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

The role of the major chemosensory organs in the host-seeking activity of Anopheles coluzzii (Diptera: Culicidae)

Anopheles coluzzii (Coetzee & Wilkerson) and its sibling species Anopheles gambiae s.s. (Giles) are highly anthropophilic and among the major malaria vectors in sub-Saharan Africa. Mosquitoes use various senses to find hosts, but rely primarily on olfaction. Therefore, the mosquito olfactory system has been studied extensively, including a variety of studies comparing chemosensory gene expression between An. coluzzii and its zoophilic sibling species Anopheles quadriannulatus (Theobald). These studies revealed species-specific chemosensory gene expression in the antennae and maxillary palps, which raised the question of a potential role for the palps in determining species-specific host preferences. To answer this question, we mechanically ablated the antennae, maxillary palps, and labella, and ran both control and ablated mosquitoes through a dual-port olfactometer. While we aimed to identify the organs responsible for vertebrate host choice, the ablated mosquitoes exclusively responded to human odor, so we were unable to do so. However, we were able to refine our understanding of the roles of these organs in host-seeking activation (leaving the release cage) as well as odor response (entering an odor port). As expected, the antennae are the most important organs to both behaviors: activation was roughly halved and vertebrate odor response was abolished in antennae-ablated mosquitoes. Maxillary palp ablation had little impact on activation, but reduced odor response to a similar degree as the exclusion of CO2. Finally, while labellar ablation dramatically reduced activation (probably associated with the inability to feed), it had little impact on odor response, suggesting that any labellar role in host choice is likely not olfactory.

Orco mediates olfactory behavior and oviposition in the whitefly Bemisia tabaci

Chemical signals play a central role in mediating insect feeding and reproductive behavior, and serve as the primary drivers of the insect-plant interactions. The detection of chemical signals, particularly host plant volatiles, relies heavily on the insect's complex olfactory system. The Bemisia tabaci cryptic species complex is a group of globally important whitefly pests of agricultural and ornamental crops that have a wide range of host plants, but the molecular mechanism of their host plant recognition is not yet clear. In this study, the odorant coreceptor gene of the Whitefly MEAM1 cryptic species (BtOrco) was cloned. The coding sequence of BtOrco was 1413 bp in length, with seven transmembrane structural domains, and it was expressed primarily in the heads of both male and female adult whiteflies, rather than in other tissues. Knockdown of BtOrco using transgenic plant-mediated RNAi technology significantly inhibited the foraging behavior of whiteflies. This inhibition was manifested as a reduced percentage of whiteflies responding to the host plant and a prolonged foraging period. Moreover, there was a substantial suppression of egg-laying activity among adult female whiteflies. These results indicate that BtOrco has the potential to be used as a target for the design of novel active compounds for the development of environmentally friendly whitefly control strategies.

Studies on the sensory sensilla on the tarsi and external genitalia of the Asian tiger mosquito, Aedes albopictus (Skuse)

Dengue fever is a worldwide health problem transmitted mainly by Aedes albopictus. The ability of female mosquitoes to transmit disease depends on their behavioral responses to locate their host. The presence of different types of sensory sensilla with different morphological characteristics is the most distinctive feature of Ae. albopictus, in addition to the main sensory organs. The sensory sensilla are chemosensory contacts that Ae. albopictus uses to locate long-distance hosts and oviposition sites by receiving well-defined cues such as changes in temperature and humidity, mechanical impact, and odors. As a result, it is crucial to study the sensory sensilla of Ae. albopictus. Although the sensory sensilla of Ae. aegypti have been extensively studied, but there is still a lack of knowledge about the sensory sensilla of the Asian tiger mosquito, Ae. albopictus. This study aimed to investigate the different types of sensory sensilla on the tarsi and external genitalia of both sexes of Ae. albopictus and their morphological features using a scanning electron microscope. The results demonstrated that the tarsi have five types of sensory sensilla: sensilla chaetica, grooved pegs, sensilla campaniform, sensilla basiconica, and sensilla coeloconica. There are two thick, grooved sensilla on the tarsal joints. Sensilla coeloconica on the tarsus are hidden under scales in a raised pit and have a diameter of 13.69 ± 0.071 μm, which is significantly larger than those on the external genitalia of males, which have a diameter of 7.65 ± 0.065 μm. Sensilla coeloconica is absent from the external genitalia of females. Two types of senillae chaetica are found on the tarsomeres, near the claws, and on the joints, which differ significantly in morphology and morphometric parameters. The ovulation-supporting cerci are provided with three pairs of long sensilla chaetica at the apex, measuring 111.36 ± 0.020 μm in length. Two groups of four apical sensilla and short spiny subapical sensilla are randomly distributed on the ovipositor. The female external genitalia have two types of sensilla chaetica, which are arranged in rows and are significantly smaller than those of the male external genitalia. RESEARCH HIGHLIGHTS: Aedes albopictus showed various sensory structures distributed on the tarsi and genitalia of both sexes. Adequate information is available in this study to understand chemoreception and odor detection by different kinds of sensilla that help to evaluate the ability of female mosquitoes to transmit diseases.

RNAi verifications on olfactory defects of an essential biocontrol agent Agasicles hygrophila (Coleoptera: Chrysomelidae) regarding mating and host allocation

Alligator weed Alternanthera philoxeroides is a perennial, worldwide pernicious weed. The beetle Agasicles hygrophila is considered to be a classical biological agent used to control A. philoxeroides. In the insect peripheral olfactory system, the odorant receptor co-receptor (ORco) plays an important function in the perception of odors in insects. However, the function of ORco in the mating and host-finding behaviors of A. hygrophila remains unclear. In this study, we characterized the odorant receptor co-receptor of A. hygrophila (AhygOrco). Real-time quantitative PCR (qRT–PCR) showed that AhygOrco was predominantly expressed in the antennae of both male and female adults, and the difference between male and female antennae was not significant. The RNA interference (RNAi) results showed that compared to the control, the injection of AhygOrco dsRNA strongly reduced the expression of AhygOrco by 90% in male beetles and 89% in female beetles. The mate-seeking and feeding behavior of AhygOrco-silenced beetles were significantly inhibited. Male adults were significantly less successful in finding a mate compared to the control group. Furthermore, host allocation abilities toward A. philoxeroides of both adults were significantly repressed. These results indicated that AhygOrco is associated with A. hygrophila feeding and mate-seeking and that inhibition of AhygOrco expression is one of the causes of reduced host and mate recognition in A. hygrophila. Meanwhile, the study provides support for exploring gene functions based on RNAi.

Analysis of the Antennal Transcriptome and Identification of Tissue-specific Expression of Olfactory-related Genes in Micromelalopha troglodyta (Lepidoptera: Notodontidae)

Micromelalopha troglodyta (Graeser) has been one of the most serious pests on poplars in China. We used Illumina HiSeq 2000 sequencing to construct an antennal transcriptome and identify olfactory-related genes. In total, 142 transcripts were identified, including 74 odorant receptors (ORs), 32 odorant-binding proteins (OBPs), 13 chemosensory proteins (CSPs), 20 ionotropic receptors (IRs), and 3 sensory neuron membrane proteins (SNMPs). The genetic relationships were obtained by the phylogenetic tree, and the tissue-specific expression of important olfactory-related genes was determined by quantitative real-time PCR (qRT-PCR). The results showed that most of these genes are abundantly expressed in the antennae and head. In most insects, olfaction plays a key role in foraging, host localization, and searching for mates. Our research lays the foundation for future research on the molecular mechanism of the olfactory system in M. troglodyta. In addition, this study provides a theoretical basis for exploring the relationship between M. troglodyta and their host plants, and for the biological control of M. troglodyta using olfactory receptor as targets.

