Sensilla Morphology and Complex Expression Pattern of Odorant Binding Proteins in the Vetch Aphid Megoura viciae (Hemiptera: Aphididae)

The morphology and molecular mechanisms of enhanced olfaction in the grain aphid Sitobion miscanthi

Winged aphids develop more sensitive olfaction than the wingless phenotype to identify potential habitat from afar. Two types of olfactory sensilla, primary rhinarium (PRh) and secondary rhinarium (SRh) are responsible for aphid olfactory perception, of which, SRh is involved in the perception of both E-β-farnesene (EBF) and plant volatiles. Odorant binding proteins (OBPs) play a vital role in the response of insect olfactory nerves located in the rhinarium to external odor stimuli. However, the role of SRhs in winged aphids have received little attention to date and the underlying mechanisms of how OBPs work for the olfactory enhancement in winged aphids are unclear. Here, we compared and analyzed the number of SRhs on the antennae of winged and wingless aphids in the grain aphid Sitobion miscanthi, then highly expressed OBPs in antennae of winged aphids were screened based on both transcriptomic and proteomic data and identified by parallel reaction monitoring parallel reaction monitoring (PRM). Furthermore, the affinity of highly expressed OBPs was tested. The results showed that winged aphids have more SRhs, and the three highly expressed OBPs, SmisOBP6/7/10, in winged antennae shared broad and overlapped ligands spectra with plant volatiles and pheromones. Furthermore, OBP9, as an OBP with high expression in antennae, not only binds EBF, but also shows extensive affinity to various ligand types. Our results highlight the importance of the higher number of SRhs as the morphological basis of enhanced olfactory perception in winged aphid and further indicates the involvement of OBP6/7/10 as the molecular support of this process.

Key amino acids in odorant-binding protein OBP7 enable Bradysia odoriphaga to recognize host plant volatiles

Bradysia odoriphaga (Diptera: Sciaridae) is a devastating underground pest that can cause serious economic losses. Odorant binding proteins (OBPs) are crucial components of the insect olfactory system, playing key roles in locating host plants, oviposition sites, and mates. Therefore, they are considered potential targets for pest control. Here, we obtained one OBP gene (BodoOBP7) from the antennal transcriptome of B. odoriphaga, and observed that the expression level of BodoOBP7 was primarily in the antennae of both sexes, with significantly higher expression level in females than in males. Fluorescence competitive binding assays indicated that BodoOBP7 exhibited strong binding affinities for the six host plant volatiles, including propyl disulfide, dipropyl trisulfide, dimethyl trisulfide, 2-tridecanone, 2-undecanone and alpha-ionone. Subsequently, homology modeling, molecular docking and site-directed mutagenesis revealed that four key amino acid residues (Phe79, Phe99, Ile96, Leu100) participate in the binding of BodoOBP7 with six host plant volatiles. Our results demonstrate that BodoOBP7 is involved in olfactory recognition in B. odoriphaga. These findings may enhance our understanding of the interaction mechanisms between host plants and B. odoriphaga, potentially offering new perspectives for the development of effective green control strategies.

Three-dimensional reconstruction of insect chemosensory sensillum

The olfactory system is involved in food and mate recognition in insects. However, 3D structures of chemosensory sensilla in insects are unexplored yet. Here, the internal structures of an olfactory sensillum on the antenna of the brown planthopper, Nilaparvata lugens (Hemiptera: Delphacidae), one of the most important rice pests, are examined and imaged using focused ion beam scanning electron microscopy. Based on these images, a 3D structure is reconstructed in this study. We find that the trichoid olfactory sensillum possesses a multiporous wall encircling a lumen with one sensory cell. Besides, there are three accessory cells (ACs) and a glia cell with different cell contents surrounding the sensory cell. The abundant tubular membrane structures in the tormogen cell suggest its role in secreting proteins like odorant binding proteins into the receptor lymph, while three auxiliary cells with simpler cellular content closely enfold the sensory cell, probably to prevent leaking of the receptor lymph into the surrounding epidermis. In the sensory cell, the microtubules and two tandem basal bodies at the base of the microtubules are also reconstructed. They are considered as a propulsive engine to ensure dendrite vibration or spinning in the receptor lymph, so that the proteins and odorant molecules move faster in the receptor lymph, which improves recognition of environmental odors and enables the insect to immediately respond to this information.

