Structural basis for odorant recognition of the insect odorant receptor OR-Orco heterocomplex

Olfactory receptors in neural regeneration in the central nervous system

Olfactory receptors are crucial for detecting odors and play a vital role in our sense of smell, influencing behaviors from food choices to emotional memories. These receptors also contribute to our perception of flavor and have potential applications in medical diagnostics and environmental monitoring. The ability of the olfactory system to regenerate its sensory neurons provides a unique model to study neural regeneration, a phenomenon largely absent in the central nervous system. Insights gained from how olfactory neurons continuously replace themselves and reestablish functional connections can provide strategies to promote similar regenerative processes in the central nervous system, where damage often results in permanent deficits. Understanding the molecular and cellular mechanisms underpinning olfactory neuron regeneration could pave the way for developing therapeutic approaches to treat spinal cord injuries and neurodegenerative diseases like Alzheimer's disease. Olfactory receptors are found in almost any cell of every organ/tissue of the mammalian body. This ectopic expression provides insights into the chemical structures that can activate olfactory receptors. In addition to odors, olfactory receptors in ectopic expression may respond to endogenous compounds and molecules produced by mucosal colonizing microbiota. The analysis of the function of olfactory receptors in ectopic expression provides valuable information on the signaling pathway engaged upon receptor activation and the receptor's role in proliferation and cell differentiation mechanisms. This review explores the ectopic expression of olfactory receptors and the role they may play in neural regeneration within the central nervous system, with particular attention to compounds that can activate these receptors to initiate regenerative processes. Evidence suggests that olfactory receptors could serve as potential therapeutic targets for enhancing neural repair and recovery following central nervous system injuries.

Key Volatiles and ORs Mediating Oviposition Preference for Maize Plants in Chinese Population of Spodoptera frugiperda

Spodoptera frugiperda is a global pest that shows distinct preferences between maize and rice hosts. Concerning the prevailing S. frugiperda corn strains in China, we identified five maize-specific volatiles through GC-MS analysis of volatiles from maize and rice seedlings, in which benzothiazole and nonadecane were confirmed as key volatiles mediating attractants for female moths. Then, we screened for candidate odorant receptors (ORs) by analyzing differentially expressed genes between the moths exposed and unexposed to the active volatiles. Furthermore, we revealed via a Xenopus oocyte system that OR58 is narrowly tuned to benzothiazole, while OR25 exhibits a broader ligand spectrum with weak response to benzothiazole. Finally, molecular docking demonstrated OR58 and OR25 bind benzothiazole via hydrogen bonds at Thr307 and Trp157 residues, respectively, explaining their different selectivity patterns. These results provide molecular insights into host-plant preference mechanisms in S. frugiperda and identify potential targets for developing novel pest management approaches.

Odorant receptor 75 is essential for attractive response to plant volatile p-anisaldehyde in Western flower thrips

The Western flower thrip (WFT), Frankliniella occidentalis, is a major pest of many vegetable crops and also a vector for the tomato spotted wilt virus, causing devastating damage worldwide. Odorant receptors (ORs) play an important role in host plant searching, however, specific functions of those ORs in WFT remain unclear. In this study, the attractive activity of four plant volatiles ((S)-(-)-verbenone, p-anisaldehyde, methyl isonicotinate, and benzaldehyde) to WFT was confirmed using a Y-tube olfactometer. Then, the specific receptor, OR75, was screened out as the candidate OR for these odorants, as its expression was significantly upregulated upon exposure to these odorants. Further in vitro functional assays with Xenopus oocyte expression system confirmed sensitivity of OR75 to p-anisaldehyde and three other odorants (β-ionone, undecanal and cinnamaldehyde). Of the three odorants, β-ionone was also attractive to WFT. Further, in vivo RNA interference experiments showed that the dsOR75 treated thrips lost their attractive response to p-anisaldehyde, but retained response to β-ionone. Finally, 3-D structures prediction and molecular docking showed that OR75 formed a hydrogen bond with p-anisaldehyde at His150 residue, while no hydrogen bond formed with β-ionone, undecanal or cinnamaldehyde. Taken together, OR75 plays a crucial role in perception of p-anisaldehyde, which helps us understand the host-seeking mechanisms of WFT, and provides a basis for development of olfactory based pest control strategies. This is the first report of an OR playing roles in sensing p-anisaldehyde in thrips.

Atomevo-odor: A database for understanding olfactory receptor-odorant pairs with multi-artificial intelligence methods

Interactions between olfactory receptors (ORs) and specific odorant molecules encode many distinct odors through intricate activation patterns. In this study, in order to enhance our understanding of olfactory perception, Atomevo-Odor (http://cslodordatabase.7fx.cn/), a comprehensive database for odorants, ORs, and high-quality OR-odorant responses combining experimental data and artificial intelligence prediction, was constructed. Moreover, graph theory and unsupervised learning methods were employed to classify the odorants, and the relationship between odorant functional groups and fragrance types was examined, along with the recognition mechanism of ORs for different odorant functional groups. Furthermore, a CNN-based model was developed for the OR-odorant response prediction. Finally, predictions of unseen data facilitated the identification of potentially responsive OR-odorant pairs, which allowed for further analysis of the response and recognition mechanisms of odorants by ORs. This study provides valuable insights into the design and guidance for subsequent experiments.