Validation of an Optogenetic Approach to the Study of Olfactory Behavior in the T-Maze of Drosophila melanogaster Adults

Simple Summary

The fruit fly (Drosophila melanogaster) has been used as a model organism to study the olfactory system of insects thanks to the wide range of genetic tools available in this species. Among these tools, optogenetics allows the immediate alteration of the functioning of certain cells with light by the targeted expression of light receptor proteins in these cells. Thus, by successively expressing these receptors in different elements of the behavioral circuit, it is possible to evaluate their effect on the final behavior of the organism. However, the use of optogenetics to dissect the receptor elements of adult olfactory behavior presents a challenge because most odorants elicit gradual attraction or avoidance depending on their concentration, complicating the representative substitution of odor by light. In this work, we explore a dual excitation model in which the subject responds to various odorant concentrations while the olfactory receptor neurons are activated by light. The dose–response curve in these flies remains odorant concentration dependent, but with reduced sensitivity compared to olfactory stimulation alone. The existence of an effect associated with each of the two stimuli, odor and light, allows us to explore the quantitative contribution of the receptor elements to olfactory behavior also by optogenetics.

Abstract

Optogenetics enables the alteration of neural activity using genetically targeted expression of light activated proteins for studying behavioral circuits in several species including Drosophila. The main idea behind this approach is to replace the native behavioral stimulus by the light-induced electrical activation of different points of the circuit. Therefore, its effects on subsequent steps of the circuit or on the final behavior can be analyzed. However, the use of optogenetics to dissect the receptor elements of the adult olfactory behavior presents a challenge due to one additional factor: Most odorants elicit attraction or avoidance depending on their concentration; this complicates the representative replacement of odor activation of olfactory sensory neurons (OSNs) by light. Here, we explore a dual excitation model where the subject is responding to odors while the OSNs are optogenetically activated. Thereby, we can assess if and how the olfactory behavior is modified. We measure the effects of light excitation on the response to several odorant concentrations. The dose-response curve of these flies still depends on odor concentration but with reduced sensitivity compared to olfactory stimulation alone. These results are consistent with behavioral tests performed with a background odor and suggest an additive effect of light and odor excitation on OSNs.

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.

A small number of male-biased candidate pheromone receptors are expressed in large subsets of the olfactory sensory neurons in the antennae of drones from the European honey bee Apis mellifera

In the European honey bee (Apis mellifera), the olfactory system is essential for foraging and intraspecific communication via pheromones. Honey bees are equipped with a large repertoire of olfactory receptors belonging to the insect odorant receptor (OR) family. Previous studies have indicated that the transcription level of a few OR types including OR11, a receptor activated by the queen-released pheromone compound (2E)-9-oxodecenoic acid (9-ODA), is significantly higher in the antenna of males (drones) than in female workers. However, the number and distribution of antennal cells expressing male-biased ORs is elusive. Here, we analyzed antennal sections from bees by in situ hybridization for the expression of the male-biased receptors OR11, OR18, and OR170. Our results demonstrate that these receptors are expressed in only moderate numbers of cells in the antennae of females (workers and queens), whereas substantially higher cell numbers express these ORs in drones. Thus, the reported male-biased transcript levels are due to sex-specific differences in the number of antennal cells expressing these receptors. Detailed analyses for OR11 and OR18 in drone antennae revealed expression in two distinct subsets of olfactory sensory neurons (OSNs) that in total account for approximately 69% of the OR-positive cells. Such high percentages of OSNs expressing given receptors are reminiscent of male-biased ORs in moths that mediate the detection of female-released sex pheromone components. Collectively, our findings indicate remarkable similarities between male antennae of bees and moths and support the concept that male-biased ORs in bee drones serve the detection of female-emitted sex pheromones.

Color Preference for Host-Seeking Activity of Aedes albopictus and Culex pipiens (Diptera: Culicidae)

A cue for long-range vision allows mosquitoes to identify hosts and differentiate the ecological niches (e.g., habitats). However, the visual factors involved in attracting mosquitoes to a host are complex and have not been fully understood. Therefore, we assessed color preference to Aedes albopictus (Skuse) and Culex pipiens (Conquillett) as diurnal and nocturnal species, respectively, using seven fundamental colors including black, white, red, yellow, green, blue, and purple with each trap at 100 lux in a laboratory. We used a binary behavioral assay using the Mosquito Preference Index (MPI) as a preference ratio with a range of 0-1. Our analyses showed that Ae. albopictus had a greater response to black (MPIs, 0.7), followed closely by red, blue, and purple (MPIs, 0.6). We also found that red, blue, and purple were significantly higher (P < 0.05) than those of green (MPI, 0.5), white (MPI, 0.3), and yellow (MPI, 0.2). Similarly, the MPIs for Cx. pipiens were significantly higher at black and red (MPIs, 0.7; P < 0.05) compare to those of white and yellow (MPIs, 0.3; P < 0.05). The color preference of Ae. albopictus showed significant correlation to luminous intensities (L-value) (r = -0.640; P = 0.000) and blue intensities (b-value) (r = -0.372; P = 0.000) for all seven colors. In addition, Cx. pipiens negatively correlated (r = -0.703; P = 0.000) between color preference and L-value. Our analyses provide a greater understanding of how color plays a role in visual sensory stimuli, and how that could potentially affect mosquito host-seeking behavior.

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.

The Expression of Chemosensory Genes in Male Maxillary Palps of Anopheles coluzzii (Diptera: Culicidae) and An. quadriannulatus

Because of its importance as a malaria vector, Anopheles coluzzii's Coetzee & Wilkerson olfactory system has been studied extensively. Among this work is a series of studies comparing the expression of chemosensory genes in olfactory organs in females and/or males of these species. These have identified species- and female-biased chemosensory gene expression patterns. However, many questions remain about the role of chemosensation in male anopheline biology. To pave the way for future work we used RNAseq to compare chemosensory gene expression in the male maxillary palps of An. coluzzii and its sibling species An. quadriannulatus Theobald. As expected, the chemosensory gene repertoire is small in the male maxillary palps. Both species express the tuning receptors Or8 and Or28 at relatively high levels. The CO2 receptor genes Gr22-Gr24 are present in both species as well, although at much lower level than in females. Additionally, several chemoreceptors are species-specific. Gr37 and Gr52 are exclusive to An. coluzzii, whereas Or9 and Gr60 were detected only in An. quadriannulatus. Furthermore, several chemosensory genes show differential expression between the two species. Finally, several Irs, Grs, and Obps that show strong differential expression in the female palps, are absent or lowly expressed in the male palps. While many questions remain about the role of chemosensation in anopheline male biology, these results suggest that the male maxillary palps could have both a sex- and species-specific role in the perception of chemical stimuli. This work may guide future studies on the role of the male maxillary palp in these species.

Olfaction in Anopheles mosquitoes

As vectors of disease, mosquitoes are a global threat to human health. The Anopheles mosquito is the deadliest mosquito species as the insect vector of the malaria-causing parasite, which kills hundreds of thousands every year. These mosquitoes are reliant on their sense of smell (olfaction) to guide most of their behaviors, and a better understanding of Anopheles olfaction identifies opportunities for reducing the spread of malaria. This review takes a detailed look at Anopheles olfaction. We explore a range of topics from chemosensory receptors, olfactory neurons, and sensory appendages to behaviors guided by olfaction (including host-seeking, foraging, oviposition, and mating), to vector management strategies that target mosquito olfaction. We identify many research areas that remain to be addressed.

