The role of the coreceptor Orco in insect olfactory transduction

The fruit fly, Drosophila melanogaster, as a microrobotics platform

Engineering small autonomous agents capable of operating in the microscale environment remains a key challenge, with current systems still evolving. Our study explores the fruit fly, Drosophila melanogaster, a classic model system in biology and a species adept at microscale interaction, as a biological platform for microrobotics. Initially, we focus on remotely directing the walking paths of fruit flies in an experimental arena. We accomplish this through two distinct approaches: harnessing the fruit flies' optomotor response and optogenetic modulation of its olfactory system. These techniques facilitate reliable and repeated guidance of flies between arbitrary spatial locations. We guide flies along predetermined trajectories, enabling them to scribe patterns resembling textual characters through their locomotion. We enhance olfactory-guided navigation through additional optogenetic activation of attraction-inducing mushroom body output neurons. We extend this control to collective behaviors in shared spaces and navigation through constrained maze-like environments. We further use our guidance technique to enable flies to carry a load across designated points in space, establishing the upper bound on their weight-carrying capabilities. Additionally, we demonstrate that visual guidance can facilitate novel interactions between flies and objects, showing that flies can consistently relocate a small spherical object over significant distances. Last, we demonstrate multiagent formation control, with flies alternating between distinct spatial patterns. Beyond expanding tools available for microrobotics, these behavioral contexts can provide insights into the neurological basis of behavior in fruit flies.

Evolutionary Process Underlying Receptor Gene Expansion and Cellular Divergence of Olfactory Sensory Neurons in Honeybees

Olfaction is crucial for animals' survival and adaptation. Unlike the strict singular expression of odorant receptor (OR) genes in vertebrate olfactory sensory neurons (OSNs), insects exhibit complex OR gene expression patterns. In honeybees (Apis mellifera), a significant expansion of OR genes implies a selection preference for the olfactory demands of social insects. However, the mechanisms underlying receptor expression specificity and their contribution to OSN divergence remain unclear. In this study, we used single-nucleus multiomics profiling to investigate the transcriptional regulation of OR genes and the cellular identity of OSNs in A. mellifera. We identified three distinct OR expression patterns, singular OR expression, co-expression of multiple OR genes with a single active promoter, and co-expression of multiple OR genes with multiple active promoters. Notably, ∼50% of OSNs co-expressed multiple OR genes, driven by polycistronic transcription of tandemly duplicated OR genes via a single active promoter. In these OSNs, their identity was determined by the first transcribed receptor. The divergent activation of the promoter for duplicated OR genes ensures the coordinated increased divergence of OSN population. By integrating multiomics data with genomic architecture, we illustrate how fundamental genetic mechanisms drive OR gene expansion and influence flanking regulatory elements, ultimately contributing to the cellular divergence of OSNs. Our findings highlight the interplay between gene duplication and regulatory evolution in shaping OSN diversity, providing new insights into the evolution and adaptation of olfaction in social insects. This study also sheds light on how genetic innovations contribute to the evolution of complex traits.

Hawkmoth Pheromone Transduction Involves G-Protein-Dependent Phospholipase Cβ Signaling

Evolutionary pressures adapted insect chemosensation to their respective physiological needs and tasks in their ecological niches. Solitary nocturnal moths rely on their acute olfactory sense to find mates at night. Pheromones are detected with maximized sensitivity and high temporal resolution through mechanisms that are mostly unknown. While the inverse topology of insect olfactory receptors and heteromerization with the olfactory receptor coreceptor suggest ionotropic transduction via odorant-gated receptor-ion channel complexes, contradictory data propose amplifying G-protein-coupled transduction. Here, we used in vivo tip-recordings of pheromone-sensitive sensilla of male Manduca sexta hawkmoths at specific times of day (rest vs activity). Since the olfactory receptor neurons distinguish signal parameters in three consecutive temporal windows of their pheromone response (phasic; tonic; late, long-lasting), respective response parameters were analyzed separately. Disruption of G-protein-coupled transduction and block of phospholipase C decreased and slowed the phasic response component during the activity phase of hawkmoths without affecting any other component of the response during activity and rest. A more targeted disruption of Gα subunits by blocking Gαo or sustained activation of Gαs using bacterial toxins affected the phasic pheromone response, while toxins targeting Gαq and Gα12/13 were ineffective. Consistent with these data, the expression of phospholipase Cβ4 depended on zeitgeber time, which indicates circadian clock-modulated metabotropic pheromone transduction cascades that maximize sensitivity and temporal resolution of pheromone transduction during the hawkmoth's activity phase. Thus, discrepancies in the literature on insect olfaction may be resolved by considering circadian timing and the distinct odor response components.

Gene Silencing of Olfactory Receptor Coreceptor by Systemic RNA Interference in Callosobruchus maculatus

Seed beetles are pernicious pests of leguminous seeds and are distributed globally. They cause great economic losses, particularly in developing countries. Of this genus, the cowpea weevil (Callosobruchus maculatus) is the most destructive and common species of this beetle. However, there are no effective and sustainable control strategies available for this species. Nevertheless, sustainable pest management strategies using sex pheromone compounds have been proposed, as C. maculatus uses species-specific sex pheromone signals for pre-mating isolation. Therefore, this study aimed to investigate the attractive capacity of male C. maculatus after RNA interference (RNAi)-based gene knockdown of olfactory receptor coreceptor (Cmac\Orco). The RNAi effect showed more than 90% reduction in transcripts, and a behavioral bioassay using a Y-tube olfactometer indicated that knocking down males impaired sexual attraction toward females, which would be a useful tool for further screening of target molecules for odorant chemical communication.

The conserved IR75 subfamily mediates carboxylic acid detection in insects of public health and agricultural importance

Insects perceive and respond to carboxylic acids (CAs), amines, and aldehydes primarily via conserved ionotropic receptors (IRs). These receptors form the basis for a second olfactory system distinct from the well-characterized odorant receptors. Neurons expressing IRs are housed in dedicated sensilla and innervate glomeruli, separate from those innervated by odorant receptor neurons. The IR8a co-receptor is highly conserved across insect orders and, together with ionotropic receptor tuning receptors, primarily detects CAs. The conservation of genes and the anatomical separation of neural pathways underscore the importance of these compound classes and CAs, specifically in insect chemical ecology. We provide a summary of carboxylic acid detection in insects, focusing on dipteran and lepidopteran species of significance to public health and agriculture. An overview of insect behavior toward CAs is provided, as well as a comprehensive update on carboxylic acid receptor function in insects. Phylogenetic analysis of publicly available genome databases reveals several species that encode and express homologs of previously deorphanized carboxylic acid receptors, highlighting avenues for future research.

Template-based modeling of insect odorant receptors outperforms AlphaFold3 for ligand binding predictions

Insects rely on odorant receptors (ORs) to detect and respond to volatile environmental cues, so the ORs are attracting increasing interest as potential targets for pest control. However, experimental analysis of their structures and functions faces significant challenges. Computational methods such as template-based modeling (TBM) and AlphaFold3 (AF3) could facilitate the structural characterisation of ORs. This study first showed that both models accurately predicted the structural fold of MhOR5, a jumping bristletail OR with known experimental 3D structures, although accuracy was higher in the extracellular region of the protein and binding mode of their cognate ligands with TBM. The two approaches were then compared for their ability to predict the empirical binding evidence available for OR-odorant complexes in two economically important fruit fly species, Bactrocera dorsalis and B. minax. Post-simulation analyses including binding affinities, complex and ligand stability and receptor-ligand interactions (RLIs) revealed that TBM performed better than AF3 in discriminating between binder and non-binder complexes. TBM's superior performance is attributed to hydrophobicity-based helix-wise multiple sequence alignment (MSA) between available insect OR templates and the ORs for which the binding data were generated. This MSA identified conserved residues and motifs which could be used as anchor points for refining the alignments.