Structural Optimization of Aphid Repellents Based on the Low-Energy Conformation of (E)-β-Farnesene

Aphids cause significant agricultural damage, prompting the search for eco-friendly control methods. Although pheromones offer a natural alternative to synthetic insecticides, the low stability of aphid alarm pheromones limits their use. To develop more stable repellents, we computed the low-energy conformations of (E)-β-farnesene (EBF), revealing five conformations with a conserved spatial region, and used these insights to modify the para-pheromone IV-30. Molecular superimposition confirmed that IV-30 mimics EBF's conformations. A series of analogues were synthesized, and experiments showed that an ester group on the six-membered ring's left side is crucial for repellent activity, while electron-rich groups on the right side enhance it. Notably, compound V-9 demonstrated repellent efficacy comparable to EBF, deepening the understanding of structure-activity relationships and supporting the development of novel aphid repellents for integrated pest management.

Structural Basis and Recognition Mechanism of Host-Derived Volatiles by Olfactory Proteins in the Agricultural Pest Bactrocera correcta (Diptera: Tephritidae)

Host-derived volatiles play a critical role in mediating plant-insect interactions. Bactrocera correcta is a destructive pest of fruit crops. In this study, we investigated the recognition mechanisms of three key volatiles─β-caryophyllene, ethyl decanoate, and hexyl hexanoate─derived from the host fruits of B. correcta. Using transcriptomic analysis, fluorescence binding assays, molecular docking, and molecular dynamics simulations, we identified BcorOBP19d-2 as a key odorant-binding protein that binds multiple volatiles and facilitates their stabilization and transport. Odorant receptors (BcorOR7a-13, BcorOR74a-3, and BcorOR7a-3) selectively recognize these volatiles, with hydrophobic interactions as the primary driving force for binding. β-Caryophyllene exhibited the highest binding specificity with BcorOR7a-13, ethyl decanoate demonstrated the strongest binding affinity with BcorOR74a-3, and hexyl hexanoate showed moderate stability with BcorOR7a-3. These findings provide structural insights into volatile recognition in polyphagous insects and offer a basis for developing attractants or repellents for pest management.

New dimensions in the molecular genetics of insect chemoreception

Chemoreception is the foundation of olfaction and taste, which in insects underlie the detection of humans to whom they spread disease and crops that they ravage. Recent advances have provided clear and in some cases surprising new insights into the molecular genetics of chemoreception. We describe mechanisms that govern the choice of a single Odorant receptor gene by an olfactory receptor neuron in Drosophila. We highlight genetic and epigenetic mechanisms by which chemoreceptor expression can be modulated. Exitrons, RNA editing, and pseudo-pseudogenes in chemosensory systems are described. We summarize key insights from the recent structural determinations of odorant and taste receptors. Finally, new molecular components of chemosensory systems, including long noncoding RNAs, are described.

Transcriptome Analysis and Identification of Chemosensory Membrane Proteins in the Head of Euplatypus parallelus

Euplatypus parallelus is a polyphagous pest capable of harming multiple plant species. Adult beetles invade tree trunks by boring holes, which negatively impacts the trees' growth and may result in tree death. E. parallelus depends on plant volatiles to identify and locate appropriate hosts for feeding or reproduction, with its olfactory system playing a vital role in volatile detection. In this work, we applied transcriptomics, phylogenetic analysis, and expression analysis to investigate four chemosensory membrane protein gene families that play a role in olfaction in E. parallelus. Based on the annotation analysis, 41 odorant receptors (ORs), 12 gustatory receptors (GRs), 14 ionotropic receptors (IRs), and 4 sensory neuron membrane proteins (SNMPs) were identified in the head. We used differential gene expression (DGE) and fragments per kilobase per million (FPKM) values to compare the transcription levels of chemosensory membrane protein gene families between males and females. The data indicate that the chemosensory membrane protein gene families in E. parallelus exhibit different expression levels in male and female heads, with some genes showing significant differences and displaying sex-biased expression. These results offer a basis for future exploration of the functions of chemosensory membrane protein gene families in E. parallelus and offer a theoretical framework for designing innovative eco-friendly control technologies.

Molecular mechanisms based on peripheral level of vanillin recognition in Orthaga achatina (Lepidoptera: Pyralidae)

Orthaga achatina (Lepidoptera Pyralidae) is a specialist pest of the camphor tree Cinnamomum camphora. Vanillin is a volatile compound found in many plants, and its effects on insects can be either attractive or repellent, depending on the species. However, the behavioral response of Orthaga achatina to vanillin, a volatile compound emitted by camphor trees, remains unexplored. In this study, we found that vanillin attracts both male and female O. achatina adults. Fluorescence competitive binding assays further revealed that among the five odorant-binding proteins (OBPs) highly expressed in both male and female antennae, OachOBP7 exhibited the most prominent binding affinity with vanillin. Furthermore, by employing the Xenopus oocyte expression and two-electrode voltage clamp recording system (XOE-TEVC) to conduct a functional characterization of 40 ORs, vanillin was the optimal ligand for OachOR7 among all tested ligands. In addition, with the 3D structure modeling and molecular docking techniques, it was revealed that OachOR7 displayed a relatively high binding affinity (-5.5 kcal/mol), and Gln84 and Asn189 were predicted to be key amino acid residues for binding vanillin. Finally, the two amino acids were verified by site-specific mutagenesis followed by XOE-TEVC, showing that the binding ability of OR7 to vanillin was significantly reduced to 9.23 × 10-2 μM after the mutation of two amino acids. This study demonstrate vanillin's behavioral attraction to O. achatina and reveal its molecular basis, offering new possibilities for targeted pest management using this compound.