Altered functional properties of the codling moth Orco mutagenized in the intracellular loop-3

Amino acid substitutions within the conserved polypeptide sequence of the insect olfactory receptor co-receptor (Orco) have been demonstrated to influence its pharmacological properties. By sequence analysis and phylogenetic investigation, in the Lepidopteran subgroup Ditrysia we identified a fixed substitution in the intracellular loop-3 (ICL-3) of a conserved histidine to glutamine. By means of HEK293 cells as a heterologous system, we functionally expressed Orco from the Ditrysian model Cydia pomonella (CpomOrco) and compared its functional properties with a site-directed mutagenized version where this ICL-3-glutamine was reverted to histidine (CpomOrco^Q417H). The mutagenized CpomOrco^Q417H displayed decreased responsiveness to VUAA1 and reduced response efficacy to an odorant agonist was observed, when co-transfected with the respective OR subunit. Evidence of reduced responsiveness and sensitivity to ligands for the mutagenized Orco suggest the fixed glutamine substitution to be optimized for functionality of the cation channel within Ditrysia. In addition, contrary to the wild type, the mutagenized CpomOrco^Q417H preserved characteristics of VUAA-binding when physiologic conditions turned to acidic. Taken together, our findings provide further evidence of the importance of ICL-3 in forming basic functional properties of insect Orco- and Orco/OR-channels, and suggest involvement of ICL-3 in the potential functional adaptation of Ditrysian Orcos to acidified extra-/intracellular environment.

Olfactory systems across mosquito species

There are 3559 species of mosquitoes in the world (Harbach 2018) but, so far, only a handful of them have been a focus of olfactory neuroscience and neurobiology research. Here we discuss mosquito olfactory anatomy and function and connect these to mosquito ecology. We highlight the least well-known and thus most interesting aspects of mosquito olfactory systems and discuss promising future directions. We hope this review will encourage the insect neuroscience community to work more broadly across mosquito species instead of focusing narrowly on the main disease vectors.

Silencing Orco Impaired the Ability to Perceive Trail Pheromones and Affected Locomotion Behavior in Two Termite Species

Sophisticated social behaviors in termite colonies are mainly regulated via chemical communication of a wide range of pheromones. Trail pheromones play important roles in foraging behavior and building tunnels and nests in termites. However, it is almost unclear how termites perceive trail pheromones. Here, we cloned and sequenced of olfactory co-receptor (Orco) genes from the two termites Reticulitermes chinensis Snyder (Isoptera: Rhinotermitidae) and Odontotermes formosanus (Shiraki) (Isoptera: Termitidae), and then examined their responses to trail pheromones after silencing Orco through RNA interference (RNAi). We found that Orco knockdown impaired their ability to perceive trail pheromones and resulted in the disability of following pheromone trails in the two termite species. Our locomotion behavior assays further showed that Orco knockdown significantly decreased the distance and velocity in the two termite species, but significantly increased the angular velocity and turn angle in the termite R. chinensis. These findings strongly demonstrated that Orco is essential for termites to perceive their trail pheromones, which provides a potential way to control termite pests by damaging olfactory system.

Mutagenesis of the orco odorant receptor co-receptor impairs olfactory function in the malaria vector Anopheles coluzzii

Mosquitoes rely heavily on their olfactory systems for host seeking, selection of oviposition sites, and avoiding predators and other environmental dangers. Of these behaviors, the preferential selection of a human blood-meal host drives the vectorial capacity of anthropophilic female Anopheles coluzzii mosquitoes. Olfactory receptor neurons (ORNs) are dispersed across several appendages on the head and express an obligate odorant receptor co-receptor (Orco) coupled with a "tuning" odorant receptor (OR) to form heteromeric, odor-gated ion channels in the membrane of these neurons. To examine the mechanistic and functional contributions of Orco/OR complexes to the chemosensory processes of An. coluzzii, we utilized CRISPR/Cas9 gene editing to create a line of homozygous, Orco-knockout, mutant mosquitoes. As expected, orco-/- ORNs across both adult and larval stages of An. coluzzii display significantly lower background activity and lack nearly all odor-evoked responses. In addition, blood-meal-seeking, adult female, orco-/- mutant mosquitoes exhibit severely reduced attraction to human- and non-human-derived odors while gravid females are significantly less responsive to established oviposition attractants. These results reinforce observations in other insects that Orco is crucial in maintaining the activity of ORNs. In that light, it significantly influences a range of olfactory-driven behaviors central to the anthropophilic host preference that is critical to the vectorial capacity of An. coluzzii as a primary vector for human malaria.

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.

Molecular Profiling of the Drosophila Antenna Reveals Conserved Genes Underlying Olfaction in Insects

Repellent odors are widely used to prevent insect-borne diseases, making it imperative to identify the conserved molecular underpinnings of their olfactory systems. Currently, little is known about the molecules supporting odor signaling beyond the odor receptors themselves. Most known molecules function in one of two classes of olfactory sensilla, single-walled or double-walled, which have differing morphology and odor response profiles. Here, we took two approaches to discover novel genes that contribute to insect olfaction in the periphery. We transcriptionally profiled Drosophila melanogasteramos mutants that lack trichoid and basiconic sensilla, the single-walled sensilla in this species. This revealed 187 genes whose expression is enriched in these sensilla, including pickpocket ion channels and neuromodulator GPCRs that could mediate signaling pathways unique to single-walled sensilla. For our second approach, we computationally identified 141 antennal-enriched (AE) genes that are more than ten times as abundant in D. melanogaster antennae as in other tissues or whole-body extracts, and are thus likely to play a role in olfaction. We identified unambiguous orthologs of AE genes in the genomes of four distantly related insect species, and most identified orthologs were expressed in the antenna of these species. Further analysis revealed that nearly half of the 141 AE genes are localized specifically to either single or double-walled sensilla. Functional annotation suggests the AE genes include signaling molecules and enzymes that could be involved in odorant degradation. Together, these two resources provide a foundation for future studies investigating conserved mechanisms of odor signaling.

DEET and other repellents are inhibitors of mosquito odorant receptors for oviposition attractants

In addition to its primary function as an insect repellent, DEET has many "off-label" properties, including a deterrent effect on the attraction of gravid female mosquitoes. DEET negatively affects oviposition sites. While deorphanizing odorant receptors (ORs) using the Xenopus oocyte recording system, we have previously observed that DEET generated outward (inhibitory) currents on ORs sensitive to oviposition attractants. Here, we systematically investigated these inhibitory currents. We recorded dose-dependent outward currents elicited by DEET and other repellents on ORs from Culex quinquefasciatus, Aedes aegypti, and Anopheles gambiae. Similar responses were observed with other plant-derived and plant-inspired compounds, including methyl jasmonate and methyl dihydrojasmolate. Inward (regular) currents elicited by skatole upon activation of CquiOR21 were modulated when this oviposition attractant was coapplied with a repellent. Compounds that generate outward currents in ORs sensitive to oviposition attractants elicited inward currents in a DEET-sensitive receptor, CquiOR136. The best ligand for this receptor, methyl dihydrojasmolate, showed repellency activity but was not as strong as DEET in our test protocol.

Chemosensory Gene Families in the Oligophagous Pear Pest Cacopsylla chinensis (Hemiptera: Psyllidae)

Chemosensory systems play an important role in insect behavior, and some key associated genes have potential as novel targets for pest control. Cacopsylla chinensis is an oligophagous pest and has become one of the main pests of pear trees, but little is known about the molecular-level means by which it locates its hosts. In this study, we assembled the head transcriptome of C. chinensis using Illumina sequencing, and 63,052 Unigenes were identified. A total of 36 candidate chemosensory genes were identified, including five different families: 12 odorant binding proteins (OBPs), 11 chemosensory proteins (CSPs), 7 odorant receptors (ORs), 4 ionotropic receptors (IRs), and 2 gustatory receptors (GRs). The number of chemosensory gene families is consistent with that found in other Hemipteran species, indicating that our approach successfully obtained the chemosensory genes of C. chinensis. The tissue expression of all genes using quantitative real-time PCR (qRT-PCR) found that some genes displayed male head, female head, or nymph-biased specific/expression. Our results enrich the gene inventory of C. chinensis and provide valuable resources for the analysis of the functions of some key genes. This will help in developing molecular targets for disrupting feeding behavior in C. chinensis.