The function of OforOrco in the allogrooming behavior of Odontotermes formosanus (Shiraki) induced by Serratia marcescens Bizio (SM1)

Termites are consistently confronted with a complex microbial environment. In addition to the role of their innate immune system in resisting pathogen infection, social immune behavior also plays a significant role in helping termites withstand the stress caused by pathogenic microorganisms. The allogrooming behavior among different individuals is commonly observed in termites, and it plays a crucial role in the social immune interaction network. In the case of Odontotermes formosanus (Shiraki), Orco is specifically involved in detecting pheromones and volatile chemicals released by termites to communicate with each other. Nonetheless, the function of Orco in the social immunity remains unreported in O. formosanus. Consequently, in this study, we recorded the allogrooming behavior of O. formosanus workers under SM1 stress. The results indicated a significant increase in allogrooming behavior due to SM1 infection. The allogrooming behavior of workers under SM1 stress was significantly increased after the addition of soldiers. Compared with pronotum group treated by SM1, SM1 treatment of workers' heads significantly reduced the allogrooming behavior among workers. In addition, we found that SM1 could greatly increase the expression of OforOrco. Furthermore, interfering with OforOrco could markedly reduce the allogrooming behavior among workers under SM1 stress, and increase the mortality of worker under SM1 stress. This study demonstrated the significant role of OforOrco in the social immunity of O. formosanus, which offers a theoretical foundation for the advancement of research on termite RNA biopesticides, and the integration of RNA interference (RNAi) with pathogens. This study is valuable for elucidating the social immune behavior and interaction network of termites.

Functional Characterization of Peripheral Neurons and a Receptor Recognizing Sex Pheromones in Hyphantria cunea (Erebidae)

Hyphantria cunea (Lepidoptera: Erebidae) is difficult and costly to control as a quarantine pest found globally. Sex pheromone trapping is an effective measure for its population monitoring and control; however, the peripheral neural mechanism of sex pheromone recognition in H. cunea remains unclear. An electrophysiological analysis showed that both male and female moths of H. cunea responded to four components of sex pheromones and the responses of male moths were stronger than those of the female moths. We identified three types of trichoid sensilla (ST) responsive to sex pheromones using the single sensillum recording technique. Each type was involved in recognizing 9R, 10S-epoxy-1, Z3, Z6-heneicosatriene (1, Z3, Z6-9S, 10R-epoxy-21Hy). Four peripheral neurons involved in the olfactory encoding of sex pheromones were identified. Four candidate pheromone receptor (PR) genes, HcunPR1a, HcunPR1b, HcunPR3, and HcunPR4, were screened by transcriptome sequencing. All of them were highly expressed in the antennae of males, except for HcunPR4, which was highly expressed in the antennae of females. Functional identification showed that HcunPR1a responded to sex pheromone. Other HcunPRs were not functionally identified. In summary, neurons involved in sex pheromone recognition of H. cunea were located in the ST, and HcunPR1a recognized secondary pheromone components 1, Z3, Z6-9S, 10R-epoxy-21Hy. Interestingly, PRs that recognize the main components of the sex pheromone may be located in an unknown branch of the olfactory receptor and merit further study. Our findings provide a better understanding of the peripheral neural coding mechanism of type II sex pheromones, and HcunPR1a may provide a target for the subsequent development of highly effective and specific biopesticides for H. cunea.

Response Plasticity of Drosophila Olfactory Sensory Neurons

In insect olfaction, sensitization refers to the amplification of a weak olfactory signal when the stimulus is repeated within a specific time window. In the vinegar fly, Drosophila melanogaster, this occurs already at the periphery, at the level of olfactory sensory neurons (OSNs) located in the antenna. In our study, we investigate whether sensitization is a widespread property in a set of seven types of OSNs, as well as the mechanisms involved. First, we characterize and compare the differences in spontaneous activity, response velocity and response dynamics, among the selected OSN types. These express different receptors with distinct tuning properties and behavioral relevance. Second, we show that sensitization is not a general property. Among our selected OSN types, it occurs in those responding to more general food odors, while OSNs involved in very specific detection of highly specific ecological cues like pheromones and warning signals show no sensitization. Moreover, we show that mitochondria play an active role in sensitization by contributing to the increase in intracellular Ca2+ upon weak receptor activation. Thus, by using a combination of single sensillum recordings (SSRs), calcium imaging and pharmacology, we widen the understanding of how the olfactory signal is processed at the periphery.

Residues of chlorpyrifos in the environment induce resistance in Aedes albopictus by affecting its olfactory system and neurotoxicity

Aedes albopictus, a virus-vector pest, is primarily controlled through the use of insecticides. In this study, we investigated the mechanisms of resistance in Ae. albopictus in terms of chlorpyrifos neurotoxicity to Ae. albopictus and its effects on the olfactory system. We assessed Ca2+-Mg2+-ATP levels, choline acetyltransferase (ChAT), Monoamine oxidase (MAO), odorant-binding proteins (OBPs), and olfactory receptor (OR7) gene expression in Ae. albopictus using various assays including Y-shaped tube experiments and DanioVision analysis to evaluate macromotor behavior. Our findings revealed that cumulative exposure to chlorpyrifos reduced the activity of neurotoxic Ca2+-Mg2+-ATPase and ChAT enzymes in Ae. albopictus to varying degrees, suppressed MAO-B enzyme expression, altered OBPs and OR7 expression patterns, as well as affected evasive response, physical mobility, and cumulative locomotor time under chlorpyrifos stress conditions for Ae. albopictus individuals. Consequently, these changes led to decreased feeding ability, reproductive capacity, and avoidance behavior towards natural enemies in Ae. albopictus populations exposed to chlorpyrifos stressors over time. To adapt to unfavorable living environments, Ae. albopictus may develop certain tolerance mechanisms against organophosphorus pesticides. This study provides valuable insights for guiding rational insecticide usage or dosage adjustments targeting the nervous system of Ae. albopictus.

Odorant receptors for floral- and plant-derived volatiles in the yellow fever mosquito, Aedes aegypti (Diptera: Culicidae)

Adult mosquitoes require regular sugar meals, including nectar, to survive in natural habitats. Both males and females locate potential sugar sources using sensory proteins called odorant receptors (ORs) activated by plant volatiles to orient toward flowers or honeydew. The yellow fever mosquito, Aedes aegypti (Linnaeus, 1762), possesses a large gene family of ORs, many of which are likely to detect floral odors. In this study, we have uncovered ligand-receptor pairings for a suite of Aedes aegypti ORs using a panel of environmentally relevant, plant-derived volatile chemicals and a heterologous expression system. Our results support the hypothesis that these odors mediate sensory responses to floral odors in the mosquito's central nervous system, thereby influencing appetitive or aversive behaviors. Further, these ORs are well conserved in other mosquitoes, suggesting they function similarly in diverse species. This information can be used to assess mosquito foraging behavior and develop novel control strategies, especially those that incorporate mosquito bait-and-kill technologies.

Binding properties of chemosensory protein 4 in Riptortus pedestris to aggregation pheromones

In insects, chemosensory proteins (CSPs) play an important role in the perception of the external environment and have been widely used for protein-binding characterization. Riptortus pedestris has received increased attention as a potential cause of soybean staygreen syndrome in recent years. In this study, we found that RpedCSP4 expression in the antennae of adult R. pedestris increased with age, with no significant difference in expression level observed between males and females, as determined through quantitative real-time polymerase chain reaction (qRT-PCR). Subsequently, we investigated the ability of RpedCSP4 to bind various ligands (five aggregated pheromone components and 13 soybean volatiles) using a prokaryotic expression system and fluorescence competitive binding assays. We found that RpedCSP4 binds to three aggregated pheromone components of R. pedestris, namely, ((E)-2-hexenyl (Z)-3-hexenoate (E2Z3), (E)-2-hexenyl (E)-2-hexenoate (E2E2), and (E)-2-hexenyl hexenoate (E2HH)), and that its binding capacities are most stable under acidic condition. Finally, the structure and protein-ligand interactions of RpedCSP4 were further analyzed via homology modeling, molecular docking, and targeted mutagenesis experiments. The L29A mutant exhibited a loss of binding ability to these three aggregated pheromone components. Our results show that the olfactory function of RpedCSP4 provides new insights into the binding mechanism of RpedCSPs to aggregation pheromones and contributes to discover new target candidates that will provide a theoretical basis for future population control of R. pedestris.