High Nitrogen Enhances Maize Susceptibility to Holotrichia parallela via β-Caryophyllene-Mediated Olfactory Recognition and Jasmonate Suppression

Excessive nitrogen application not only affects plant development but also significantly influences plant-pest interactions. This study investigates how nitrogen fertilization modifies the metabolism of maize (Zea mays) and its interaction with Holotrichia parallela, a key soil-dwelling pest. High-nitrogen (HN) conditions increased the emission of volatile organic compounds (VOCs), with β-caryophyllene and palmitic acid strongly attracting larvae behavior selection, and β-caryophyllene attracting female oviposition. RNA interference targeting HparOR19 and HparOR22 genes confirmed the role of β-caryophyllene in pest olfactory recognition. Conversely, low-nitrogen (LN) conditions stimulated jasmonic acid (JA)-related defenses, while HN promoted the production of primary metabolites such as glucose, fructose, and sucrose, which act as feeding stimulants. These findings highlight the intricate relationship among nitrogen fertilization, maize metabolism, and pest behavior, underscoring the necessity of integrating metabolic insights into effective pest management strategies.

Identification of mosquito olfactory receptors capable of detecting nitro compounds

Insects possess an advanced olfactory system capable of detecting a wide range of odors through seven-transmembrane olfactory receptors (ORs). These ORs form heteromeric complexes with olfactory receptor co-receptor, Orco, and upon binding to specific ligands, they trigger the intracellular influx of ions such as sodium and calcium. Identifying ORs that respond to chemical molecules released from explosives holds significant importance for the development of biosensors for security and humanitarian purposes. In this study, screening of 196 mosquito ORs in HEK293FT cells for intracellular calcium flux on nitro compound administrations identified ORs as sensors for 2,4-dinitrotoluene, 2-nitroaniline, 2,3-dinitrotoluene, 2,6-dinitrotoluene, and 4-amino-2,6-dinitrotoluene. The different odor response profiles exhibited by naturally occurring polymorphisms or indels in the single OR gene that we had cloned were also explored. Sequence comparisons on these natural genetic variations and heterologous expression of each variant resulted in the identification of the amino acid positions involved critically in the gain and loss of odor sensitivity. Furthermore, we found that various combinations of the identified positions and different amino acid residues artificially evolve the OR with a higher sensitivity to nitro compounds. Our findings pave the way for the development of high-performance explosive detection biosensors, significantly contributing to technological advancements in landmine clearance and other areas. Additionally, our established screening system suggests the potential for identifying insect ORs that could serve as elements for various biosensors beyond explosive detection.

Development and Perfection of Marine-Based Insecticide Biofilm for Pea Seed Protection: Experimental and Computational Approaches

This work aims to develop an insecticidal biofilm based on Calothrixin A, collagen, and chitosan for the protection of pea seeds. The main objective is to improve the ingredient concentrations maximizing the insecticidal activity of the biofilm and to study the desorption of Calothrixin A according to the diffusion parameters. Eight biofilm formulations were prepared with different concentrations of the components and tested on Sitona lineatus and Bruchus pisorum. The results show that a high concentration of Calothrixin A tended to increase insecticidal activity, although this increase was not always significant, while a higher concentration of collagen and chitosan reduced insecticidal activity, probably by limiting the diffusion of the active ingredient. The prediction models for insecticidal activity showed that the interaction of the factors had no significant impact on the responses, but the model for Sitona lineatus presented better accuracy. The diffusion tests revealed that the CB3C-5 biofilm, with high diffusion parameters, correlated with insecticidal activity. The characterization of the CB3C-5 biofilm showed adequate physical, mechanical, thermal, and structural properties for agricultural seed storage application. Moreover, the computational approach showed that Calothrixin A interacts more efficiently with the OR5-Orco complex than with the small OBP, disrupting the olfactory detection of insects. This mechanism highlights the targeting of the olfactory complex as a potential strategy to control insect pests. This research contributes to the understanding of the role of marine-based biofilms for seed protection and opens perspectives for the development of ecological solutions against insect pests, particularly in the field of sustainable agriculture.

Functional Investigation of the Receptor to the Major Pheromone Component in the C-Strain and the R-Strain of the Fall Armyworm Spodoptera frugiperda

The fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) is an important invasive polyphagous crop pest that has been invading the world since 2016. This species consists of two strains adapted to different host plants, the corn strain and the rice strain, which also exhibit differences in their mating behavior, pheromone composition, and pheromone receptor sequences. A way to monitor invasion and control this pest is the use of synthetic sex pheromones to trap adults and disturb the mate-finding process via the release of large amounts of pheromones in the air. However, the efficiency of these methods depends on the specificity and sensitivity of the corresponding pheromone receptors. Yet, only pheromone receptors of the corn strain have been characterized, and nothing is known about the specificity and sensitivity of the rice strain orthologues. To address this gap, we functionally expressed the receptors to the major sex pheromone component of the two strains in Drosophila olfactory sensory neurons and challenged them with a large panel of pheromone compounds using single-sensillum recordings. Although their sequences present subtle mutations, we revealed that they share similar response spectra and sensitivity. The implications of these results on pheromone-based pest management strategies are discussed.