Use of machine learning to identify novel, behaviorally active antagonists of the insect odorant receptor co-receptor (Orco) subunit

Olfaction is a key component of the multimodal approach used by mosquitoes to target and feed on humans, spreading various diseases. Current repellents have drawbacks, necessitating development of more effective agents. In addition to variable odorant specificity subunits, all insect odorant receptors (ORs) contain a conserved odorant receptor co-receptor (Orco) subunit which is an attractive target for repellent development. Orco directed antagonists allosterically inhibit odorant activation of ORs and we previously showed that an airborne Orco antagonist could inhibit insect olfactory behavior. Here, we identify novel, volatile Orco antagonists. We functionally screened 83 structurally diverse compounds against Orco from Anopheles gambiae. Results were used for training machine learning models to rank probable activity of a library of 1280 odorant molecules. Functional testing of a representative subset of predicted active compounds revealed enrichment for Orco antagonists, many structurally distinct from previously known Orco antagonists. Novel Orco antagonist 2-tert-butyl-6-methylphenol (BMP) inhibited odorant responses in electroantennogram and single sensillum recordings in adult Drosophila melanogaster and inhibited OR-mediated olfactory behavior in D. melanogaster larvae. Structure-activity analysis of BMP analogs identified compounds with improved potency. Our results provide a new approach to the discovery of behaviorally active Orco antagonists for eventual use as insect repellents/confusants.

Insect Pheromone Receptors - Key Elements in Sensing Intraspecific Chemical Signals

Pheromones are chemicals that serve intraspecific communication. In animals, the ability to detect and discriminate pheromones in a complex chemical environment substantially contributes to the survival of the species. Insects widely use pheromones to attract mating partners, to alarm conspecifics or to mark paths to rich food sources. The various functional roles of pheromones for insects are reflected by the chemical diversity of pheromonal compounds. The precise detection of the relevant intraspecific signals is accomplished by specialized chemosensory neurons housed in hair-like sensilla located on the surface of body appendages. Current data indicate that the extraordinary sensitivity and selectivity of the pheromone-responsive neurons (PRNs) is largely based on specific pheromone receptors (PRs) residing in their ciliary membrane. Besides these key elements, proper ligand-induced responses of PR-expressing neurons appear to generally require a putative co-receptor, the so-called "sensory neuron membrane protein 1" (SNMP1). Regarding the PR-mediated chemo-electrical signal transduction processes in insect PRNs, ionotropic as well as metabotropic mechanisms may be involved. In this review, we summarize and discuss current knowledge on the peripheral detection of pheromones in the olfactory system of insects with a focus on PRs and their specific role in the recognition and transduction of volatile intraspecific chemical signals.

Candidate olfactory genes identified in Heortia vitessoides (Lepidoptera: Crambidae) by antennal transcriptome analysis

Heortia vitessoides Moore is the most severe defoliating pest of Aquilaria sinensis (Lour.) Gilg (Thymelaeaceae) forests. Olfaction in insects is essential for host identification, mating, and oviposition, in which olfactory proteins, including odorant-binding proteins (OBPs), chemosensory proteins (CSPs), olfactory receptors (ORs), ionotropic receptors (IRs), and sensory neuron membrane proteins (SNMPs), are responsible for chemical signaling. Here, we determined the transcriptomes of male and female adult antennae of H. vitessoides. We assembled 52,383 unigenes and annotated their putative gene functions based on the gene ontology (GO), eukaryotic ortholog groups (KOG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. Overall, 61 olfactory-related transcripts, including nine OBPs, 10 CSPs, 28 ORs, 12 IRs, and two SNMPs, were identified. Expression patterns of OBPs and CSPs in the female antennae, male antennae, and legs were performed using reverse transcription quantitative PCR (RT-qPCR). The results revealed that HvitOBP1, HvitOBP6, and HvitGOBP1 were enriched in the female antennae, while HvitOBP2, HvitOBP3, HvitOBP5, HvitGOBP2, and HvitPBP1 were enriched in the male antennae. HvitOBP4 was expressed at nearly the same level in the antennae of both males and females. Four CSPs (HvitCSP3, HvitCSP5, HvitCSP7, and HvitCSP10) and two CSPs (HvitCSP1 and HvitCSP4) were expressed at higher levels in the female and male antennae, respectively. HvitCSP6 was expressed at higher levels both in the female antennae and legs. Three CSP genes (HvitCSP2, HvitCSP8, and HvitCSP9) were expressed at higher levels in the legs. These results provide a basis for further studies on the molecular olfactory mechanisms of H. vitessoides.

Evolutionary Analysis of the Highly Conserved Insect Odorant Coreceptor (Orco) Revealed a Positive Selection Mode, Implying Functional Flexibility

Odorant coreceptor (Orco) represents one of the essential genes in the insect olfactory system, which facilitates signal transduction and heterodimerization with different odorant receptors (Ors) in the insect antennal dendritic membrane. Evolutionary analysis by detecting positive selection is important to examine the functional flexibility of Orco that potentially supports insect survival. The maximum likelihood codon substitution model was applied using CODEML program as implemented in PAML ver 4.9e package across 59 Orco codon sequences available from GenBank. These sequences represented five major insect orders and two reproductive systems (holometabola and nonholometabola). In the site model that identified common ω values for Orco, it was clearly shown that Orco was under strong purifying selection, indicated by the ω value that was far from 1 (ω: 0.03). However, in to the branch model, positive selection was detected to be acting on Dipteran lineages, whereas in the branch-site model, several sites were under significant positive selection occurring in the following four clades: Coleoptera, Diptera, Lepidoptera, and Psocodea. The typical evolutionary mode acting on Orco was consistent with the entropy value [H(x)], confirming that 48.9% of the Orco site was under conservation (H(x) < 0.5), whereas 26.9% of the Orco sites was under high variation (H(x) > 1). These findings confirmed that Orco genes are generally highly conserved and can possibly be used for the manipulation of insect pest control programs. However, positive selection that acts on certain lineages suggested future adaptive evolutionary ability of Orco to anticipate flexible functions for successful olfactory processes.

Membrane Proteins Mediating Reception and Transduction in Chemosensory Neurons in Mosquitoes

Mosquitoes use chemical cues to modulate important behaviors such as feeding, mating, and egg laying. The primary chemosensory organs comprising the paired antennae, maxillary palps and labial palps are adorned with porous sensilla that house primary sensory neurons. Dendrites of these neurons provide an interface between the chemical environment and higher order neuronal processing. Diverse proteins located on outer membranes interact with chemicals, ions, and soluble proteins outside the cell and within the lumen of sensilla. Here, we review the repertoire of chemosensory receptors and other membrane proteins involved in transduction and discuss the outlook for their functional characterization. We also provide a brief overview of select ion channels, their role in mammalian taste, and potential involvement in mosquito taste. These chemosensory proteins represent targets for the disruption of harmful biting behavior and disease transmission by mosquito vectors.