Potential of volatile organic compounds in the management of insect pests and diseases of food legumes: a comprehensive review

Cool season legumes (Faba bean, chickpea, lentil, pea, and grass pea) are important protein harvests for food and nutrition security in many countries. They play key roles in sustainable cereal production through their ecological benefits. However, diseases and pests attack continue to have a substantial impact on crop yield and quality. Although growers used different control options to manage these biotic stresses such as pesticide application, cultural practices, and resistant varieties, there is a pressing need for the development of new, more cost-effective and environmentally friendly solution to help farmers in facing the existing environmental issues. Recently, there is a growing interest among researchers in exploiting Volatile Organic Compounds (VOCs) for the elaboration of disease and pest control strategies in food legumes and other crops. These compounds have important functions in ecological relationships occurring between plants and their surrounding environment, as well as plants and others species, such as pests and pathogens. Due to their unique properties, VOCs can be employed in improving management alternatives for food legume diseases and pests. In this assessment, we investigated the role of VOCs in plant-pest and plant-pathogen interactions and their present applications in pest and diseases control strategies. We emphasized the ecological importance of employing plant VOCs in legume farming and crop breeding. Additionally, we highlighted the potential of microbial VOCs in facilitating microbe-microbe, microbe-plant and microbe-plant-pest interactions, along with their role in food legume protection.

Odorant-binding protein 19 in Monochamus alternatus involved in the recognition of a volatile strongly emitted from ovipositing host pines

Monochamus alternatus is the primary carrier of pine wood nematodes, which pose a serious threat to Pinus spp. in many countries. Newly emerging M. alternatus adults feed on heathy host pines, while matured adults transfer to stressed host pines for mating and oviposition. Several odorant-binding proteins (OBPs) of M. alternatus have been proved to aid in the complex process of host location. To clarify the corresponding relations between OBPs and pine volatiles, more OBPs need to be studied. In this research, MaltOBP19 showed a specific expression in the antennae and mouthparts of M. alternatus, and it was marked in 4 types of antenna sensilla by immunolocalization. Fluorescence binding assays demonstrated the high binding affinity of MaltOBP19 with camphene and myrcene in vitro. In Y-tube olfactory experiments, M. alternatus adults were attracted by camphene and RNAi of OBP19 via microinjection significantly decreased their attraction index. Myrcene induced phobotaxis, but RNAi had no significant effect on this behavior. Further, we found that ingesting dsOBP19 produced by a bacteria-expressed system with a newly constructed vector could lead to the knockdown of MaltOBP19. These results suggest that MaltOBP19 may play a role in the process of host conversion via the recognition of camphene, which has been identified to be strongly released in stressed host pines. In addition, it is proved that knockdown of OBP can be achieved by oral administration of bacteria-expressed double-stranded RNA in M. alternatus adults, providing a new perspective in the control of M. alternatus.

Functional characterization of alkaline phosphatases involved alarm pheromone in the vetch aphid Megoura viciae

The alkaline phosphatases (ALPs) are highly promiscuous enzymes and have been extensively investigated in mammals for their medical significance, but their functional promiscuity is relatively poorly understood in insects. Here, we first identified four ALP genes (designated as MvALP1-4) in the vetch aphid Megoura viciae that contained one alkaline phosphatase site, three metal-binding sites, and varied other functional sites. Phylogenetic analysis, molecular docking and the spatiotemporal expression profiling of MvALP1-4 were very different, indicating a promiscuous functionality. We also found that MvALP4 involved the biosynthesis of aphid alarm pheromones (EβF) in vitro and in vivo. Finally, transcriptome analysis in the stimulated and unstimulated aphids supported the involvement of MvALPs in the biosynthesis of aphid alarm pheromones. Our study identified a multifunctional ALP involved terpene synthase enzyme activity in the aphid, which contributes to the understanding of the functional plasticity of ALPs in insects.