Functional Characterization of an Antenna-Biased Odorant Receptor AaOr96 Involved in Tea Tree Oil Repellency Against Aedes aegypti

Numerous essential oils have been well acknowledged as eco-friendly alternatives to combat insect pests due to synthetic insecticide-induced pest resistance and environment pollution. As a highly commercial essential oil, tea tree oil exhibits excellent insecticidal and repellent activities. However, the molecular mechanism of the olfactory system mediating the tea tree oil-induced repellency against insect pests remains unknown. In our study, mosquito was used as a suitable model to examine the molecular mechanism of tea tree oil-induced repellency against insect pests. The results showed that tea tree oil exhibited excellent spatial and oviposition repellency against Aedes aegypti adults and outstanding repellency against larvae, which were conferred by the main constituent terpinen-4-ol. The reduced repellency in the Orco-/- mutant strain revealed that tea tree oil-induced repellency against mosquitoes was dependent on odorant receptor(s). Moreover, we identified one antenna-biased odorant receptor, AaOr96, that was involved in detecting constituents of tea tree oil to elicit repellency, and the predicted protein-ligand complex indicated that AaOr96 interacted with terpinen-4-ol via van der Waals forces from five key residues. Finally, knocking out AaOr96 resulted in a reduced spatial repellency against A. aegypti by tea tree oil and terpinen-4-ol, and a reduced oviposition repellency by terpinen-4-ol, but not by tea tree oil. Our study not only reveals that tea tree oil has great potential in pest management but also provides more insights into the molecular basis of repellency of essential oils.

Deorphanization of Pheromone Receptors and Discovery of a Novel Agonist for Sex Pheromone Communication in Diamondback Moths

Sex pheromone communication is an essential component of mate recognition in moths. In this study, we heterologously expressed male-biased pheromone receptors (PRs) of diamondback moths, Plutella xylostella, in Drosophila OR67d neurons and determined their responses toward sex pheromonal compounds. The neurons expressing PxylOR59, PxylOR13, and PxylOR46 specifically responded to three sex pheromone components, Z11-16:Ald, Z11-16:Ac, and Z11-16:OH, respectively. The most effective ligands of other three PRs, PxylOR47, PxylOR49, and PxylOR73 were Z11-14:Ac, Z9,E12-14:Ac, and Z9,E11-14:Ac, respectively. Interestingly, the last two PRs were also tuned to Z11-14:Ac, which was not present in the pheromone glandular extract of P. xylostella in previous studies. Y-tube olfactometer assays revealed that the sex pheromone blend mixed with Z11-14:Ac at a ratio of 100:0.1 attracted more virgin males compared to the sex pheromone blend. These findings improve our understanding of the olfactory coding mechanisms in this important pest and provide promising potential for enhancing insect capture of pheromone traps.

Novel neonicotinoid insecticide cycloxaprid exhibits sublethal toxicity to honeybee (Apis mellifera L.) workers by disturbing olfactory sensitivity and energy metabolism

The risk of neonicotinoid insecticides to honeybees is a global issue. Cycloxaprid (CYC) is a novel neonicotinoid insecticide with outstanding activities, good safety profiles, and no cross-resistance with other neonicotinoids. Information on the environmental risks of CYC is limited, especially its effects on honeybees. Herein, the acute and chronic toxicities of CYC on honeybees were evaluated, and the underlying mechanisms were explored via transcriptomics and molecular docking. The results indicate that CYC had high toxicity to honeybees, with a 48-h oral median lethal dose of 32.8 ng/bee. Over a 10-days of chronic exposure to CYC at sublethal concentration 30 μg/L, the honeybees showed significantly decreased survival rates and food consumption. Additionally, the sensitivity of honeybees to sucrose and odors and CO2 production was significantly reduced. Furthermore, molecular docking revealed that CYC has higher binding affinity than odors to odorant-binding proteins, and the olfactory and metabolism pathways gene expression was negatively affected at transcriptome level. These findings indicate that CYC at sublethal concentration can pose risks to honeybees by affecting their olfactory function and energy metabolic balance. Further study and consideration are needed to fully exploit the benefits of this pesticide.

Not All Bark Beetles Smell the Same: Population-Level Functional Olfactory Polymorphisms in Ips typographus Pheromone Receptor ItypOR33

Eurasian spruce bark beetle Ips typographus, a natural part of forest ecosystems, is a major threat to Norway spruce forests during outbreaks. Olfaction plays a crucial role in the survival and range expansion of these beetles, amid forest disturbances and climate change. As the current management strategies are suboptimal for controlling outbreaks, the reverse chemical ecology approaches based on pheromone receptors offer promising alternatives. While the search for pheromone receptors is in progress, recently found chromosomal inversions indicates signs of adaptation in this species. Our attempts to characterise one of the highly expressed odorant receptors, ItypOR33, located in an inversion, led to the discovery of polymorphic variants distributed with similar frequency across 18 European populations. Deorphanizing ItypOR33 and its variant ItypOR33a using the Drosophila empty-neuron system (DeNS) revealed ItypOR33 tuned to amitinol, a heterospecific pheromone component in Ips spp., whereas its variant tuned to (S)-(-)-ipsenol, a conspecific pheromone component of I. typographus. The in silico approaches revealed the structural basis of variations by predicting putative ligand-binding sites, tunnels and ligand-receptor interactions. However, no sex-specific differences were found in the ItypOR33 expression, and its ligand amitinol elicited behavioural and electrophysiological responses. Reporting population-level functional olfactory polymorphisms for the first time in a non-model organism-bark beetles, provides key evidence for further exploring their survival and adaptation in forests. Additionally, these findings indicate potential long-term complexities of managing bark beetles in forests.