The Two Main Olfactory Receptor Families in Drosophila, ORs and IRs: A Comparative Approach

Most insect species rely on the detection of olfactory cues for critical behaviors for the survival of the species, e.g., finding food, suitable mates and appropriate egg-laying sites. Although insects show a diverse array of molecular receptors dedicated to the detection of sensory cues, two main types of molecular receptors have been described as responsible for olfactory reception in Drosophila, the odorant receptors (ORs) and the ionotropic receptors (IRs). Although both receptor families share the role of being the first chemosensors in the insect olfactory system, they show distinct evolutionary origins and several distinct structural and functional characteristics. While ORs are seven-transmembrane-domain receptor proteins, IRs are related to the ionotropic glutamate receptor (iGluR) family. Both types of receptors are expressed on the olfactory sensory neurons (OSNs) of the main olfactory organ, the antenna, but they are housed in different types of sensilla, IRs in coeloconic sensilla and ORs in basiconic and trichoid sensilla. More importantly, from the functional point of view, they display different odorant specificity profiles. Research advances in the last decade have improved our understanding of the molecular basis, evolution and functional roles of these two families, but there are still controversies and unsolved key questions that remain to be answered. Here, we present an updated review on the advances of the genetic basis, evolution, structure, functional response and regulation of both types of chemosensory receptors. We use a comparative approach to highlight the similarities and differences among them. Moreover, we will discuss major open questions in the field of olfactory reception in insects. A comprehensive analysis of the structural and functional convergence and divergence of both types of receptors will help in elucidating the molecular basis of the function and regulation of chemoreception in insects.

Host-location behavior of the tea green leafhopper Empoasca vitis Göthe (Hemiptera: Cicadellidae): olfactory and visual effects on their orientation

The tea green leafhopper, Empoasca vitis Göthe, is one of the most serious pests in tea growing areas. This study investigated the roles played by olfaction and vision in host orientation behavior. The compound eye of E. vitis was found to be a photopic eye; few olfactory sensilla were found on the antennae, while abundant gustatory sensilla were recorded on the mouthparts. Three opsin genes (EV_LWop, EV_UVop, EV_Bop) were isolated and found to be mainly expressed in the compound eye compared with other parts of the body. Immunolocalization indicated that the opsins mainly located in the different regions of rhabdom. The transcription levels of EV_LWop, EV_Bop and EV_UVop were reduced by 77.3, 70.0 and 40.0%, respectively, by RNA interference induced by being fed a special RNA-rich diet for 6 days. The rate of tropism to host color was effectively impaired by 67.6 and 29.5% in the dsEV_LWop and dsEV_Bop treatment groups, but there was no significant change in the dsEV_UVop group. The determination of the cause of the tropism indicated that odors from the host over long distances were unable to attract E. vitis and were only detected when the insects were close to the host. The developed compound eye of E. vitis plays a leading role in host location, and the long-wavelength opsin significantly affects the tropism to host color; the lack of olfactory sensilla results in long-distance odors not being able to be detected until the insect is near to the host-plant. The understanding of these behavioral mechanisms, especially the importance of opsin genes is expected to be useful for pest management.

Characterization of Chemosensory Responses on the Labellum of the Malaria Vector Mosquito, Anopheles coluzzii

Anopheles gambiae coluzzii (An. coluzzii) uses olfaction to modulate a range of critical behaviors that are essential for survival and reproduction; most notably, host preference and selection underlie its vectorial capacity for human malaria. As is the case for all mosquitoes, An. coluzzii has three specialized peripheral olfactory appendages—the antennae, maxillary palps and labella—which are used to detect and orient in response to a large variety of olfactory cues. Of these, neither the molecular nor the physiological significance of the labellum have been thoroughly characterized despite suggestions that labial-derived odorant reception is critical for close-range host attraction. Here we report global chemoreceptor transcriptome profiles together with a systematic electrophysiological analysis of labial T2 sensilla, and associated behavioral responses of female An. coluzzii. Single sensillum recordings of the T2 sensilla revealed robust responses to odorants previously associated with human sweat and oviposition sites and identified a 10-component blend that elicited attraction in a dual-choice landing bioassay designed to mimic host seeking in which non-blood fed females were significantly more attracted to the labial-responsive odorant blend as compared to gravid females. Taken together, these data suggest that, in An. coluzzii, olfactory responses derived from the labellum contribute to host-seeking.

Access to the odor world: olfactory receptors and their role for signal transduction in insects

The sense of smell enables insects to recognize and discriminate a broad range of volatile chemicals in their environment originating from prey, host plants and conspecifics. These olfactory cues are received by olfactory sensory neurons (OSNs) that relay information about food sources, oviposition sites and mates to the brain and thus elicit distinct odor-evoked behaviors. Research over the last decades has greatly advanced our knowledge concerning the molecular basis underlying the reception of odorous compounds and the mechanisms of signal transduction in OSNs. The emerging picture clearly indicates that OSNs of insects recognize odorants and pheromones by means of ligand-binding membrane proteins encoded by large and diverse families of receptor genes. In contrast, the mechanisms of the chemo-electrical transduction process are not fully understood; the present status suggests a contribution of ionotropic as well as metabotropic mechanisms. In this review, we will summarize current knowledge on the peripheral mechanisms of odor sensing in insects focusing on olfactory receptors and their specific role in the recognition and transduction of odorant and pheromone signals by OSNs.

Mutant cycle analysis identifies a ligand interaction site in an odorant receptor of the malaria vector Anopheles gambiae

Lack of information about the structure of insect odorant receptors (ORs) hinders the development of more effective repellants to control disease-transmitting insects. Mutagenesis and functional analyses using agonists to map the odorant-binding sites of these receptors have been limited because mutations distant from an agonist-binding site can alter agonist sensitivity. Here we use mutant cycle analysis, an approach for exploring the energetics of protein-protein or protein-ligand interactions, with inhibitors, to identify a component of the odorant-binding site of an OR from the malaria vector, Anopheles gambiae The closely related odorant-specificity subunits Agam/Or15 and Agam/Or13 were each co-expressed with Agam/Orco (odorant receptor co-receptor subunit) in Xenopus oocytes and assayed by two-electrode voltage clamp electrophysiology. We identified (-)-fenchone as a competitive inhibitor with different potencies at the two receptors and used this difference to screen a panel of 37 Agam/Or15 mutants, surveying all positions that differ between Agam/Or15 and Agam/Or13 in the transmembrane and extracellular regions, identifying position 195 as a determinant of (-)-fenchone sensitivity. Inhibition by (-)-fenchone and six structurally related inhibitors of Agam/Or15 receptors containing each of four different hydrophobic residues at position 195 served as input data for mutant cycle analysis. Several mutant cycles, calculated from the inhibition of two receptors by each of two ligands, yielded coupling energies of ≥1 kcal/mol, indicating a close, physical interaction between the ligand and residue 195 of Agam/Or15. This approach should be useful in further expanding our knowledge of odorant-binding site structures in ORs of disease vector insects.

Chemosensory gene expression in olfactory organs of the anthropophilic Anopheles coluzzii and zoophilic Anopheles quadriannulatus

BACKGROUND

Anopheles (An.) coluzzii, one of Africa's primary malaria vectors, is highly anthropophilic. This human host preference contributes greatly to its ability to transmit malaria. In contrast, the closely related An. quadriannulatus prefers to feed on bovids and is not thought to contribute to malaria transmission. The diverged preference for host odor profiles between these sibling species is likely reflected in chemosensory gene expression levels in the olfactory organs. Therefore, we compared the transcriptomes of the antennae and maxillary palps between An. coluzzii and An. quadriannulatus, focusing on the major chemosensory gene families.

RESULTS

While chemosensory gene expression is strongly correlated between the two species, various chemosensory genes show significantly enhanced expression in one of the species. In the antennae of An. coluzzii the expression of six olfactory receptors (Ors) and seven ionotropic receptors (Irs) is considerably enhanced, whereas 11 Ors and 3 Irs are upregulated in An. quadriannulatus. In the maxillary palps, leaving aside Irs with very low level of expression, one Ir is strongly enhanced in each species. In addition, we find divergence in odorant binding protein (Obp) gene expression, with several highly expressed Obps being enhanced in the antennae and palps of An. coluzzii. Finally, the expression of several gustatory receptors (Grs) in the palps appears to be species-specific, including a homolog of a sugar-sensing Drosophila Gr.