Secondary Rhinaria Contribute to Major Sexual Dimorphism of Antennae in the Aphid Semiaphis heraclei (Takahashi)

Sexual generation is an important generation in the life cycle of host-alternating aphids, and its population size determines the intensity of the peak in the next spring. Although male trapping techniques based on olfactory stimuli have been successfully established in the field, the biological basis of olfactory perception in males is unclear. In this study, we compared the morphology of antennae and the types, sizes, numbers, and distribution of sensilla between males and sexual females in the host-alternating aphid Semiaphis heraclei (Hemiptera: Aphididae). We found that flagellum length differentiation contributed to the majority of the sexual dimorphism of antennae. Most sensillum types or subtypes, including trichoid sensilla subtype I, campaniform sensilla, and primary rhinaria subtypes I and II, were enlarged in males. In addition, males bore more trichoid sensilla subtype I than sexual females. In particular, secondary rhinaria were present in males only and could not be detected in sexual females. These results revealed the structural basis of male olfactory perception. Our findings provide insight into the mechanism underlying chemical communication between sexual aphids and could thus be useful for pest control.

Volatile Organic Compounds: A Promising Tool for Bed Bug Detection

The recent decades' resurgence of bed bugs as a public health concern in industrialized countries has driven an increased interest on new sustainable insecticide-free methods to monitor and control these ectoparasites. Current methods of detection rely mainly on visual inspection or canine scent detection, which are methods that are time-consuming, require experience, are non-specific or require costly mission repetitions. Volatile organic compounds (VOCs) are considered an environmentally friendly alternative and a promising approach for bed bug detection. An overview of the released literature on VOCs, their chemical characteristics and their role in bed bugs' intra- and inter-species communications allowed us to highlight the identification of 49 VOCs in Cimex lectularius (23 molecules) and C. hemipterus (26), which are emitted by both sexes during diverse compartments including aggregation (46), mating (11), defense (4), etc., and all life stages including exuviae or dead bed bugs as a principal indicator of infestation. The latter has a great importance for application of these semiochemicals in successful detection and control management of bed bugs and to prevent their further dispersion. This approach has the advantage of more reliability compared to conventional detection methods with no need for repeated inspections, household furniture moving or resident rehousing for bed bugs' VOC detection, which are commonly performed by active or passive sampling with absorbing tubes and analyzed by gas chromatography-based analytical platforms.

Functional analysis of odorant-binding proteins for the parasitic host location to implicate convergent evolution between the grain aphid and its parasitoid Aphidius gifuensis

(E)-β-farnesene (EBF) is a typical and ecologically important infochemical in tri-trophic level interactions among plant-aphid-natural enemies. However, the molecular mechanisms by which parasitoids recognize and utilize EBF are unclear. In this study, we functionally characterized 8 AgifOBPs in Aphidifus gifuensis, one dominant endo-parasitoid of wheat aphid as well as peach aphid in China. Among which, AgifOBP6 was the only OBP upregulated by various doses of EBF, and it showed a strong binding affinity to EBF in vitro. The lack of homology between AgifOBP6 and EBF-binding proteins from aphids or from other aphid natural enemies supported that this was a convergent evolution among insects from different orders driven by EBF. Molecular docking of AgifOBP6 with EBF revealed key interacting residues and hydrophobic forces as the main forces. AgifOBP6 is widely expressed among various antennal sensilla. Furthermore, two bioassays indicated that trace EBF may promote the biological control efficiency of A. gifuensis, especially on winged aphids. In summary, this study reveals an OBP (AgifOBP6) that may play a leading role in aphid alarm pheromone detection by parasitoids and offers a new perspective on aphid biological control by using EBF. These results will improve our understanding of tri-trophic level interactions among plant-aphid-natural enemies.