Identification of a new lineage of pheromone receptors in mirid bugs (Heteroptera: Miridae)

Sex pheromones, typically released by females are crucial signals for the reductive biology of insects, primarily detected by sex pheromone receptors (PRs). A clade of PRs in three mirid bugs, Apolygus lucorum, Adelphocoris lineolatus, and Adelphocoris suturalis, has been found to respond to pheromones, (E)-2-hexenyl butyrate (E2HB) and hexyl butyrate (HB), with higher sensitivity to E2HB. In this study, we aimed to identify PRs responsible for the other two pheromone components, HB and (E)-4-oxo-2-hexenal (4-OHE), by using a combination of phylogenetic analyses, sequence similarity analyses, and in vitro functional studies. As a result, five new candidate PRs (AlucOR34, AlinOR9, AlinOR10, AsutOR9, and AsutOR10) positioned outside of the previously known PR clade were identified. All five PRs were found to respond to both E2HB and HB, with some PRs exhibiting a significant and sensitive binding to HB. However, PRs for 4-OHE remains unidentified. Overall, our study suggests that mirid bugs have evolved two distinct lineages of PRs with similar response profiles. This research offers valuable insights into sex pheromone recognition within the peripheral olfactory system and contributes to the identification of PRs in mirid bugs, providing new targets for developing the behavioral regulators for these insects.

A plant virus manipulates both its host plant and the insect that facilitates its transmission

Tomato yellow leaf curl virus (TYLCV), a devastating pathogen of tomato crops, is vectored by the whitefly Bemisia tabaci, yet the mechanisms underlying TYLVC epidemics are poorly understood. We found that TYLCV triggers the up-regulation of two β-myrcene biosynthesis genes in tomato, leading to the attraction of nonviruliferous B. tabaci. We also identified BtMEDOR6 as a key whitefly olfactory receptor of β-myrcene involved in the distinct preference of B. tabaci MED for TYLCV-infected plants. TYLCV inhibits the expression of BtMEDOR6, canceling this preference and thereby facilitating TYLCV transmission to uninfected plants. Greenhouse experiments corroborated the role of β-myrcene in whitefly attraction. These findings reveal a sophisticated viral strategy whereby TYLCV modulates both host plant attractiveness and vector olfactory perception to enhance its spread.

Transcriptome Profiling of Euproctis pseudoconspersa Reveals Candidate Olfactory Genes for Type III Sex Pheromone Detection

The tea tussock moth (Euproctis pseudoconspersa) is a common tea plantation pest with Type III sex pheromone components (SPCs). However, the olfactory genes involved in the perception of Type III SPCs remain unknown. To identify the olfactory genes involved in E. pseudoconspersa olfactory perception, we sequenced the transcriptomes of different tissues from male and female moths. We identified 27 chemosensory proteins, 39 odorant-binding proteins (OBPs), 28 ionotropic receptors (IRs), and 67 odorant receptors (ORs). Phylogenetic and antennal abundance analyses showed that EpseOR12, EpseOR13, EpseOR15, EpseOR16, and EpseOR18 belonged to the pheromone receptor clades of Type II moths, with predominant expression in male antennae. Besides these EpseORs, EpseOR14 and EpseOR32 were two of the most abundant EpseORs in male antennae, where they were predominantly expressed. Four pheromone-binding proteins (PBPs) were identified, with higher expression in male antennae. EpseORs and EpsePBPs may be involved in Type III SPC detection. Additionally, a few EpseOBPs, EpseIRs, and EpseORs were predominantly expressed in either male or female antennae. These genes may play important roles in olfaction and may be involved in detecting host plant volatiles and pheromones. These results provide a foundation for further exploration of the molecular mechanisms of E. pseudoconspersa olfaction.

Insect olfactory neurons: receptors, development, and function

Insects represent the most diverse group of animals in the world. While the olfactory systems of different species share general principles of organization, they also exhibit a wide range of structural and functional diversity. Scientists have gained tremendous insight into olfactory neural development and function, notably in Drosophila, but also in other insect species (see reviews by Benton, 2022; Robertson, 2019; Yan et al., 2020). In the last few years, new evidence has steadily mounted, for example, the stoichiometry of odorant receptor and co-receptor (OR-Orco) complex. This review aims to highlight the recent progress on four aspects: (1) the structure and function of the OR-Orco complex, (2) chemosensory gene co-expression, (3) diverse neural developmental processes, and (4) the role of genes and neurons in olfactory development and olfactory-mediated behavior.

Prospects on non-canonical olfaction in the mosquito and other organisms: why co-express?

The Aedes aegypti mosquito utilizes olfaction during the search for humans to bite. The attraction to human body odor is an innate behavior for this disease-vector mosquito. Many well-studied model species have olfactory systems that conform to a particular organization that is sometimes referred to as the 'one-receptor-to-one-neuron' organization because each sensory neuron expresses only a single type of olfactory receptor that imparts the neuron's chemical selectivity. This sensory architecture has become the canon in the field. This review will focus on the recent finding that the olfactory system of Ae. aegypti has a different organization, with multiple olfactory receptors co-expressed in many of its olfactory sensory neurons. We will discuss the canonical organization and how this differs from the non-canonical organization, examine examples of non-canonical olfactory systems in other species, and discuss the possible roles of receptor co-expression in odor coding in the mosquito and other organisms.

The Hoverfly Attracting Property of a Methyl Salicylate-Containing (E)-β-Farnesene Analog (3e) and Potential Mechanism by Mediating the EcorOBP15 and EcorOR3

Using natural enemies provides a sustainable method to control major agricultural pests. Hoverflies are significant natural enemies of aphids and efficient pollinators. Herbivore-induced plant volatiles (HIPVs), including (E)-β-farnesene (EBF) and methyl salicylate (MeSA), are key olfactory cues mediating hoverflies behavior. Our previous work identified compound 3e, an EBF analog containing a MeSA moiety, exhibited aphid-repelling and ladybug-attracting activities. However, whether 3e can attract hoverflies remains unknown. Therefore, this study explored the attractant property and potential mechanism of 3e toward hoverflies. Laboratory bioassays and field trials indicated 3e has an obvious hoverfly attracting property. The attraction mechanism studies demonstrate that, similar to EBF, 3e can interact with EcorOBP15 and EcorOR3, with its greater chemical softness, larger hydrophobic and charge regions enhancing these interactions. Furthermore, 3e exhibited low toxicity to honeybees (Apis mellifera) and hoverflies (Eupeodes corollae). Consequently, 3e could be a promising eco-friendly behavioral regulator for integrated aphid management in sustainable agriculture.