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.

Identification and functional characterization of female antennae-biased odorant receptor 23 involved in acetophenone detection of the Indian meal moth Plodia interpunctella

The Indian meal moth, Plodia interpunctella (Lepidoptera: Pyralidae), a globally distributed storage pest, relies on odors that are emitted from stored foods to select a suitable substrate for oviposition. However, the molecular mechanism underlying the chemical communication between P. interpunctella and its host remains elusive. In this study, 130 chemosensory genes were identified from the transcriptomes of 7 P. interpunctella tissues, and the quantitative expression levels of all 56 P. interpunctella odorant receptor genes (PintORs) were validated using real-time quantitative polymerase chain reaction. The functional characteristics of 5 PintORs with female antennae-biased expression were investigated using 2-electrode voltage clamp recordings in Xenopus laevis oocytes. PintOR23 was found to be specifically tuned to acetophenone. Acetophenone could elicit a significant electrophysiological response and only attracted mated females when compared with males and virgin females. In addition, molecular docking predicted that the hydrogen bonding sites, TRP-335 and ALA-167, might play key roles in the binding of PintOR23 to acetophenone. Our study provides valuable insights into the olfactory mechanism of oviposition substrate detection and localization in P. interpunctella and points toward the possibility of developing eco-friendly odorant agents to control pests of stored products.

Odorant receptors for floral- and plant-derived volatiles in the yellow fever mosquito, Aedes aegypti (Diptera: Culicidae)

Adult mosquitoes require regular sugar meals, usually floral nectar, to survive and flourish in natural habitats. Both males and females locate potential sugar sources using sensory proteins called odorant receptors activated by plant volatiles that facilitate orientation toward flowers or honeydew. The Yellow Fever mosquito, Aedes aegypti (Linnaeus, 1762), possesses a large repertoire of odorant receptors, many of which are likely to support floral odor detection and nectar-seeking. In this study, we have employed a heterologous expression system and the two-electrode voltage clamping technique to identify environmentally relevant chemical compounds that activate specific odorant receptors. Importantly, we have uncovered ligand-receptor pairings for a suite of Aedes aegypti odorant receptors likely to mediate appetitive or aversive behavioral responses, thus shaping a critical aspect of the life history of a medically important mosquito. Moreover, the high degree of conservation of these receptors in other disease-transmitting species suggests common mechanisms of floral odor detection. This knowledge can be used to further investigate mosquito foraging behavior to either enhance existing, or develop novel, control strategies, especially those that incorporate mosquito bait-and-kill or attractive toxic sugar bait technologies.

The Orco gene involved in recognition of host plant volatiles and sex pheromone in the chive maggot Bradysia odoriphaga

The insect olfactory recognition system plays a crucial role in the feeding and reproductive behaviors of insects. The odorant receptor co-receptor (Orco), as an obligatory chaperone, is critical for odorant recognition by way of forming heteromeric complexes with conventional odorant receptors (ORs). To investigate the biological functions of Orco in perceiving host plant volatiles and sex pheromone, the Orco gene was identified from the chive maggot Bradysia odoriphaga transcriptome data. Multiple sequence alignment reveals that BodoOrco exhibits an extremely high sequence identity with Orcos from other dipteran insects. The expression of BodoOrco is significantly higher in adults than in larvae and pupae, and the BodoOrco gene is primarily expressed in the antennae of both sexes. Furthermore, the Y-tube assay indicated that knockdown of BodoOrco leads to significant reductions in B. odoriphaga adults' response to all tested host plant volatiles. The dsOrco-treated unmated male adults show less attraction to unmated females and responded slowly compared with dsGFP control group. These results indicated that BodoOrco is involved in recognition of sex pheromone and host plant volatiles in B. odoriphaga and has the potential to be used as a target for the design of novel active compounds for developing ecofriendly pest control strategies.

Biosensors for Odor Detection: A Review

Animals can easily detect hundreds of thousands of odors in the environment with high sensitivity and selectivity. With the progress of biological olfactory research, scientists have extracted multiple biomaterials and integrated them with different transducers thus generating numerous biosensors. Those biosensors inherit the sensing ability of living organisms and present excellent detection performance. In this paper, we mainly introduce odor biosensors based on substances from animal olfactory systems. Several instances of organ/tissue-based, cell-based, and protein-based biosensors are described and compared. Furthermore, we list some other biological materials such as peptide, nanovesicle, enzyme, and aptamer that are also utilized in odor biosensors. In addition, we illustrate the further developments of odor biosensors.

Rational engineering approaches for establishing insect olfaction reporters in yeast

Insect olfaction directly impacts insect behavior and thus is an important consideration in the development of smart farming tools and in integrated pest management strategies. Insect olfactory receptors (ORs) have been traditionally studied using Drosophila empty neuron systems or with expression and functionalization in HEK293 cells or Xenopus laevis oocytes. Recently, the yeast Saccharomyces cerevisiae (S. cerevisiae) has emerged as a promising chassis for the functional expression of heterologous seven transmembrane receptors. S. cerevisiae provides a platform for the cheap and high throughput study of these receptors and potential deorphanization. In this study, we explore the foundations of a scalable yeast-based platform for the functional expression of insect olfactory receptors by employing a genetically encoded calcium sensor for quantitative evaluation of fluorescence and optimized experimental parameters for enhanced functionality. While the co-receptor of insect olfactory receptors remains non-functional in our yeast-based system, we thoroughly evaluated various experimental variables and identified future research directions for establishing an OR platform in S. cerevisiae.

Task-specific odorant receptor expression in worker antennae indicates that sensory filters regulate division of labor in ants

Division of labor (DOL) is a characteristic trait of insect societies, where tasks are generally performed by specialized individuals. Inside workers focus on brood or nest care, while others take risks by foraging outside. Theory proposes that workers have different thresholds to perform certain tasks when confronted with task-related stimuli, leading to specialization and consequently DOL. Workers are presumed to vary in their response to task-related cues rather than in how they perceive such information. Here, we test the hypothesis that DOL instead stems from workers varying in their efficiency to detect stimuli of specific tasks. We use transcriptomics to measure mRNA expression levels in the antennae and brain of nurses and foragers of the ant Temnothorax longispinosus. We find seven times as many genes to be differentially expressed between behavioral phenotypes in the antennae compared to the brain. Moreover, half of all odorant receptors are differentially expressed, with an overrepresentation of the 9-exon gene family upregulated in the antennae of nurses. Nurses and foragers thus apparently differ in the perception of their olfactory environment and task-related signals. Our study supports the hypothesis that antennal sensory filters predispose workers to specialize in specific tasks.