A Recipe for Success: Three Key Strategies Used by Aphids and Pseudomonas syringae to Colonize the Phyllosphere

Aphids and Pseudomonas syringae are a permanent challenge for agriculture, causing severe losses to the crop industry worldwide. Despite the obvious phylogenetic distance between them, both have become predominant colonizers of the plant kingdom. In this study, we reviewed three key steps of spread and colonization that aphids and P. syringae have mastered to successfully colonize the phyllosphere. These steps involve (i) plant-to-plant movement for locating new nutritional sources, (ii) disruption and modification of the apoplast to facilitate nutrient acquisition, and (iii) suppression of host defenses through effector proteins. In addition, we will provide insights about the direct interaction between aphids and P. syringae and how this yet underrated phenomenon could bring new ecological implications for both organisms beyond their pathogenicity.

Comparative morphology and plant volatile responses of antennal sensilla in Cinara cedri (Hemiptera: Lachninae), Eriosoma lanigerum (Hemiptera: Eriosomatinae), and Therioaphis trifolii (Hemiptera: Calaphidinae)

Olfaction is important for mediating aphid behaviors and is involved in host location and mating. Antennal primary rhinaria play a key role in the chemoreception of aphids. The function of the peripheral olfactory system in the subfamily Aphidinae has been intensively studied, but little is known about other subfamilies of Aphididae. Therefore, three aphid species were selected to study the olfactory reception of plant volatiles: Cinara cedri (Lachninae), Eriosoma lanigerum (Eriosomatinae), and Therioaphis trifolii (Calaphidinae). In this study, the morphology and distribution of the antennal sensilla of apterous adults were observed by scanning electron microscopy. Three morphological types were identified (placoid sensilla, coeloconic sensilla, and trichoid sensilla); the first two were distributed on the antennal primary rhinaria. A pattern of primary rhinaria in C. cedri was found that differed from that of E. lanigerum and T. trifolii and consists of 1 large placoid sensillum (LP) on the 4th segment, 2 LPs on the 5th segment, and a group of sensilla on the 6th antennal segments. Later, we recorded and compared neuronal responses of the distinct placoid sensilla in the primary rhinaria of the three aphid species to 18 plant volatiles using a single sensillum recording (SSR) technique. The results indicated that the functional profiles based on the tested odorants of the primary rhinaria of the three investigated aphid species were clustered into three classes, and exhibited excitatory responses to certain types of odorants, especially terpenes. In C. cedri, the ORNs in LP6 exhibited the highest responses to (±)-citronellal across all tested chemicals, and showed greater sensitivity to (±)-citronellal than to (+)-limonene. ORNs in LP5 were partially responsive to α-pinene and (-)-β-pinene in a dose-dependent manner. Across different species, E. lanigerum showed significantly stronger neuronal responses of LP5 to several terpenes, such as (-)-linalool and α-terpineol, compared to other species. In T. trifolii, the neuronal activities in LP6 showed a greater response to methyl salicylate as compared to LP5. Overall, our results preliminarily illustrate the functional divergence of ORNs in the primary rhinaria of aphids from three subfamilies of Aphididae and provide a basis for better understanding the mechanism of olfactory recognition in aphids.

EsigGOBP1: The Key Protein Binding Alpha-Phellandrene in Endoclita signifer Larvae

Endoclita signifer larvae show olfactory recognition towards volatiles of eucalyptus trunks and humus soils. Further, EsigGOBP1 was identified through larval head transcriptome and speculated as the main odorant-binding proteins in E. signifer larvae. In this study, the highest expression of EsigGOBP1 was only expressed in the heads of 3rd instar larvae of E. signifer, compared with the thorax and abdomen; this was consistent with the phenomenon of habitat transfer of 3rd instar larvae, indicating that EsigGOBP1 was a key OBP gene in E. signifer larvae. Results of fluorescence competition binding assays (FCBA) showed that EsigGOBP1 had high binding affinities to eight GC-EAD active ligands. Furthermore, screening of key active odorants for EsigGOBP1 and molecular docking analysis, indicated that EsigGOBP1 showed high binding activity to alpha-phellandrene in 3rd instar larvae of E. signifer. Conformational analysis of the EsigGOBP1-alpha-phellandrene complex, showed that MET49 and GLU38 were the key sites involved in binding. These results demonstrated that EsigGOBP1 is a key odorant-binding protein in E. signifer larvae, which recognizes and transports eight key volatiles from eucalyptus trunk, especially the main eucalyptus trunks volatile, alpha-phellandrene. Taken together, our results showed that EsigGOBP1 is involved in host selection of E. signifer larvae, which would aid in developing EsigGOBP1 as molecular targets for controlling pests at the larval stage.