Harnessing Insect Chemosensory and Mechanosensory Receptors Involved in Feeding for Precision Pest Management

Chemosensation and mechanosensation are vital to insects' survival and behavior, shaping critical physiological processes such as feeding, metabolism, mating, and reproduction. During feeding, insects rely on diverse chemosensory and mechanosensory receptors to distinguish between nutritious and harmful substances, enabling them to select suitable food sources while avoiding toxins. These receptors are distributed across various body parts, allowing insects to detect environmental cues about food quality and adjust their behaviors accordingly. A deeper understanding of insect sensory physiology, especially during feeding, not only enhances our knowledge of insect biology but also offers significant opportunities for practical applications. This review highlights recent advancements in research on feeding-related sensory receptors, covering a wide range of insect species, from the model organism Drosophila melanogaster to agricultural and human pests. Additionally, this review examines the potential of targeting insect sensory receptors for precision pest control. Disrupting behaviors such as feeding and reproduction emerges as a promising strategy for pest management. By interfering with these essential behaviors, we can effectively control pest populations while minimizing environmental impacts and promoting ecological balance.

The Role of (E)-β-Farnesene in Tritrophic Interactions: Biosynthesis, Chemoreception, and Evolution

(E)-β-farnesene (EBF) stands out as a crucial volatile organic compound, exerting significant influence on the complex interactions between plants, aphids, and predator insects. Serving as an alarm signal within aphids, EBF is also emitted by plants as a defense mechanism to attract aphid predators. This review delves into EBF sources, functions, biosynthesis, detection mechanisms, and its coevolutionary impacts on aphids and insect predators. The exploration underscores the need to comprehend the biophysical and structural foundations of EBF receptors in aphids, emphasizing their role in unraveling the intricate patterns and mechanisms of interaction between EBF and target receptors. Furthermore, we advocate for adopting structure-based or machine-learning methodologies to anticipate receptor-ligand interactions. On the basis of this knowledge, we propose future research directions aiming at designing, optimizing, and screening more stable and efficient active odorants. A pivotal outcome of this comprehensive investigation aims to contribute to the development of more effective aphid-targeted control strategies.

Binary mixtures of Vanderbilt University allosteric agonist thermolysis components act as volatile spatial repellents for malaria vector mosquitoes

BACKGROUND

The development of economically viable and environmentally neutral tools to control insects that consume or damage over 20% of global agriculture or vector human and animal disease represents one of the most important challenges of the 21st century. The suite of chemical-based strategies currently employed to control insect populations rely primarily on insecticides, which are subject to rapid resistance and often have harmful off-target environmental and health-related impacts, and, to a lesser degree, repellents, which typically rely on masking attractive odors. The discovery and characterization of Vanderbilt University allosteric agonists (VUAAs), a family of small-molecule agonists that target the highly conserved, insect-specific odorant receptor coreceptor (Orco), raise the potential for the development of a novel repellent paradigm for vector/pest management. VUAAs have the potential to target nearly all insect olfactory sensory neurons, leading to highly aversive behavioral responses, but importantly have limited volatility, thereby reducing their utility as spatial repellents.

RESULTS

We have characterized VUAA thermolysis components and identified a suite of volatiles (VUAA-based active ingredients, VUAIs) that act specifically in novel binary combinations as robust and long-lasting spatial repellents against Anopheline mosquitoes. In mobility-based behavioral experiments, VUAIs act synergistically as effective spatial repellents and outperform parent VUAA compounds against host-seeking Anopheline mosquitoes.

CONCLUSIONS

VUAIs are volatile alternatives to Vanderbilt University allosteric agonists (VUAAs) that have the potential for use as spatial repellents in disease vector and agricultural pest control. The repellency observed is odorant receptor coreceptor (Orco)-dependent, supporting the hypothesis that VUAIs and VUAAs similarly target an allosteric Orco recognition site. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Identification of a Soybean Volatile Attractive for Riptortus pedestris Using Reverse Chemical Ecology Approach

The bean bug Riptortus pedestris is a major soybean pest and a cause of the stay-green symptoms. However, the molecular mechanisms underlying its olfaction-mediated host-seeking behavior remain unclear. In this study, we compared the antennae transcriptomes of starved and nonstarved adult R. pedestris, identifying four differentially expressed odorant receptor (OR) genes. Among these, RpedOR13 showed a strong response to the host volatile 2-phenylethanol (2-PE) in Xenopus oocyte assays, while electroantennography and behavioral tests confirmed 2-PE as an effective attractant. Next, phylogenetic analysis identified RpedOR72b as a paralog of RpedOR13, with subsequent Xenopus oocyte assays confirming its specific response to 2-PE. Additionally, RNA interference experiments highlighted the crucial role of RpedOR72b in detecting 2-PE. Taken together, these findings provide new insights into the molecular mechanisms of host-seeking behavior in R. pedestris and highlight the successful application of reverse chemical ecology in OR-based screening of bioactive compounds.