Binding properties of chemosensory protein 12 in Riptortus pedestris to aggregation pheromone (E)-2-hexenyl (Z)-3-hexenoate

Riptortus pedestris (bean bug), a common soybean pest, has a highly developed olfactory system to find hosts for feeding and oviposition. Chemosensory proteins (CSPs) have been identified in many insect species; however, their functions in R. pedestris remain unknown. In this study, quantitative real time-polymerase chain reaction (qRT-PCR) revealed that the expression of RpedCSP12 in the adult antennae of R. pedestris increased with age. Moreover, a significant difference in the expression levels of RpedCSP12 was observed between male and female antennae at one and three days of age. We also investigated the binding ability of RpedCSP12 to different ligands using a prokaryotic expression system and fluorescence competitive binding assays. We found that RpedCSP12 only bound to one aggregation pheromone, (E)-2-hexenyl (Z)-3-hexenoate, and its binding decreased with increasing pH. Furthermore, homology modelling, molecular docking, and site-directed mutagenesis revealed that the Y27A, L74A, and L85A mutants lost their binding ability to (E)-2-hexenyl (Z)-3-hexenoate. Our findings highlight the olfactory roles of RpedCSP12, providing insights into the mechanism by which RpedCSPs bind to aggregation pheromones. Therefore, our study can be used as a theoretical basis for the population control of R. pedestris in the future.

Genome-wide identification of candidate chemosensory receptors in the bean bug Riptortus pedestris (Hemiptera: Alydidae) and the functional verification of its odorant receptor co-receptor (Orco) in recognizing aggregation pheromone

A sophisticated and sensitive olfactory system plays a vital role in the survival and reproduction of insects. Chemosensory receptors are indispensable for the molecular recognition and discrimination of semiochemicals. Riptortus pedestris is a notorious pest of legume plants, resulting in yield losses and quality decreases in soybeans. It is well accepted that R. pedestris highly relies on its olfactory system in detecting aggregation pheromones, host volatiles, and pesticides; however, little research focused on its chemosensory receptors. In the present study, we identified 237 odorant receptors (ORs), 42 gustatory receptors (GRs), and 31 ionotropic receptors (IRs) from the reported genome of R. pedestris, and analyzed their phylogenetic relationship with other hemipteran species. Through the results of RNA-seq and real-time quantitative PCR (qRT-PCR), we found that RpedORs displayed different expression levels in the antennae of R. pedestris at different development stages. To further verify the function of odorant receptor co-receptor (Orco), an obligate and unique insect OR, we silenced RpedOrco by RNA interference (RNAi) method. The results showed that silencing RpedOrco could significantly impair the response to aggregation pheromone in R. pedestris, indicating that RpedOrco plays an essential role in odorant detection. Our results can provide the theoretical foundations for revealing the olfactory recognition mechanism of R. pedestris and help explore and develop novel olfactory-based agents against this pest.

Ligand-binding properties of chemosensory protein 1 in Callosobruchus chinensis to mung bean volatiles

Callosobruchus chinensis (Coleoptera: Fabaceae) is a worldwide pest that feeds exclusively on legumes, and is the most serious pest affecting mung beans. Usually, the insect olfactory system plays a predominant role in searching for host plants and egg-laying locations. Chemosensory proteins (CSPs), are mainly responsible for transporting specific odour molecules from the environment. In this study, we found that the CSP1 gene of adult C. chinensis displayed antennae-biased expression using quantitative real-time PCR (qRT-PCR) analysis. The binding properties of 23 mung bean volatiles were then determined through several analyses of in vitro recombinant CSP1 protein, including fluorescence competitive binding assay, homology modelling, molecular docking, and site-directed mutagenesis. Fluorescence competitive binding assays showed that CchiCSP1 protein could bind to four mung bean volatiles and was most stable at pH 7.4. After site-directed mutation of three key amino acid bases (L39, V25, and Y35), their binding affinities to each ligand were significantly decreased or lost. This indicated that these three amino acid residues may be involved in the binding of CchiCSP1 to different ligands. We further used Y-tube behavioural bioassays to find that the four mung bean volatiles had a significant attraction or repulsion response in adult C. chinensis. The above findings confirm that the CchiCSP1 protein may be involved in the response of C. chinensis to mung bean volatiles and plays an important role in olfactory-related behaviours. The four active volatiles are expected to develop into new behavioural attractants or repellents in the future.

Genome-wide identification and characterization of the chemosensory relative protein genes in Rhus gall aphid Schlechtendalia chinensis

BACKGROUND

The Rhus gall aphid Schlechtendalia chinensis specially uses the only species Rhus chinensis and certain moss species (Mniaceae) as its primary host plant and secondary host plants, respectively. Rhus galls are formed on the primary host by the sucking of aphids, and used in traditional medicine as well as other various areas due to their high tannin contents. Chemoreception is critical for insect behaviors such as host searching, location and identification of mates and reproductive behavior. The process of chemoreception is mediated by a series of protein gene families, including odorant-binding proteins (OBPs), chemosensory proteins (CSPs), olfactory receptors (ORs), gustatory receptors (GRs), ionotropic receptors (IRs), and sensory neuron membrane proteins (SNMPs). However, there have been no reports on the analysis of molecular components related to the chemoreception system of S. chinensis at the genome level.

RESULTS

We examined the genes of eight OBPs, nine CSPs, 24 ORs, 16 GRs, 22 IRs, and five SNMPs in the S. chinensis genome using homological searches, and these chemosensory genes appeared mostly on chromosome 1. Phylogenetic and gene number analysis revealed that the gene families, e.g., ORs, GRs, CSPs and SNMPs in S. chinensis, have experienced major contractions by comparing to Myzus persicae, while the two gene families OBPs and IRs had slight expansion. The current results might be related to the broader host range of M. persicae versus the specialization of S. chinensis on only a host plant. There were 28 gene pairs between genomes of S. chinensis and Acyrthosiphon pisum in the chemoreceptor gene families by collinear comparison. Ka/Ks ratios (< 1) indicated that the genes of S. chinensis were mainly affected by purification selection during evolution. We also found the lower number and expression level of chemoreception genes in S. chinensis than in other 11 aphid species, such as ORs, GRs and IRs, which play an important role in host search.

CONCLUSION

Our study firstly identified the genes of the different chemosensory protein gene families in the S. chinensis genome, and analyzed their general features and expression profile, demonstrating the importance of chemoreception in the aphid and providing new information for further functional research.

Orco mutagenesis causes deficiencies in olfactory sensitivity and fertility in the migratory brown planthopper, Nilaparvata lugens

BACKGROUND

The migratory brown planthopper (BPH), Nilaparvata lugens (Hemiptera: Delphacidae), is the most destructive pest affecting rice plants in Asia and feeds exclusively on rice. Studies have investigated the olfactory response of BPHs to the major rice volatile compounds in rice. The insect olfactory co-receptor (Orco) is a crucial component of the olfactory system and is essential for odorant detection. Functional analysis of the Orco gene in BPHs would aid in the identification of their host preference.

RESULTS

We identified the BPH Orco homologue (NlOrco) by Blast searching the BPH transcriptome with the Drosophila Orco gene sequence. Spatiotemporal analysis indicated that NlOrco is first expressed in the later egg stage, and is expressed mainly in the antennae in adult females. A NlOrco-knockout line (NlOrco-/- ) was generated through clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9-mediated mutagenesis. The NlOrco-/- mutants showed no response to rice volatile compounds and consequently no host-plant preference. In addition, NlOrco-/- mutants exhibited extended nymphal duration and impaired fecundity compared with wild-type BPHs.

CONCLUSION

Our findings indicated that BPHs exhibit strong olfactory responses to major rice volatile compounds and suggest that NlOrco is required for the maximal fitness of BPHs. Our results may facilitate the identification of potential target genes or chemical compounds for BPH control applications. © 2022 Society of Chemical Industry.