Functional characterization of two different decaprenyl diphosphate synthases in the vetch aphid Megoura viciae

Long-chain polyprenyl diphosphate synthases play a critical role in the formation of the prenyl side-chain of ubiquinones, but up to date, their functions have scarcely been characterized in insects. Here, we first cloned the complementary DNAs encoding the subunits of decaprenyl diphosphate synthase (DPPS) in the vetch aphid Megoura viciae, an important agricultural pest insect. The results showed that there existed three DPPS subunits, designated as MvDPPS1, MvDPPS2a, and MvDPPS2b, with an open reading frame of 1218, 1275, and 1290 bp, and a theoretical isoelectric point of 7.91, 6.63, and 9.62, respectively. The sequences of MvDPPS1s from different aphid species were nearly identical, while the sequences of MvDPPS2a and MvDPPS2b shared only moderate sequence similarity. Phylogenetic analysis clearly separated MvDPPS2a and MvDPPS2b, indicating a functional differentiation between them. Functional coexpression analysis in Escherichia coli showed that MvDPPS1 plus MvDPPS2a and MvDPPS1 plus MvDPPS2b, respectively, catalyzed the formation of the prenyl side-chain of the ubiquinone coenzyme Q10 (CoQ10). Interestingly, MvDPPS1 plus MvDPPS2b catalyzed the formation of the prenyl side-chain of a ubiquinone other than CoQ10. RNA interference-mediated knockdown of MvDPPS2a imposed no significant effect on MvDPPS2b, and vice versa, suggesting no compensatory action between them. In the end, we detected the product CoQ10 in the aphid, the first identification of CoQ10 in an insect species. Taken together, we characterized two functional DPPSs in M. viciae, one of which might be multifunctional. Our study helps to understand the functional plasticity of the terpenoid backbone biosynthesis pathway in insects.

The genome of the rice planthopper egg parasitoid wasps Anagrus nilaparvatae casts light on the chemo- and mechanosensation in parasitism

BACKGROUND

Mymaridae is an ancient insect group and is a basal lineage of the superfamily Chalcidoidea. Species of Mymaridae have great potential for biological control. Anagrus nilaparvatae, a representative species of Mymaridae, is ideal for controlling rice planthopper due to its high rate of parasitism and ability to find hosts efficiently in paddy ridges and fields.

RESULTS

Using both PacBio single-molecule real-time and Illumina sequencing, we sequenced and assembled the whole genome of A. nilaparvatae, a first for the family Mymaridae. The assembly consists of 394 scaffolds, totaling 488.8 Mb. The assembly is of high continuity and completeness, indicated by the N50 value of 25.4 Mb and 98.2% mapping rate of Benchmarking Universal Single-Copy Orthologs. In total, 16,894 protein-coding genes in the genome were annotated. A phylogenomic tree constructed for A. nilaparvatae and other 12 species of Hymenoptera confirmed that the family Mymaridae is sister to all remaining chalcidoids. The divergence time between A. nilaparvatae and the other seven Chalcidoidea species was dated at ~ 126.9 Mya. Chemoreceptor and mechanoreceptor genes are important in explaining parasitic behavior. We identified 17 odorant binding proteins, 11 chemosensory proteins, four Niemann-Pick type C2 proteins, 88 olfactory receptors, 12 gustatory receptors, 22 ionotropic receptors and 13 sensory neuron membrane proteins in the genome of A. nilaparvatae, which are associated with the chemosensory functions. Strikingly, there is only one pickpocket receptors and nine transient receptor potential genes in the genome that have a mechanosensory function.