CONCLUSIONS

A considerable number of Ors and Irs are differentially expressed between these two closely related species with diverging host preference. These chemosensory genes could play a role in the human host preference of the malaria vector An. coluzzii. Additionally, divergence in Obp expression between the two species suggests a possible role of these odor carrier proteins in determining host preference. Finally, divergence in chemosensory expression in the palps may point towards a possible role for the maxillary palps in host differentiation.

Olfactory Ionotropic Receptors in Mosquito Aedes albopictus (Diptera: Culicidae)

Ionotropic glutamate receptors (iGluRs) are a conserved family of ligand-gated ion channels that primarily function to mediate neuronal communication at synapses. A variant subfamily of iGluRs, the ionotropic receptors (IRs), was recently identified in insects and proved with the function in odorant recognition. Ionotropic receptors participate in a distinct olfactory signaling pathway that is independent of olfactory receptors activity. In the present study, we identify 102 putative IR genes, dubbed as AalbIr genes, in mosquito Aedes albopictus (Skuse) by in silico comparative sequence analysis. Among AalbIr genes, 19 show expression in the female antenna by RT-PCR. These putative olfactory AalbIRs share four conservative hydrophobic domains of amino acids, similar to the transmembrane and ion channel pore regions found in conventional iGluRs. To determine the potential function of these olfactory AalbIRs in host-seeking, we compared their transcript expression levels in the antennae of blood-fed females with that of non-blood-fed females by quantitative real-time RT-PCR. Three AalbIr genes showed downregulation when the mosquito finished a bloodmeal. These results may help to improve our understanding of the IR-mediated olfactory signaling in mosquitoes.

orco Mutagenesis Causes Loss of Antennal Lobe Glomeruli and Impaired Social Behavior in Ants

Life inside ant colonies is orchestrated with diverse pheromones, but it is not clear how ants perceive these social signals. It has been proposed that pheromone perception in ants evolved via expansions in the numbers of odorant receptors (ORs) and antennal lobe glomeruli. Here, we generate the first mutant lines in the clonal raider ant, Ooceraea biroi, by disrupting orco, a gene required for the function of all ORs. We find that orco mutants exhibit severe deficiencies in social behavior and fitness, suggesting they are unable to perceive pheromones. Surprisingly, unlike in Drosophila melanogaster, orco mutant ants also lack most of the ∼500 antennal lobe glomeruli found in wild-type ants. These results illustrate that ORs are essential for ant social organization and raise the possibility that, similar to mammals, receptor function is required for the development and/or maintenance of the highly complex olfactory processing areas in the ant brain. VIDEO ABSTRACT.

Profiles of soluble proteins in chemosensory organs of three members of the afro-tropical Anopheles gambiae complex

In female mosquitoes, host-seeking and preference as well as several other important behaviors are largely driven by olfaction. Species of the Afrotropical Anopheles gambiae complex display divergent host-preference that are associated with significant differences in their vectorial capacity for human malaria. Olfactory sensitivity begins with signal transduction and activation of peripheral sensory neurons that populate the antennae, maxillary palps and other appendages. We have used shotgun proteomics to characterize the profile of soluble proteins of antennae and maxillary palps of three different species: An. coluzzii, An. arabiensis and An. quadriannulatus that display remarkable differences in anthropophilic behavior. This analysis revealed interspecific differences in the abundance of several proteins that comprise cuticular components, glutathione S-transferase and odorant binding proteins, the latter of which known to be directly involved in odor recognition.

Molecular and functional characterization of a candidate sex pheromone receptor OR1 in Spodoptera litura

Olfaction is primarily mediated by highly specified olfactory receptors (ORs). Here, we cloned and identified an olfactory receptor, named SlituOR1 (Genbank no. JN835269), from Spodoptera litura and found evidence that it is a candidate pheromone receptor. It exhibited male-biased expression in the antennae, where it was localized at the base of sensilla trichoidea, the antennal sensilla mainly responsive to pheromones in moths. Conserved orthologues of this receptor, found among known pheromone receptors within the Lepidoptera, and SlituOR1 were placed among a clade of candidate pheromone receptors in a phylogeny tree of insect OR gene sequences. SlituOR1 showed differential expression in S. litura populations attracted to traps baited with different ratios of the two sex pheromone components (9Z,11E)-tetradecadienyl acetate (Z9E11-14:OAc) and (9Z,12E)-tetradecadienyl acetate (Z9E12-14:OAc). Knocking down of SlituOR1 by RNA interference reduced the electroantennogram (EAG) response to Z9E11-14:OAc, and this result is consistent with the field trapping experiment. We infer that variation in transcription levels of olfactory receptors may modulate sex pheromone perception in male moths and could provide some of the flexibility required to maintain the functionality of communication with females when a population is adapting to a new niche and reproductive isolation becomes an advantage.

Functional and Nonfunctional Forms of CquiOR91, an Odorant Selectivity Subunit of Culex quinquefasciatus

In Culex quinquefasciatus, CquiOR91 is the ortholog of 2 larvae-specific odorant receptors (ORs) from Anopheles gambiae (Agam\Or40, previously shown to respond to several odorant ligands including the broad-spectrum repellent N,N-diethyl-3-methylbenzamide, DEET) and Aedes aegypti (Aaeg\Or40). When we cloned full-length CquiOR91 from a Culex quinquefasciatus larval head RNA sample, we found 2 alleles of this OR, differing at 9 residues. Functional analysis using the Xenopus oocyte expression system and 2-electrode voltage clamp electrophysiology revealed one allele (CquiOR91.1) to be nonfunctional, whereas the other allele (CquiOR91.2) was functional. Receptors formed by CquiOR91.2 and Cqui\Orco responded to (-)-fenchone, (+)-fenchone, and DEET, similar to what has been reported for Agam\Or40. We also identified 5 novel odorant ligands for the CquiOR91.2 + Cqui\Orco receptor: 2-isobutylthiazole, veratrole, eucalyptol, d-camphor, and safranal, with safranal being the most potent. To explore possible reasons for the lack of function for CquiOR91.1, we generated a series of mutant CquiOR91.2 subunits, in which the residue at each of the 9 polymorphic residue positions was changed from what occurs in CquiOR91.2 to what occurs in CquiOR91.1. Eight of the 9 mutant versions of CquiOR91.2 formed functional receptors, responding to (-)-fenchone. Only the CquiOR91.2 Y183C mutant was nonfunctional. The reverse mutation (C183Y) conferred function on CquiOR91.1 , which became responsive to (-)-fenchone and safranal. These results indicate that the "defect" in CquiOR91.1 that prevents function is the cysteine at position 183.

Inhibition of insect olfactory behavior by an airborne antagonist of the insect odorant receptor co-receptor subunit

Response to volatile environmental chemosensory cues is essential for insect survival. The odorant receptor (OR) family is an important class of receptors that detects volatile molecules; guiding insects towards food, mates, and oviposition sites. ORs are odorant-gated ion channels, consisting of a variable odorant specificity subunit and a conserved odorant receptor co-receptor (Orco) subunit, in an unknown stoichiometry. The Orco subunit possesses an allosteric site to which modulators can bind and noncompetitively inhibit odorant activation of ORs. In this study, we characterized several halogen-substituted versions of a phenylthiophenecarboxamide Orco antagonist structure. Orco antagonist activity was assessed on ORs from Drosophila melanogaster flies and Culex quinquefasciatus mosquitoes, expressed in Xenopus laevis oocytes and assayed by two-electrode voltage clamp electrophysiology. One compound, OX1w, was also shown to inhibit odorant activation of a panel of Anopheles gambiae mosquito ORs activated by diverse odorants. Next, we asked whether Orco antagonist OX1w could affect insect olfactory behavior. A Drosophila melanogaster larval chemotaxis assay was utilized to address this question. Larvae were robustly attracted to highly diluted ethyl acetate in a closed experimental chamber. Attraction to ethyl acetate was Orco dependent and also required the odorant specificity subunit Or42b. The addition of the airborne Orco antagonist OX1w to the experimental chamber abolished larval chemotaxis towards ethyl acetate. The Orco antagonist was not a general inhibitor of sensory behavior, as behavioral repulsion from a light source was unaffected. This is the first demonstration that an airborne Orco antagonist can alter olfactory behavior in an insect. These results suggest a new approach to insect control and emphasize the need to develop more potent Orco antagonists.