Involvement of three chemosensory proteins in perception of host plant volatiles in the tea green leafhopper, Empoasca onukii

Chemosensory proteins (CSPs) can bind and transport odorant molecules, which are believed to be involved in insect chemoreception. Here, we investigated three CSPs in perception of volatiles in Empoasca onukii. Expression profiles showed that although EonuCSP4, EonuCSP 6-1 and EonuCSP6-2 were ubiquitously expressed in heads, legs, thoraxes and abdomen, they were all highly expressed in the antennae of E. onukii. Further, fluorescence competitive binding assays revealed that EonuCSP4 and 6-1 had binding affinities for three plant volatiles, suggesting their possible involvement in the chemosensory process. Among them, EonuCSP6-1 showed relatively high binding affinities for benzaldehyde. Behavioral assays revealed that the adults of E. onukii showed a significant preference for two compounds including benzaldehyde. The predicted three-dimensional (3D) structures of these 3 CSP have the typical six α-helices, which form the hydrophobic ligand-binding pocket. We therefore suggest that Eoun6-1 might be involved in the chemoreception of the host-related volatiles for E. onukii. Our data may provide a chance of finding a suitable antagonist of alternative control strategies which block the perception of chemosensory signals in pest, preventing the food- orientation behaviors.

The odorant receptor co-receptor gene contributes to mating and host-searching behaviors in parasitoid wasps

BACKGROUND

Biological control of pest insects by parasitoid wasps is an effective and environmentally friendly strategy compared with the use of synthetic pesticides. Successful courtship and host-search behaviors of parasitoid wasps are important for biological control efficiency and are often mediated by chemical odorant cues. The odorant receptor co-receptor (Orco) gene has an essential role in the perception of odors in insects. However, the function of Orco in the mating and host-searching behaviors of parasitoid wasps remains underexplored.

RESULTS

We identified the full-length Orco genes of four Drosophila parasitoid species in the genus Leptopilina, namely L. heterotoma, L. boulardi, L. syphax and L. drosophilae. Sequence alignment and membrane-topology analysis showed that Leptopilina Orcos had similar amino acid sequences and topology structures. Phylogenetic analysis revealed that Leptopilina Orcos were highly conserved. Furthermore, the results of quantitative real-time polymerase chain reactions showed that all four Orco genes had a typical antennae-biased tissue expression pattern. After knockdown of Orco in these different parasitoid species, we found that Orco-deficient male parasitoid wasps, but not females, lost their courtship ability. Moreover, Orco-deficient female parasitoid wasps presented impaired host-searching performance and decreased oviposition rates.

CONCLUSION

Our study demonstrates that Orcos are essential in the mating and host-searching behaviors of parasitoid wasps. To our knowledge, this is the first time that the functions of Orco genes have been characterized in parasitoid wasps, which broadens our understanding of the chemoreception basis of parasitoid wasps and contributes to developing advanced pest management strategies. © 2022 Society of Chemical Industry.

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.

Silencing the odorant receptor co-receptor impairs olfactory reception in a sensillum-specific manner in the cockroach

Insects detect odors via a large variety of odorant receptors (ORs) expressed in olfactory sensory neurons (OSNs). The insect OR is a heteromeric complex composed of a ligand-specific receptor and the co-receptor (ORco). In this study, we identified the ORco gene of the cockroach, Periplaneta americana (PameORco), and performed RNAi-based functional analysis of PameORco. All OSNs in the basiconic sensilla expressed PameORco and received a large variety of odors including sex pheromones. In trichoid sensilla, a PameORco-positive OSN was consistently paired with a PameORco-negative OSN tuned to acids. In adult cockroaches injected with PameORco dsRNA at the nymphal stage, the expression of PameORco, odor receptions via ORs, and its central processing were strongly suppressed. These results provide new insights into the molecular basis of olfactory reception in the cockroach. The long-lasting and irreversible effects of PameORco RNAi would be an effective method for controlling the household pest.

•

Whole sequence of ORco in the American cockroach (PameORco) was characterized

•

PameORco expressed in olfactory sensory neurons in a sensillar type-specific manner

•

RNAi chronically and irreversibly suppressed the PameORco expression beyond molts

•

PameORco was essential for receptions of sex pheromones and general odors

Molecular characterization and expression variation of the odorant receptor co-receptor in the Formosan subterranean termite

Subterranean termites live in underground colonies with a division of labor among castes (i.e., queens and kings, workers, and soldiers). The function of social colonies relies on sophisticated chemical communication. Olfaction, the sense of smell from food, pathogens, and colony members, plays an important role in their social life. Olfactory plasticity in insects can be induced by long- and short-term environmental perturbations, allowing adaptive responses to the chemical environment according to their physiological and behavioral state. However, there is a paucity of information on the molecular basis of olfaction in termites. In this study, we identified an ortholog encoding the odorant receptor co-receptor (Orco) in the Formosan subterranean termite, Coptotermes formosanus, and examined its expression variation across developmental stages and in response to social conditions. We found that C. formosanus Orco showed conserved sequence and structure compared with other insects. Spatial and temporal analyses showed that the Orco gene was primarily expressed in the antennae, and it was expressed in eggs and all postembryonic developmental stages. The antennal expression of Orco was upregulated in alates (winged reproductives) compared with workers and soldiers. Further, the expression of Orco decreased in workers after starvation for seven days, but it was not affected by the absence of soldiers or different group sizes. Our study reveals the molecular characteristics of Orco in a termite, and the results suggest a link between olfactory sensitivity and nutritional status. Further studies are warranted to better understand the role of Orco in olfactory plasticity and behavioral response.

Identification and Tissue Expression Profiles of Odorant Receptor Genes in the Green Peach Aphid Myzus persicae

The green peach aphid Myzus persicae (Hemiptera: Aphididae) relies heavily on its olfactory system to locate plant hosts, find mates, and avoid parasitoids or predators. The insect odorant receptors (ORs) have been proven to play a critical role in the perception of odorants from the environment. In the present study, 33 odorant receptor candidate genes including the Orco gene were identified from the antennal, head, legs and body transcriptomes of M. persicae. Phylogenetic analysis of ORs from seven different orders of insect species suggests that ORs from different insect species are highly divergent and most ORs from the same species formed monophyletic groups. In addition, the aphid ORs were clustered into six different sub-clades in the same clade. Furthermore, the genomic structure of the OR genes also tends to be consistent, suggesting that ORs from the family Aphididae have a relatively close evolutionary relationship. Reads per kilobase per million (RPKM) and tissue expression profiles analyses revealed that 27 out of the 33 MperORs were uniquely or primarily expressed in the antennae, indicating their putative roles in chemoreception. This work provides a foundation to further investigate the molecular and ecological functions of MperORs in the aphid–aphid, aphid–plant and aphid–natural enemy interactions.

Time-Dependent Odorant Sensitivity Modulation in Insects

Simple Summary

Insects, including blood-feeding female mosquitoes, can transmit deadly diseases, such as malaria, encephalitis, dengue, and yellow fever. Insects use olfaction to locate food sources, mates, and hosts. The nature of odorant plumes poses a challenge for insects in locating odorant sources in the environment. In order to modulate the system for the detection of fresh stimuli or changes in odorant concentrations, the olfaction system desensitizes to different concentrations and durations of stimuli. Without this ability, the chemotaxis behaviors of insects are defective. Thus, understanding how insects adjust their olfactory response dynamics to parse the chemical language of the external environment is not only a basic biology question but also has far-reaching implications for repellents and pest control.

Abstract

Insects use olfaction to detect ecologically relevant chemicals in their environment. To maintain useful responses over a variety of stimuli, olfactory receptor neurons are desensitized to prolonged or high concentrations of stimuli. Depending on the timescale, the desensitization is classified as short-term, which typically spans a few seconds; or long-term, which spans from minutes to hours. Compared with the well-studied mechanisms of desensitization in vertebrate olfactory neurons, the mechanisms underlying invertebrate olfactory sensitivity regulation remain poorly understood. Recently, using a large-scale functional screen, a conserved critical receptor phosphorylation site has been identified in the model insect Drosophila melanogaster, providing new insight into the molecular basis of desensitization in insects. Here, we summarize the progress in this area and provide perspectives on future directions to determine the molecular mechanisms that orchestrate the desensitization in insect olfaction.