CONCLUSIONS

We obtained a high-quality genome assembly for A. nilaparvatae using PacBio single-molecule real-time sequencing, which provides phylogenomic insights for its evolutionary history. The small numbers of chemo- and mechanosensory genes in A. nilaparvatae indicate the species-specific host detection and oviposition behavior of A. nilaparvatae might be regulated by relatively simple molecular pathways.

Enantiomeric Discrimination in Insects: The Role of OBPs and ORs

Olfaction is a complex recognition process that is critical for chemical communication in insects. Though some insect species are capable of discrimination between compounds that are structurally similar, little is understood about how this high level of discrimination arises. Some insects rely on discriminating between enantiomers of a compound, demonstrating an ability for highly selective recognition. The role of two major peripheral olfactory proteins in insect olfaction, i.e., odorant-binding proteins (OBPs) and odorant receptors (ORs) has been extensively studied. OBPs and ORs have variable discrimination capabilities, with some found to display highly specialized binding capability, whilst others exhibit promiscuous binding activity. A deeper understanding of how odorant-protein interactions induce a response in an insect relies on further analysis such as structural studies. In this review, we explore the potential role of OBPs and ORs in highly specific recognition, specifically enantiomeric discrimination. We summarize the state of research into OBP and OR function and focus on reported examples in the literature of clear enantiomeric discrimination by these proteins.

Computational investigation of aphid odorant receptor structure and binding function

Odorant receptors (OR) play a critical role in signal transduction and olfactory recognition in insects. Unfortunately, insect ORs are difficult to express and purify, and limited structural data are available. Computational methods were used to predict models for aphid ORs, and binding interactions with aphid pheromones and other semiochemicals were investigated. Previously functionally characterised ORs from the pea aphid, Acyrthosiphon pisum, ApisOR4 and ApisOR5, were screened against functional ligands. ApisOR5 had a defined binding site, and had predicted interactions with the aphid alarm pheromone, (E)-β-farnesene. ApisOR4 had multiple distinct binding sites and showed broad tuning to multiple odorants. Screening of six other highly conserved ORs showed some interactions and potential enantiomeric discrimination between the aphid sex pheromone components (4aS,7S,7aR)-nepetalactone and (1R,4aS,7S,7aR)-nepetalactol. These results indicate that specific binding sites may be more critical to understanding olfactory activity of ligands and ORs than kinetic data, and greater knowledge of the method of action of ORs is required.Communicated by Ramaswamy H. Sarma.

Transcriptome analysis and molecular characterization of soluble chemical communication proteins in the parasitoid wasp Anagrus nilaparvatae (Hymenoptera: Mymaridae)

Anagrus nilaparvatae is an important egg parasitoid wasp of pests such as the rice planthopper. Based on the powerful olfactory system of sensing chemical information in nature, A. nilaparvatae shows complicated life activities and behaviors, such as feeding, mating, and hosting. We constructed a full‐length transcriptome library and used this to identify the characteristics of soluble chemical communication proteins. Through full‐length transcriptome sequencing, splicing, assembly, and data correction by Illumina, we obtained 163.59 Mb of transcriptome data and 501,179 items with annotation information. We then performed Gene Ontology (GO) functional classification of the transcriptome's unigenes. We analyzed the sequence characteristics of soluble chemical communication protein genes and identified eight genes: AnilOBP2, AnilOBP9, AnilOBP23, AnilOBP56, AnilOBP83, AnilCSP5, AnilCSP6, and AnilNPC2. After sequence alignment and conserved domain prediction, the eight proteins encoded by the eight genes above were found to be consistent with the typical characteristics of odorant‐binding proteins (OBPs), chemosensory proteins (CSPs), and Niemann‐pick type C2 proteins (NPC2s) in other insects. Phylogenetic tree analysis showed that the eight genes share low homology with other species of Hymenoptera. Quantitative real‐time polymerase chain reaction (RT‐qPCR) was used to analyze the expression responses of the eight genes in different sexes and upon stimulation by volatile organic compounds. The relative expression levels of AnilOBP9, AnilOBP26, AnilOBP83, AnilCSP5, and AnilNPC2 in males were significantly higher than those in females, while the relative expression level of AnilCSP6 was higher in females. The expression levels of AnilOBP9 and AnilCSP6 were significantly altered by the stimulation of β‐caryophyllene, suggesting that these two genes may be related to host detection. This study provides the first data for A. nilaparvatae's transcriptome and the molecular characteristics of soluble chemical communication proteins, as well as an opportunity for understanding how A. nilaparvatae behaviors are mediated via soluble chemical communication proteins.