Comparation of pheromone-binding proteins 1 and 2 of Spodoptera frugiperda in perceiving the three sex pheromone components Z9-14:Ac, Z7-12: Ac and Z11-16: Ac

Pheromone-binding proteins (PBPs) are mainly responsible for binding and transporting hydrophobic pheromone molecules across the aqueous sensilla lymph to the receptor proteins. The preference of each PBP is believed to be different for each pheromone component within a single species. Significantly higher expression level of PBP1 and PBP2 in the male antennae of Spodoptera frugiperda suggesting that SfruPBP1 and SfruPBP2 might play important roles in pheromone perception. However, the preference of these two PBP to the three main pheromone components Z9-14: Ac, Z7-12: Ac and Z11-16: Ac have not been determined. In this study, a fluorescence competitive binding assay revealed that the binding intensities of SfruPBP1 and SfruPBP2 to Z9-14: Ac or Z7-12: Ac was comparable. We then used the CRISPR/Cas9 system to individually or simultaneously knock out PBP1 and PBP2 in S.frugiperda. The result of courtship behavior indicated that SfruPBP1 and SfruPBP2 were indispensable and played equal roles in perceiving the pheromones Z9-14: Ac and Z7-12: Ac for orientation, wing vibration, and hair-pencil display. Compared with Z9-14:Ac and Z7-12: Ac, Z11-16: Ac showed higher or medium binding intensities with SfruPBP1 and SfruPBP2 but played a minor role in inducing the wing vibration behavior. The results of this study are valuable for elucidating the mechanisms involved in sex pheromone perception and may facilitate the development of PBP-targeted pest control techniques.

Natural volatiles preventing mosquito biting: An integrated screening platform for accelerated discovery of ORco antagonists

Insect olfactory receptors are heteromeric ligand-gated cation channels composed of an obligatory receptor subunit, ORco, and one of many variable subunits, ORx, in as yet undefined molar ratios. When expressed alone ex vivo, ORco forms homotetrameric channels gated by ORco-specific ligands acting as channel agonists. Using an insect cell-based system as a functional platform for expressing mosquito odorant receptors ex vivo, we identified small molecules of natural origin acting as specific ORco channel antagonists, orthosteric or allosteric relative to a postulated ORco agonist binding site, which cause severe inhibition of olfactory function in mosquitoes. In the present communication, we have compiled common structural features of such orthosteric antagonists and developed a ligand-based pharmacophore whose properties are deemed necessary for binding to the agonist binding site and causing inhibition of ORco's biological function. In silico screening of an available collection of natural volatile compounds with the pharmacophore resulted in identification of several ORco antagonist hits. Cell-based functional screening of the same compound collection resulted in the identification of several compounds acting as orthosteric and allosteric antagonists of ORco channel function ex vivo and inducing anosmic behaviors to Aedes albopictus mosquitoes in vivo. Comparison of the in silico screening results with those of the functional assays revealed that the pharmacophore predicted correctly seven out of the eight confirmed orthosteric antagonists and none of the allosteric ones. Because the pharmacophore screen produced additional hits that did not cause inhibition of the ORco channel function, we also generated a support vector machine (SVM) model based on two descriptors of all pharmacophore hits. Training of the SVM on the ex vivo validated compound collection resulted in the selection of the confirmed orthosteric antagonists with a very low cross-validation out-of-sample misclassification rate. Employment of the combined pharmacophore-SVM platform for in silico screening of a larger collection of olfaction-relevant volatiles produced several new hits. Functional validation of randomly selected hits and rejected compounds from this screen confirmed the power of this virtual screening platform as a convenient tool for accelerating the pace of discovery of novel vector control agents. To the best of our knowledge, this study is the first one that combines a pharmacophore with a SVM model for identification of AgamORco antagonists and specifically orthosteric ones.

BEGAN: Boltzmann-Reweighted Data Augmentation for Enhanced GAN-Based Molecule Design in Insect Pheromone Receptors

Identifying small molecules that bind strongly to target proteins in rational molecular design is crucial. Machine learning techniques, such as generative adversarial networks (GAN), are now essential tools for generating such molecules. In this study, we present an enhanced method for molecule generation using objective-reinforced GANs. Specifically, we introduce BEGAN (Boltzmann-enhanced GAN), a novel approach that adjusts molecule occurrence frequencies during training based on the Boltzmann distribution exp(-ΔU/τ), where ΔU represents the estimated binding free energy derived from docking algorithms and τ is a temperature-related scaling hyperparameter. This Boltzmann reweighting process shifts the generation process toward molecules with higher binding affinities, allowing the GAN to explore molecular spaces with superior binding properties. The reweighting process can also be refined through multiple iterations without altering the overall distribution shape. To validate our approach, we apply it to the design of sex pheromone analogs targeting Spodoptera frugiperda pheromone receptor SfruOR16, illustrating that the Boltzmann reweighting significantly increases the likelihood of generating promising sex pheromone analogs with improved binding affinities to SfruOR16, further supported by atomistic molecular dynamics simulations. Furthermore, we conduct a comprehensive investigation into parameter dependencies and propose a reasonable range for the hyperparameter τ. Our method offers a promising approach for optimizing molecular generation for enhanced protein binding, potentially increasing the efficiency of molecular discovery pipelines.

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.