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

Simple Summary

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

Abstract

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

Ligand-binding properties of odorant-binding protein 6 in Athetis lepigone to sex pheromones and maize volatiles

BACKGROUND

Athetis lepigone, a noctuid moth feeding on more than 30 different crops worldwide, has evolved a sophisticated, sensitive, and specific chemosensory system to detect and discriminate exogenous chemicals. Odorant-binding proteins (OBPs) are the most important agent in insect chemosensory systems to be explored as an alternative target for environmentally friendly approaches to pest management.

RESULTS

To investigate the olfactory function of A. lepigone OBPs (AlepOBPs), AlepOBP6 was identified and expressed in Escherichia coli. The binding affinity of the recombinant OBP to 20 different ligands was then examined using a competitive binding approach. The results revealed that AlepOBP6 can bind to two sex pheromones and ten maize volatiles, and its conformation stability is pH dependent. We also carried out a structure-function study using different molecular approaches, including structure modeling, molecular docking, and a mutation functional assay to identify amino acid residues (M39, V68, W106, Q107, and Y114) involved in the binding of AlepOBP6 to both sex pheromones and maize volatiles in A. lepigone.

CONCLUSION

These results suggest that AlepOBP6 is likely involved in mediating the responses of A. lepigone to sex pheromones and maize volatiles, which may play a pivotal function in mating, feeding, and oviposition behaviors. This study not only provides new insight into the binding mechanism of OBPs to sex pheromones and host volatiles in moths, but also contributes to the discovery of novel target candidates for developing efficient behavior disruptors to control A. lepigone in the future. © 2021 Society of Chemical Industry.

RNAi-Mediated Screening of Selected Target Genes Against Culex quinquefasciatus (Diptera: Culicidae)

Culex quinquefasciatus, a member of the Culex pipiens complex, is widespread in Saudi Arabia and other parts of the world. It is a vector for lymphatic filariasis, Rift Valley fever, and West Nile virus. Studies have shown the deleterious effect of RNA interference (RNAi)-mediated knockdown of various lethal genes in model and agricultural pest insects. RNAi was proposed as a tool for mosquito control with a focus on Aedes aegypti and Anopheles gambiae. In this study, we examined the effect of RNAi of selected target genes on both larval mortality and adult emergence of Cx. quinquefasciatus through two delivery methods: soaking and nanoparticles. Ten candidate genes were selected for RNAi based on their known lethal effect in other insects. Disruption of three genes, chitin synthase-1, inhibitor of apoptosis 1, and vacuolar adenosine triphosphatase, resulted in the highest mortality among the selected genes using the two treatment methods. Silencing the other seven genes resulted in a medium to low mortality in both assays. These three genes are also active against a wide range of insects and could be used for RNAi-based mosquito control in the future.

Three chemosensory proteins from the sweet potato weevil, Cylas formicarius, are involved in the perception of host plant volatiles

BACKGROUND

Chemosensory proteins (CSPs) play important roles in chemical communication, but their precise physiological functions are still unclear. Cylas formicarius is the most serious pest attacking sweet potato around the world. At present, there is no effective way to control this pest.

RESULTS

Our results showed that CforCSP1, 5 and 6 genes were highly expressed in the antennae of both sexes of C. formicarius. In addition, results from a fluorescence competitive binding assay showed that the CforCSP1, 5 and 6 proteins had high binding affinities for 17 plant volatiles including eight host plant volatiles. This indicated that the three proteins may be involved in the detection of host plant volatiles. Furthermore, results from four-arm olfactometer bioassays showed that there was a significant tendency for C. formicarius to be attracted to eucalyptol, β-carotene, benzaldehyde, vanillin and phenethyl alcohol, while it was repelled by β-ionone. Finally, the levels of expression of the three CforCSPs in C. formicarius were successfully inhibited by RNA interference (RNAi). Behavioral experiments showed that CforCSP1, 5 and 6-deficient C. formicarius were partly anosmic to β-cyclocitral, benzaldehyde, octyl aldehyde, and β-ionone and exhibited a reduced ability to locate the host plant volatiles β-carotene and vanillin.

CONCLUSION

Our data suggest that CforCSP1, 5 and 6 likely are involved in the chemical communication between C. formicarius and host plant volatiles, which may play pivotal roles in oviposition and feeding site preferences. More importantly, these results could provide information for the development of monitoring and push-pull strategies for the control of C. formicarius. © 2021 Society of Chemical Industry.

Functional analysis of the odorant receptor coreceptor in odor detection in Grapholita molesta (lepidoptera: Tortricidae)

The olfactory system must detect and discriminate various semiochemicals in the environment. In response to such diversity, insects have evolved a family of odorant-gated ion channels composed of a common receptor (coreceptor, Orco) and a ligand-binding tuning odorant receptor (OR) that confers odour specificity. This study aims to examine the expression pattern of Orco gene of Grapholita molesta (GmolOrco) and to elucidate the role of GmolOrco in detecting G. molesta sex pheromone and green leaf volatiles by using gene silencing via RNA interference (RNAi) coupled antennal electrophysiological (EAG). Multiple sequence alignment showed that GmolOrco shared high sequence similarities with the Orco ortholog of lepidopterans. The results of real-time quantitative PCR detection demonstrated that GmolOrco was predominantly expressed in adult antennae and had the highest expression quantity in adult period among the different developmental stages. Compared with the noninjected controls, GmolOrco expression in GmolOrcodouble-stranded RNA (dsRNA)-injected males was reduced to 39.92% and that in females was reduced to 40.43%. EAG assays showed that the responses of GmolOrco-dsRNA injected males to sex pheromones (Z)-8-dodecenyl acetate (Z8-12:OAc) and (Z)-8-dodecenyl alcohol (Z8-12:OH) were significantly reduced, and the GmolOrco-dsRNA-injected female to green leaf volatile (Z)-3-hexenyl acetate also significantly declined. We inferred that Orco-mediated olfaction was different in male and female G. molesta adults and was mainly involved in recognizing the sex pheromones released by female moths.

The Olfactory Chemosensation of Hematophagous Hemipteran Insects

As one of the most abundant insect orders on earth, most Hemipteran insects are phytophagous, with the few hematophagous exceptions falling into two families: Cimicidae, such as bed bugs, and Reduviidae, such as kissing bugs. Many of these blood-feeding hemipteran insects are known to be realistic or potential disease vectors, presenting both physical and psychological risks for public health. Considerable researches into the interactions between hemipteran insects such as kissing bugs and bed bugs and their human hosts have revealed important information that deepens our understanding of their chemical ecology and olfactory physiology. Sensory mechanisms in the peripheral olfactory system of both insects have now been characterized, with a particular emphasis on their olfactory sensory neurons and odorant receptors. This review summarizes the findings of recent studies of both kissing bugs (including Rhodnius prolixus and Triatoma infestans) and bed bugs (Cimex lectularius), focusing on their chemical ecology and peripheral olfactory systems. Potential chemosensation-based applications for the management of these Hemipteran insect vectors are also discussed.