Appraisal of MsepCSP14 for chemosensory functions in Mythimna separata

Chemosensory proteins (CSPs) have great contributions in performing diverse functions in insects. However, physiological appraisal of chemosensory protein genes still remains elusive in insects. We studied expression patterns and binding affinities of MsepCSP14, a chemosensory protein, in Mythimna separata. The distinct functions of MsepCSP14 were validated by employing different molecular techniques. The MsepCSP14 had high resemblance of sequence with chemosensory proteins of other insect family members. The MsepCSP14 expression was higher in antennal tissues of females than other tissues. Fluorescence binding assay validated that binding of nine out of 21 ligands to MsepCSP14 was higher at pH 7.4 than at pH 5.0. Three dimensional modeling (3D) and docking analysis predicted that amino acid residues of MsepCSP14 were involved in binding of compounds, and behavior assay displayed that adults of M. separata considerably responded to four volatiles from compounds demonstrating strong binding ability to MsepCSP14. Results of the present study suggest that MsepCSP14 is likely to mediate chemosensory functions in M. separata.

Polyphenism in Antennal Sensilla Among Different Adult Morphs of Nonhost-Alternating, Holocyclic Sitobion avenae (Hemiptera: Aphididae)

Aphids, mainly distributed in temperate zones, exhibit seasonal generation-alternating phenomena. Across the life cycle, different morphs are produced. Sitobion avenae (Fabricius 1775) is a major pest of wheat worldwide. To elucidate olfactory perception of morph-specific behavior across their life cycle, we investigated antennal sensilla among seven morphs using scanning electron microscopy. Trichoid, placoid, coeloconic, and campaniform sensilla were identified. Trichoid sensilla, big multiporous placoid sensilla (primary rhinarium), a group of sensilla (primary rhinaria), and campaniform sensilla showed similar distribution and resemblance among morphs, whereas small multiporous placoid sensilla (secondary rhinaria) exhibited obvious differences. Compared to apterous morphs, alate morphs possessed a greater abundance of secondary rhinaria, with the greatest found in males on antennal segments III-V. Alate virginoparae and alate sexuparae ranged from six to fourteen rhinaria on antennal segment III. Fundatrices, apterous virginoparae and apterous sexuparae only had one or two secondary rhinaria on antennal segment III while they disappeared in oviparae. Secondary rhinaria, lying in a cuticle cavity, are convex or concave in their central part. In males, both forms were present, with a greater proportion of convex form than that of the concave form. Fundatrices and virginoparae had the convex form while sexuparae had the concave form. Polyphenism of secondary rhinaria might suggest their association with the olfactory functions of morph-specific behavior. These results have improved our understanding of the adaptive evolution of the antennal sensilla in nonhost-alternating, holocyclic aphids.

Genetic knockout and general odorant-binding/chemosensory protein interactions: Revealing the function and importance of GOBP2 in the yellow peach moth's olfactory system

The GOBP2 protein has a unique function in the yellow peach moth (Conogethes punctiferalis (Guenée)). Several general odorant-binding proteins (GOBPs) have been identified in various lepidopteran species, but the functional difference between GOBP1 and GOBP2 in recognition of host plant odorants is still unknown. The functions of GOBP1 and GOBP2 in the yellow peach moth were evaluated in this study by using the CRISPR-Cas9 system. The results revealed the importance of GOBP2 in the olfaction mechanism in the yellow peach moth. The perception of the GOBP1-knockout larvae toward feeding decreased but did not reach a significant level while knocking out the GOBP2 and GOBP1/2 genes resulted in huge differences. On the other hand, electroantennograms (EAGs) and wind tunnel tests showed that the sensitivity of GOBP2 knockout adults to odorants decreased more than that of GOBP1 knockout individuals. The results of STRING database text mining grabbed our attention in protein-protein interaction studies. In this research, we first proved the existence of physical interactions between GOBPs and chemosensory proteins (CSPs) through the surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC) methods. Interestingly, GOBP1 and GOBP2 could not interact with each other, but they could interact with CSPs. The interaction results indicated that GOBP2 could physically interact with CSP15, CSP5, and OBP17, whereas GOBP1 could bind only with CSP5 and CSP10, and its association constant (k_a) was also more substantial than that of GOBP1. These results strongly suggest the importance of the function of GOBP2 in the perception of host plant odorants by the yellow peach moth.

