involved in some female-specific behaviors such as seeking oviposition site; whereas the three remaining OR-encoding genes showed male-biased expression, indicating their possible roles in sensing female sex pheromones. Our results laid a solid foundation for future studies to reveal olfactory mechanisms as well as designing strategies for controlling this rice pest.

Molecular control limiting sensitivity of sweet taste neurons in Drosophila

To assess the biological value of environmental stimuli, animals' sensory systems must accurately decode both the identities and the intensities of these stimuli. While much is known about the mechanism by which sensory neurons detect the identities of stimuli, less is known about the mechanism that controls how sensory neurons respond appropriately to different intensities of stimuli. The ionotropic receptor IR76b has been shown to be expressed in different Drosophila chemosensory neurons for sensing a variety of chemicals. Here, we show that IR76b plays an unexpected role in lowering the sensitivity of Drosophila sweet taste neurons. First, IR76b mutants exhibited clear behavioral responses to sucrose and acetic acid (AA) at concentrations that were too low to trigger observable behavioral responses from WT animals. Second, IR76b is expressed in many sweet neurons on the labellum, and these neurons responded to both sucrose and AA. Removing IR76b from the sweet neurons increased their neuronal responses as well as animals' behavioral responses to sucrose and AA. Conversely, overexpressing IR76b in the sweet neurons decreased their neuronal as well as animals' behavioral responses to sucrose and AA. Last, IR76b's response-lowering ability has specificity: IR76b mutants and WT showed comparable responses to capsaicin when the mammalian capsaicin receptor VR1 was ectopically expressed in their sweet neurons. Our findings suggest that sensitivity of Drosophila sweet neurons to their endogenous ligands is actively limited by IR76b and uncover a potential molecular target by which contexts can modulate sensitivity of sweet neurons.

Human Odour Coding in the Yellow Fever Mosquito, Aedes aegypti

Insects use their olfactory systems to obtain chemical information on mating partners, oviposition sites and food. The yellow fever mosquito Aedes aegypti, an important vector of human infectious diseases, shows strong preference for human blood meals. This study investigated the chemical basis of host detection by characterizing the neuronal responses of antennal olfactory sensilla of female Ae. aegypti to 103 compounds from human skin emanations. The effect of blood feeding on the responses of olfactory sensilla to these odorants was examined as well. Sensilla SBTII, GP, and three functional subtypes of SST (SST1, SST2, and SST3) responded to most of the compounds tested. Olfactory receptor neurons (ORNs) ‘A’ and ‘B’ in the trichoid sensilla, either activated or inhibited, were involved in the odour coding process. Compounds from different chemical classes elicited responses with different temporal structures and different response patterns across the olfactory sensilla. Except for their increased responses to several odorants, blood-fed mosquitoes generally evoked reduced responses to specific aldehydes, alcohols, aliphatics/aromatics, ketones, and amines through the SST1, SST2, SBTI, SBTII and GP sensilla. The odorants eliciting diminished responses in female mosquitoes after blood feeding may be important in Ae. aegypti host-seeking activity and thus can be candidates for mosquito attractants in the process of this disease vector management.