Cyclic nucleotide-dependent ionic currents in olfactory receptor neurons of the hawkmoth Manduca sexta suggest pull-push sensitivity modulation

Olfactory receptor neurons (ORNs) of the hawkmoth Manduca sexta sensitize via cAMP- and adapt via cGMP-dependent mechanisms. Perforated patch clamp recordings distinguished 11 currents in these ORNs. Derivatives of cAMP and/or cGMP antagonistically affected three of five K⁺ currents and two non-specific cation currents. The Ca²⁺ -dependent K⁺ current I_K(Ca ²⁺ ₎ and the sensitive pheromone-dependent K⁺ current I_K(cGMP-) , which both express fast kinetics, were inhibited by 8bcGMP, while a slow K⁺ current, I_K(cGMP+) , was activated by 8bcGMP. Furthermore, application of 8bcAMP blocked slowly activating, zero mV-reversing, non-specific cation currents, I_LL and I_cat(PKC?) , which remained activated in the presence of 8bcGMP. Their activations pull the membrane potential towards their 0-mV reversal potentials, in addition to increasing intracellular Ca²⁺ levels voltage- and I_LL -dependently. Twenty minutes after application, 8bcGMP blocked a TEA-independent K⁺ current, I_K(noTEA) , and a fast cation current, I_cat(nRP) , which both shift the membrane potential to negative values. We conclude that conditions of sensitization are maintained at high levels of cAMP, via specific opening/closure of ion channels that allow for fast kinetics, hyperpolarized membrane potentials, and low intracellular Ca²⁺ levels. In contrast, adaptation is supported via cGMP, which antagonizes cAMP, opening Ca²⁺ -permeable channels with slow kinetics that stabilize depolarized resting potentials. The antagonistic modulation of peripheral sensory neurons by cAMP or cGMP is reminiscent of pull-push mechanisms of neuromodulation at central synapses underlying metaplasticity.

Smelling in the dark: Phylogenomic insights into the chemosensory system of a subterranean beetle

The chemosensory system has experienced relevant changes in subterranean animals, facilitating the perception of specific chemical signals critical to survival in their particular environment. However, the genomic basis of chemoreception in cave-dwelling fauna has been largely unexplored. We generated de novo transcriptomes for antennae and body samples of the troglobitic beetle Speonomus longicornis (whose characters suggest an extreme adaptation to a deep subterranean environment) in order to investigate the evolutionary origin and diversification of the chemosensory gene repertoire across coleopterans through a phylogenomic approach. Our results suggested a diminished diversity of odourant and gustatory gene repertoires compared to polyphagous beetles that inhabit surface habitats. Moreover, S. longicornis showed a large diversity of odourant-binding proteins, suggesting an important role of these proteins in capturing airborne chemical cues. We identified a gene duplication of the ionotropic coreceptor IR25a, a highly conserved single-copy gene in protostomes involved in thermal and humidity sensing. In addition, no homologous genes to sugar receptors or the ionotropic receptor IR41a were detected. Our findings suggest that the chemosensory gene repertoire of this cave beetle may result from adaptation to the highly specific ecological niche it occupies, and that gene duplication and loss may have played an important role in the evolution of gene families involved in chemoreception. Altogether, our results shed light on the genomic basis of chemoreception in a cave-dwelling invertebrate and pave the road towards understanding the genomic underpinnings of adaptation to the subterranean lifestyle at a deeper level.

Antennal transcriptome sequencing and identification of candidate chemoreceptor proteins from an invasive pest, the American palm weevil, Rhynchophorus palmarum

For decades, the American palm weevil (APW), Rhynchophorus palmarum, has been a threat to coconut and oil palm production in the Americas. It has recently spread towards North America, endangering ornamental palms, and the expanding date palm production. Its behavior presents several parallelisms with a closely related species, R. ferrugineus, the red palm weevil (RPW), which is the biggest threat to palms in Asia and Europe. For both species, semiochemicals have been used for management. However, their control is far from complete. We generated an adult antennal transcriptome from APW and annotated chemosensory related gene families to obtain a better understanding of these species' olfaction mechanism. We identified unigenes encoding 37 odorant-binding proteins (OBPs), ten chemosensory proteins (CSPs), four sensory neuron membrane proteins (SNMPs), seven gustatory receptors (GRs), 63 odorant receptors (ORs), and 28 ionotropic receptors (IRs). Noticeably, we find out the R. ferrugineus pheromone-binding protein and pheromone receptor orthologs from R. palmarum. Candidate genes identified and annotated in this study allow us to compare these palm weevils' chemosensory gene sets. Most importantly, this study provides the foundation for functional studies that could materialize as novel pest management strategies.

Characterizing the Role of Orco Gene in Detecting Aggregation Pheromone and Food Resources in Protaetia brevitarsis Leiws (Coleoptera: Scarabaeidae)

An accurate olfactory system for recognizing semiochemicals and environmental chemical signals plays crucial roles in survival and reproduction of insects. Among all olfaction-related proteins, olfactory receptors (ORs) contribute to the conversion of chemical stimuli to electric signals and thereby are vital in odorant recognition. Olfactory receptor co-receptor (Orco), one of the most conserved ORs, is extremely essential in recognizing odorants through forming a ligand-gated ion channel complex with conventional ligand-binding odorant receptors. We have previously identified aggregation pheromone in Protaetia brevitarsis (Coleoptera: Scarabaeidae), a native agricultural and horticultural pest in East-Asia. However, to our best knowledge, its olfaction recognition mechanisms are still veiled. To illustrate how P. brevitarsis recognize aggregation pheromone and host plants, in the present study we cloned and sequenced the full-length Orco gene from P. brevitarsis antennae (named PbreOrco) and found that PbreOrco is highly conserved and similar to Orcos from other Coleoptera insects. Our real-time quantitative PCR (qRT-PCR) results showed that PbreOrco is mainly expressed in antenna. We also demonstrated that silencing PbreOrco using RNA interference through injecting dsOrco fragment significantly inhibited PbreOrco expression in comparison with injecting control dsGFP and subsequently revealed using electroantennogram and behavioral bioassays that decreasing PbreOrco transcript abundance significantly impaired the responses of P. brevitarsis to intraspecific aggregation pheromone and prolonged the time of P. brevitarsis spending on food seeking. Overall, our results demonstrated that PbreOrco is crucial in mediating odorant perception in P. brevitarsis.

Insights into chemosensory genes of Pagiophloeus tsushimanus adults using transcriptome and qRT-PCR analysis

Pagiophloeus tsushimanus is a new, destructive, and monophagous weevil pest that thrives on Cinnamomum camphora, found in Shanghai. The functions of chemosensory genes involved in the host location and intraspecific communication of P. tsushimanus remain unknown. The male-female transcriptomes of P. tsushimanus adults were assembled using Illumina sequencing, and we focused on all chemosensory genes in transcriptomes. In general, 58,088 unigenes with a mean length of 1018.19 bp were obtained. In total, 39 odorant binding proteins (OBPs), 10 chemosensory proteins (CSPs), 22 olfactory receptors (ORs), 16 gustatory receptors (GRs), eight ionotropic receptors (IRs), and five sensory neuron membrane proteins (SNMPs) were identified. PtsuOBPs comprised four subfamilies (20 Minus-C, one Plus-C, two Dimer, and 15 Classic). Both PtsuOBPs and PtsuCSPs contained a highly conserved sequence motif of cysteine residues. PtsuORs including one olfactory receptor co-receptors (Ptsu/Orco) comprised seven predicted transmembrane domains. Phylogenetic analysis revealed that PtsuOBPs, PtsuCSPs, and PtsuORs in P. tsushimanus exhibited low homology compared to other insect species. The results of tissue- and sex-specific expression patterns indicated that PtsuOBPs and PtsuORs were highly abundant in the antennae; whereas, PtsuCSPs were not only highly abundant in antennae, but also abdominal apexes, wings, and legs. In conclusion, these results enrich the gene database of P. tsushimanus, which may serve as a basis for identifying novel targets to disrupt olfactory key genes and may provide a reverse validation method to identify attractants for formulating potential eco-friendly control strategies for this pest.