The neurobiology of gustation in insect disease vectors: progress and potential

For insect vectors of human diseases, mealtimes are a key moment of infection. Understanding how and when such species decide on what to feed is both an interesting problem in sensory neurobiology and a source of information for intervention of these behaviors to control spread of infectious agents. Here I review the current knowledge of the molecular and cellular mechanisms of gustation in insect disease vectors, covering blood-feeders as well as scavengers that spread pathogens indirectly. I also consider how these behaviors are modulated over short and long timescales, and describe efforts to artificially modulate them. Though a relatively nascent field, gustatory neurobiology in insect vectors has much promise for future fundamental discoveries and practical applications.

Variant Ionotropic Receptors in the Malaria Vector Mosquito Anopheles gambiae Tuned to Amines and Carboxylic Acids

The principal Afrotropical human malaria vector mosquito, Anopheles gambiae, remains a significant threat to global health. A critical component in the transmission of malaria is the ability of An. gambiae females to detect and respond to human-derived chemical kairomones in their search for blood meal hosts. The basis for host odor responses resides in olfactory receptor neurons (ORNs) that express chemoreceptors encoded by large gene families, including the odorant receptors (ORs) and the variant ionotropic receptors (IRs). While ORs have been the focus of extensive investigation, functional IR complexes and the chemical compounds that activate them have not been identified in An. gambiae. Here we report the transcriptional profiles and functional characterization of three An. gambiae IR (AgIr) complexes that specifically respond to amines or carboxylic acids - two classes of semiochemicals that have been implicated in mediating host-seeking by adult females but are not known to activate An. gambiae ORs (AgOrs). Our results suggest that AgIrs play critical roles in the detection and behavioral responses to important classes of host odors that are underrepresented in the AgOr chemical space.

Organization of olfactory centres in the malaria mosquito Anopheles gambiae

Mosquitoes are vectors for multiple infectious human diseases and use a variety of sensory cues (olfactory, temperature, humidity and visual) to locate a human host. A comprehensive understanding of the circuitry underlying sensory signalling in the mosquito brain is lacking. Here we used the Q-system of binary gene expression to develop transgenic lines of Anopheles gambiae in which olfactory receptor neurons expressing the odorant receptor co-receptor (Orco) gene are labelled with GFP. These neurons project from the antennae and maxillary palps to the antennal lobe (AL) and from the labella on the proboscis to the suboesophageal zone (SEZ), suggesting integration of olfactory and gustatory signals occurs in this brain region. We present detailed anatomical maps of olfactory innervations in the AL and the SEZ, identifying glomeruli that may respond to human body odours or carbon dioxide. Our results pave the way for anatomical and functional neurogenetic studies of sensory processing in mosquitoes.

Silencing the Olfactory Co-Receptor RferOrco Reduces the Response to Pheromones in the Red Palm Weevil, Rhynchophorus ferrugineus

The red palm weevil (RPW, Rhynchophorus ferrugineus), one of the most widespread of all invasive insect pest species, is a major cause of severe damage to economically important palm trees. RPW exhibits behaviors very similar to those of its sympatric species, the Asian palm weevil (R. vulneratus), which is restricted geographically to the southern part of Southeast Asia. Although efficient and sustainable control of these pests remains challenging, olfactory-system disruption has been proposed as a promising approach for controlling palm weevils. Here, we report the cloning and sequencing of an olfactory co-receptor (Orco) from R. ferrugineus (RferOrco) and R. vulneratus (RvulOrco) and examine the effects of RferOrco silencing (RNAi) on odorant detection. RferOrco and RvulOrco encoding 482 amino acids showing 99.58% identity. The injection of double-stranded RNA (dsRNA) from RferOrco into R. ferrugineus pupae significantly reduced RferOrco gene expression and led to the failure of odor-stimulus detection, as confirmed through olfactometer and electroantennography (EAG) assays. These results suggest that olfactory-system disruption leading to reduced pheromone detection holds great potential for RPW pest-control strategies.

Inhibition of Malaria Infection in Transgenic Anopheline Mosquitoes Lacking Salivary Gland Cells

Malaria is an important global public health challenge, and is transmitted by anopheline mosquitoes during blood feeding. Mosquito vector control is one of the most effective methods to control malaria, and population replacement with genetically engineered mosquitoes to block its transmission is expected to become a new vector control strategy. The salivary glands are an effective target tissue for the expression of molecules that kill or inactivate malaria parasites. Moreover, salivary gland cells express a large number of molecules that facilitate blood feeding and parasite transmission to hosts. In the present study, we adapted a functional deficiency system in specific tissues by inducing cell death using the mouse Bcl-2-associated X protein (Bax) to the Asian malaria vector mosquito, Anopheles stephensi. We applied this technique to salivary gland cells, and produced a transgenic strain containing extremely low amounts of saliva. Although probing times for feeding on mice were longer in transgenic mosquitoes than in wild-type mosquitoes, transgenic mosquitoes still successfully ingested blood. Transgenic mosquitoes also exhibited a significant reduction in oocyst formation in the midgut in a rodent malaria model. These results indicate that mosquito saliva plays an important role in malaria infection in the midgut of anopheline mosquitoes. The dysfunction in the salivary glands enabled the inhibition of malaria transmission from hosts to mosquito midguts. Therefore, salivary components have potential in the development of new drugs or genetically engineered mosquitoes for malaria control.

Malaria, transmitted by anopheline mosquitoes, represents an important global public health challenge. The salivary glands of mosquitoes are an attractive target tissue for malaria and vector control. We produced a transgenic strain inducing cell death in the salivary glands with a cell death effector molecule in the Asian malaria vector mosquito, Anopheles stephensi. This transgenic strain contained extremely low amounts of saliva. An analysis of this strain revealed that saliva plays an important role in probing as well as malaria infection in the midgut in a rodent malaria model. The dysfunction in the salivary glands enabled the inhibition of malaria transmission to mosquito midguts. Therefore, salivary components are also important in malaria control.

Intracellular regulation of the insect chemoreceptor complex impacts odour localization in flying insects

Flying insects are well known for airborne odour tracking and have evolved diverse chemoreceptors. While ionotropic receptors (IRs) are found across protostomes, insect odorant receptors (ORs) have only been identified in winged insects. We therefore hypothesized that the unique signal transduction of ORs offers an advantage for odour localization in flight. Using Drosophila, we found expression and increased activity of the intracellular signalling protein PKC in antennal sensilla following odour stimulation. Odour stimulation also enhanced phosphorylation of the OR co-receptor Orco in vitro, while site-directed mutation of Orco or mutations in PKC subtypes reduced the sensitivity and dynamic range of OR-expressing neurons in vivo, but not IR-expressing neurons. We ultimately show that these mutations reduce competence for odour localization of flies in flight. We conclude that intracellular regulation of OR sensitivity is necessary for efficient odour localization, which suggests a mechanistic advantage for the evolution of the OR complex in flying insects.