Peripheral Coding of Sex Pheromones in the Tomato Leaf Miner, Phthorimaea absoluta (Meyrick)

Phthorimaea absoluta releases (E3,Z8,Z11)-tetradeca-3,8,11-trienyl acetate (E3,Z8,Z11-14:OAc) and (E3,Z8)-tetradeca-3,8-dienyl acetate (E3,Z8-14:OAc) with a ratio of 90:10 as the sex pheromone. However, how this pest uses pheromone receptors (PRs) to detect the two pheromone components is unknown. Here, we functionally characterize the PR repertoire of P. absoluta. First, we identified five putative PRs by transcriptome sequencing, i.e., PabsOR4, PabsOR8, PabsOR12a, PabsOR14, and PabsOR17. These receptors are predominantly expressed in the male antennae. Next, we expressed them in Drosophila OR67 neurons and investigated their responses. PabsOR14 and PabsOR8 selectively respond to the main component, E3,Z8,Z11-14:OAc with different sensitivities, while PabsOR17 is tuned to the minor component, E3,Z8-14:OAc. In addition, PabsOR4 weakly responds to both sex pheromone components. Moreover, PabsOR17 and PabsOR4 potently respond to a non-sex pheromone compound, (Z)-7-dodecenyl acetate (Z7-12:OAc). Lastly, we demonstrated that Z7-12:OAc can replace E3,Z8-14:OAc to attract virgin males. Our findings elucidate the peripheral coding of the sex pheromone in P. absoluta, providing a new perspective for controlling it.

Functional Characterization Supports Multiple Evolutionary Origins of Pheromone Receptors in Bark Beetles

Chemical communication using pheromones is thought to have contributed to the diversification and speciation of insects. The species-specific pheromones are detected by specialized pheromone receptors (PRs). Whereas the evolution and function of PRs have been extensively studied in Lepidoptera, only a few PRs have been identified in beetles, which limits our understanding of their evolutionary histories and physiological functions. To shed light on these questions, we aimed to functionally characterize potential PRs in the spruce bark beetle Ips typographus ("Ityp") and explore their evolutionary origins and molecular interactions with ligands. Males of this species release an aggregation pheromone comprising 2-methyl-3-buten-2-ol and (4S)-cis-verbenol, which attracts both sexes to attacked trees. Using two systems for functional characterization, we show that the highly expressed odorant receptor (OR) ItypOR41 responds specifically to (4S)-cis-verbenol, with structurally similar compounds eliciting minor responses. We next targeted the closely related ItypOR40 and ItypOR45. Whereas ItypOR40 was unresponsive, ItypOR45 showed an overlapping response profile with ItypOR41, but a broader tuning. Our phylogenetic analysis shows that these ORs are present in a different OR clade as compared to all other known beetle PRs, suggesting multiple evolutionary origins of PRs in bark beetles. Next, using computational analyses and experimental validation, we reveal two amino acid residues (Gln179 and Trp310) that are important for ligand binding and pheromone specificity of ItypOR41 for (4S)-cis-verbenol, possibly via hydrogen bonding to Gln179. Collectively, our results shed new light on the origins, specificity, and ligand binding mechanisms of PRs in beetles.

Antennal Transcriptome Screening and Identification of Chemosensory Proteins in the Double-Spine European Spruce Bark Beetle, Ips duplicatus (Coleoptera: Scolytinae)

The northern bark beetle, Ips duplicatus, is an emerging economic pest, reportedly infesting various species of spruce (Picea spp.), pine (Pinus spp.), and larch (Larix spp.) in Central Europe. Recent climate changes and inconsistent forest management practices have led to the rapid spread of this species, leaving the current monitoring strategies inefficient. As understanding the molecular components of pheromone detection is key to developing novel control strategies, we generated antennal transcriptomes from males and females of this species and annotated the chemosensory proteins. We identified putative candidates for 69 odorant receptors (ORs), 50 ionotropic receptors (IRs), 25 gustatory receptors (GRs), 27 odorant-binding proteins (OBPs), including a tetramer-OBP, 9 chemosensory proteins (CSPs), and 6 sensory neuron membrane proteins (SNMPs). However, no sex-specific chemosensory genes were detected. The phylogenetic analysis revealed conserved orthology in bark beetle chemosensory proteins, especially with a major forest pest and co-habitant, Ips typographus. Recent large-scale functional studies in I. typographus chemoreceptors add greater significance to the orthologous sequences reported here. Nevertheless, identifying chemosensory genes in I. duplicatus is valuable to understanding the chemosensory system and its evolution in bark beetles (Coleoptera) and, generally, insects.

Infestation of Rice Striped Stem Borer (Chilo suppressalis) Larvae Induces Emission of Volatile Organic Compounds in Rice and Repels Female Adult Oviposition

Plants regulate the biosynthesis and emission of metabolic compounds to manage herbivorous stresses. In this study, as a destructive pest, the pre-infestation of rice striped stem borer (SSB, Chilo suppressalis) larvae on rice (Oryza sativa) reduced the subsequent SSB female adult oviposition preference. Widely targeted volatilomics and transcriptome sequencing were used to identify released volatile metabolic profiles and differentially expressed genes in SSB-infested and uninfested rice plants. SSB infestation significantly altered the accumulation of 71 volatile organic compounds (VOCs), including 13 terpenoids. A total of 7897 significantly differentially expressed genes were identified, and genes involved in the terpenoid and phenylpropanoid metabolic pathways were highly enriched. Correlation analysis revealed that DEGs in terpenoid metabolism-related pathways were likely involved in the regulation of VOC biosynthesis in SSB-infested rice plants. Furthermore, two terpenoids, (-)-carvone and cedrol, were selected to analyse the behaviour of SSB and predators. Y-tube olfactometer tests demonstrated that both (-)-carvone and cedrol could repel SSB adults at higher concentrations; (-)-carvone could simultaneously attract the natural enemies of SSB, Cotesia chilonis and Trichogramma japonicum, and cedrol could only attract T. japonicum at lower concentrations. These findings provide a better understanding of the response of rice plants to SSB and contribute to the development of new strategies to control herbivorous pests.