Host plants transfer induced regulation of the chemosensory genes repertoire in the alfalfa plant bug Adelphocoris lineolatus (Goeze)

The alfalfa plant bug Adelphocoris lineolatus, an economically important pest, has representative behavioral characteristics with host plants transfer. Olfactory system is essential for insects to perceive ever-changing chemical signals in the external environment, and chemosensory genes play crucial roles in signals reception and transduction. In this work, we compared the differences in chemosensory genes expression before and after host plants transfer by constructing 12 antennal transcriptomes of male and female bugs, respectively. The results showed that the expression levels of most chemosensory genes in A. lineolatus changed to adapt to the transformation of the hosts plant. More remarkable, female bugs had more up-regulated chemosensory genes than males. Differentially expressed genes (DEGs) analysis revealed three odorant binding proteins (OBPs), three chemosensory proteins (CSPs), eight odorant receptors (ORs) and one ionotropic receptor (IR) showed significant differences when the host plant transferred. There were complex characteristics of up- and down- regulated genes in male and female adults, among which OBP19 showed higher expression in females exposing to the new host plant alfalfa, suggesting this OBP may be associated with the localization of the oviposition site. The OR54 and OR82 were up-regulated in both genders, indicating their possible roles in recognizing some alfalfa-specific volatiles. These findings will provide valuable insights in biological functions of chemosensory genes in A. lineolatus and facilitate the development of new targets for novel strategies to control the alfalfa plant bug and other herbivores.

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.

Anosmic flies: what Orco silencing does to olive fruit flies

Background

The olive fruit fly (Bactrocera oleae) is the most destructive pest of the olive cultivation worldwide causing significant production losses and olive fruit impoverishment, as its larvae feed exclusively on the olive fruit. Reproductive and sexual behavior, as well as host-plant recognition of the fly, are highly dependent on its chemosensory system. Therefore, exploring the role of genes that play a critical role in olfaction, could reveal potential molecular targets that determine species-specific features on chemical communication and could be used to impair sexual behavior.

Results

In this study we identified the gene that encodes the conserved olfactory co-receptor Orco (Odorant receptor co-receptor), which interacts with all divergent insect odorant receptors, and investigated how disruption of its expression affects chemoreception. We initially searched the expression profile of Bo-Orco in both sexes during sexual maturation, as well as pre- and post-mating communication by relative quantitative real time PCR (qRT-PCR) analysis suggesting that Bo-Orco was abundantly expressed in sexually mature adults.

We further investigated the functional role of Bo-Orco in mating and oviposition behavior via transient gene silencing that was performed through in vivo dsRNA hemolymph injections in sexually mature flies 7 days after eclosion. Orco-knockdown phenotypes in both sexes showed reduced copulation rates in mating competitiveness tests, possibly through impaired olfactory-mediated detection of sex pheromone. In addition, oviposition was significantly inhibited in dsRNA-Orco injected females in a post-mating behavior test.

Conclusions

Our results demonstrate that Orco plays a crucial role in the reproductive behavior of the olive fruit fly, since pre- and post-mating processes were affected. This is the first report in the olive fruit fly that links the chemosensory pathway with the mating behavior and the reproductive potential at a molecular basis, rendering this gene a potential target for the improvement of the olive fruit fly population control techniques.

Identification and expression of chemosensory receptor genes in the egg parasitoid Trissolcus basalis

Parasitic wasps largely rely on chemosenses to locate resources. Understanding the evolution of their chemoreceptors can help elucidate the mechanisms underlying host adaptation and speciation. Trissolcus basalis is a biological control agent of the southern green stink bug, a pantropical pest, and is ideal for investigating the evolution of chemoreceptors. We identified 34 gustatory receptors, 170 odorant receptors, one odorant co-receptor, and 23 ionotropic receptors. Comparison with other Hymenoptera revealed species-specific expansions of 21 Grs and 53 Ors. Most of these Or expansions have 9 exons. Gender- and tissue-specific analyses showed that 5 Grs and 54 Ors are expressed only in antennae in both sexes, 66 Ors in female antennae only, and 4 Ors in male antennae alone. The identification and expression profile of chemosensory receptor genes in T. basalis helps in understanding the link between the evolution of chemoreceptors and speciation in parasitic wasps.

Olfactory co-receptor is involved in host recognition and oviposition in Ophraella communa (Coleoptera: Chrysomelidae)

Common ragweed (Ambrosia artemisiifolia) is a notorious invasive weed that has spread across most temperate regions of the world. The beetle (Ophraella communa) is considered to be an effective control agent against A. artemisiifolia. As an oligophagous insect, its olfactory system is extremely important for host seeking in the wild. To the best of our knowledge, there is no report on the molecular mechanisms underlying olfaction recognition in this beetle. Hence, in this study, we characterized the odorant receptor co-receptor of O. communa and named it as 'OcomORco'. Real-time quantitative PCR (qRT-PCR) showed that, compared to the control treatment, RNA interference (RNAi) strongly reduced the expression of OcomORco by 89% in male and 90% in female beetles. Electroantennogram assay showed that the antennal response of both male and female beetles to four volatiles of A. artemisiifolia was significantly reduced. The injected male or female beetles lost their preference for plant leaves as observed in the behavioural tests. In addition, disruption of the expression of OcomORco resulted in a reduction of oviposition, while there was no difference in larval hatching rate between control and knockdown females. We demonstrated that OcomORco plays a vital role in olfactory perception and host search in O. communa, and it is involved in oviposition in an indirect way.

Genetic Underpinnings of Host Manipulation by Ophiocordyceps as Revealed by Comparative Transcriptomics

Ant-infecting Ophiocordyceps fungi are globally distributed, host manipulating, specialist parasites that drive aberrant behaviors in infected ants, at a lethal cost to the host. An apparent increase in activity and wandering behaviors precedes a final summiting and biting behavior onto vegetation, which positions the manipulated ant in a site beneficial for fungal growth and transmission. We investigated the genetic underpinnings of host manipulation by: (i) producing a high-quality hybrid assembly and annotation of the Ophiocordyceps camponoti-floridani genome, (ii) conducting laboratory infections coupled with RNAseq of O. camponoti-floridani and its host, Camponotus floridanus, and (iii) comparing these data to RNAseq data of Ophiocordyceps kimflemingiae and Camponotus castaneus as a powerful method to identify gene expression patterns that suggest shared behavioral manipulation mechanisms across Ophiocordyceps-ant species interactions. We propose differentially expressed genes tied to ant neurobiology, odor response, circadian rhythms, and foraging behavior may result by activity of putative fungal effectors such as enterotoxins, aflatrem, and mechanisms disrupting feeding behaviors in the ant.

The olfactory coreceptor IR8a governs larval feces-mediated competition avoidance in a hawkmoth

Finding a suitable oviposition site is a challenging task for a gravid female moth. At the same time, it is of paramount importance considering the limited capability of most caterpillars to relocate to alternative host plants. The hawkmoth, Manduca sexta, oviposits on solanaceous plants. Larvae hatching on a plant that is already attacked by conspecific caterpillars face food competition. Here, we show that feces from conspecific caterpillars are sufficient to deter a female M. sexta from ovipositing on a plant. Furthermore, we not only identify the responsible compound in the feces but also localize the population of sensory neurons that governs the female’s avoidance. Hence, our work increases the understanding of how animals cope with a competitive environment.

Finding a suitable oviposition site is a challenging task for a gravid female moth. At the same time, it is of paramount importance considering the limited capability of most caterpillars to relocate to alternative host plants. The hawkmoth, Manduca sexta (Sphingidae), oviposits on solanaceous plants. Larvae hatching on a plant that is already attacked by conspecific caterpillars can face food competition, as well as an increased exposure to predators and induced plant defenses. Here, we show that feces from conspecific caterpillars are sufficient to deter a female M. sexta from ovipositing on a plant and that this deterrence is based on the feces-emitted carboxylic acids 3-methylpentanoic acid and hexanoic acid. Using a combination of genome editing (CRISPR-Cas9), electrophysiological recordings, calcium imaging, and behavioral analyses, we demonstrate that ionotropic receptor 8a (IR8a) is essential for acid-mediated feces avoidance in ovipositing hawkmoths.