Insights Into the Evolution of Chemoreceptor Genes Superfamily in Tyrophagus putrescentiae (Acari: Acaridae)

All living organisms, including animals, plants, fungi, and bacteria, use the olfactory system to recognize chemicals or pheromone from their environment. Insects detect a volatile substance using odorant receptors (ORs) or gustatory receptors (GRs) and ionotropic receptors (IRs). The gene families of the olfactory system in Acari are still not clear. In this study, we identified seven ORs, one GR, and five IRs from the transcriptome of the storage mite, Tyrophagus putrescentiae Schrank. No olfactory coreceptor was found in this transcriptome. Phylogenetic analysis of these gene families with other Arthropoda species revealed the conservation of carbon dioxide receptors in all tested flying insects and T. putrescentiae Most of these ORs and GRs were unique to three mosquitoes (Anopheles gambiae Giles, Culex quinquefasciatus Say, and Aedes aegypti L.), Ixodes scapularis Say and Pediculus humanus L., indicating their involvement in specific aspects of both gustatory and olfactory perception. Some clades contained receptors obtained from all tested insect vector species, indicating a degree of conservation among some vector-dependent OR lineages. IRs family was a highly dynamic and independent original of the chemoreceptor genes subfamily. Our findings would make it possible for future research on the chemosensory recognition mechanism in Acari.

Silencing of the olfactory co-receptor gene in Dendroctonus armandi leads to EAG response declining to major host volatiles

In this study, a polymerase chain reaction (PCR) based on homology genes of Orco was utilized to identify DarmOrco, which is essential for olfaction in D. armandi. The results showed that DarmOrco shares significant sequence homology with Orco proteins had known in other insects. Quantitative real-time PCR (qRT-PCR) analysis suggested that DarmOrco was abundantly expressed in adult D. armandi; by contrast, DarmOrco showed trace amounts of expression level in other stages. Of different tissues, DarmOrco expression level was the highest in the antennae. In order to understand the functional significance of Orco, we injected siRNA of DarmOrco into the conjunctivum between the second and third abdominal segments, and evaluated its expression after siRNA injected for 24 h, 48 h and 72 h. The results of qRT-PCR demonstrated that the reduction of mRNA expression level was significant (~80%) in DarmOrco siRNA-treated D. armandi than in water-injected and non-injected controls. The electroantennogram responses of females and males to 11 major volatiles of its host, were also reduced (30~68% for females; 16~70% for males) in siRNA-treated D. armandi compared with the controls. These results suggest that DarmOrco is crucial in mediating odorant perception.

Silencing the odorant receptor co-receptor RproOrco affects the physiology and behavior of the Chagas disease vector Rhodnius prolixus

Olfaction is one of the main sensory modalities that allow insects to interpret their environment. Several proteins, including odorant-binding proteins (OBPs) and odorant receptors (ORs), are involved in this process. Odorant receptors are ion channels formed by a binding unit OR and an odorant receptor co-receptor (Orco). The main goal of this study was to characterize the Orco gene of Rhodnius prolixus (RproOrco) and to infer its biological functions using gene silencing. The full-length RproOrco gene sequence was downloaded from VectorBase. This gene has 7 introns and is located in the genome SuperContig GL563069: 1,017,713-1,023,165. RproOrco encodes a protein of 473 amino acids, with predicted 7 transmembrane domains, and is highly expressed in the antennae during all R. prolixus developmental stages. The RNAi technique effectively silenced RproOrco, reducing the gene's expression by approximately 73%. Interestingly, the effect of gene silencing persisted for more than 100 days, indicating a prolonged effect of dsRNA that was maintained even after molting. The phenotypic effects of silencing involved the following: (1) loss of the ability to find a vertebrate host in a timely manner, (2) decreased ingested blood volume, (3) delayed and decreased molt rate, (4) increased mortality rate, and (5) decreased egg laying. Our data strongly suggest that dsOrco disrupts R. prolixus host-finding behavior, which is further reflected in the blood ingestion, molting, mortality, and egg laying data. This study clearly demonstrates that Orco is an excellent target for controlling triatomine populations. Thus, the data presented here open new possibilities for the control of vector-borne diseases.

Patterns of expression of odorant receptor genes in a Chagas disease vector

Rhodnius prolixus is a triatomine bug acting as a relevant vector of Chagas disease for which the genome sequence has been recently made available. Based on this information, a set of olfactory (ORs) and ionotropic receptor (IRs) genes potentially related to olfactory processes was characterized, and the expression patterns along bug development and in different structures potentially involved in promoting chemosensory-mediated behaviors were studied. For this, diverse bioinformatic procedures were used to validate gene models analyzing their structural and functional features and designing specific primers. Evolutionary relationships among R. prolixus olfactory coreceptors (RproOrco, RproIR25a, RproIR8a and RproIR76b) and their orthologues from other insects were shown to have mostly good bootstrap support values in phylogenetic trees. Moreover, antennal expression was confirmed for most genes included in the study. Both ORs and IRs showed antennal expression along the whole development of bugs of this species, with few exceptional receptors showing gradually increasing expression or expression restricted to the antennae of adult bugs. Finally, the expression of most of the selected genes was confirmed in other structures, such as rostri, tarsi, tibial pads and genitalia, which are potentially involved in promoting chemosensory-mediated behaviors. These results are discussed in terms of their relevance to advance in the understanding of the molecular bases of triatomine behavior.

Development of a mosquito attractant blend of small molecules against host-seeking Aedes aegypti

A mosquito's dependence on olfaction in the hunt for human host could be efficiently exploited to protect humans from mosquito bites. The present study is undertaken to make the most attractant compound blend for Aedes aegypti mosquitoes to lure them to traps. Eleven molecules (M1-M11) at different dilutions were screened for attractancy against non-blood-fed adult female mosquitoes in an olfactometer. The results showed that the attractancy was dependent on both the chemical nature of the molecule and the strength of the odor. Out of 11 molecules screened, 9 showed significant attractancy (P < 0.05) when tested individually. The attractancy was in the order of M11 > M7 > M6 > M10 > M9 > M3 > M2 > M1 > M4 with attractancy indices (AIs) 86.11, 55.93, 55.17, 54, 52.94, 52, 50, 43.64, and 32, respectively, at the optimum dilutions. Seven blends (I-VII) were made and were screened for attractancy against Ae. aegypti. All the blends showed significant attractancy (P < 0.05). The attractancy was in the order of blend VII > III > IV > I > VI > V > II with AIs 96.63, 89.19, 65, 57.89, 56.1, 47.13, and 44.44, respectively. Among the seven blends, blend VII with constituent molecules M6, M9, M10, and M11 is the most promising with an AI value of 96.63. This blend will be useful in luring the host-seeking mosquitoes to traps. The field efficacy of these attractant blends may be explored in the future.

Bacteria Endosymbiont, Wolbachia, Promotes Parasitism of Parasitoid Wasp Asobara japonica

Wolbachia is the most widespread endosymbiotic bacterium that manipulates reproduction of its arthropod hosts to enhance its own spread throughout host populations. Infection with Wolbachia causes complete parthenogenetic reproduction in many Hymenoptera, producing only female offspring. The mechanism of such reproductive manipulation by Wolbachia has been extensively studied. However, the effects of Wolbachia symbiosis on behavioral traits of the hosts are scarcely investigated. The parasitoid wasp Asobara japonica is an ideal insect to investigate this because symbiotic and aposymbiotic strains are available: Wolbachia-infected Tokyo (TK) and noninfected Iriomote (IR) strains originally collected on the main island and southwest islands of Japan, respectively. We compared the oviposition behaviors of the two strains and found that TK strain females parasitized Drosophila melanogaster larvae more actively than the IR strain, especially during the first two days after eclosion. Removing Wolbachia from the TK strain wasps by treatment with tetracycline or rifampicin decreased their parasitism activity to the level of the IR strain. Morphological and behavioral analyses of both strain wasps showed that Wolbachia endosymbionts do not affect development of the host female reproductive tract and eggs, but do enhance host-searching ability of female wasps. These results suggest the possibility that Wolbachia endosymbionts may promote their diffusion and persistence in the host A. japonica population not only at least partly by parthenogenesis but also by enhancement of oviposition frequency of the host females.