Function and Characterization Analysis of BodoOBP8 from Bradysia odoriphaga (Diptera: Sciaridae) in the Recognition of Plant Volatiles and Sex Pheromones

The belowground pest Bradysia odoriphaga (Diptera: Sciaridae) has a sophisticated and sensitive olfactory system to detect semiochemical signals from the surrounding environment. In particular, odorant-binding proteins (OBPs) are crucial in capturing and transporting these semiochemical signals across the sensilla lymph to the corresponding odorant receptors. In this study, we cloned a full-length cDNA sequence of BodoOBP8 from B. odoriphaga. Real-time PCR (qRT-PCR) analysis revealed that BodoOBP8 has the highest expression levels in males, with more pronounced expression in the male antennae than in other tissues. In this study, the recombinant protein BodoOBP8 was successfully expressed by a bacterial system to explore its function. Competitive binding assays with 33 host plant volatiles and one putative sex pheromone (n-heptadecane) revealed that purified BodoOBP8 strongly bound to two sulfur compounds (methyl allyl disulfide and diallyl disulfide) and to n-heptadecane; the corresponding dissolution constants (Ki) were 4.04, 6.73, and 4.04 μM, respectively. Molecular docking indicated that Ile96, Ile103, Ala107, and Leu111, located in the hydrophobic cavity of BodoOBP8, are the key residues mediating the interaction of BodoOBP8 with two sulfur compounds (methyl allyl disulfide and diallyl disulfide) and n-heptadecane. These results show that BodoOBP8 plays a role in the recognition of plant volatiles and sex pheromones, suggesting its application as a molecular target for the screening of B. odoriphaga attractants and repellents and facilitating a new mechanism of B. odoriphaga control.

Antennal and palpal sensilla of three predatory Lispe species (Diptera: Muscidae): an ultrastructural investigation

Antennae and maxillary palps are the most important chemical reception organs of flies. So far, the morphology of antennae and maxillary palps of flies of most feeding habits have been well described, except for that of relatively rare aquatic predatory species. This study describes sensilla on antennae and maxillary palps of three aquatic predatory Lispe species: Lispe longicollis, L. orientalis and L. pygmaea. Types, distribution, and density of sensilla are characterised via light and scanning electron microscopy. One type of mechanoreceptors is found on antennal scape. Mechanoreceptors (two subtypes) and one single pedicellar button (in L. pygmaea) are located on antennal pedicel. Four types of sensilla are discovered on antennal postpedicel: trichoid sensilla, basiconic sensilla (three subtypes), coeloconic sensilla and clavate sensilla. A unique character of these Lispe species is that the coeloconic sensilla are distributed sparsely on antennal postpedicel. Mechanoreceptors and basiconic sensilla are observed on the surface of maxillary palps in all three species. We demonstrated clear sexual dimorphism of the maxillary palps in some of the Lispe species, unlike most other Muscidae species, are larger in males than females. This, along with their courtship dance behaviour, suggest their function as both chemical signal receiver and visual signal conveyer, which is among the few records of a chemical reception organ act as a signal conveyer in insects.

Novel Temporal Expression Patterns of EBF-Binding Proteins in Wing Morphs of The Grain Aphid Sitobion miscanthi

High chemosensitivity of insects to volatile organic compounds (VOC) stimuli is mediated by odorant binding proteins (OBPs). In aphids, three OBPs (OBP3, OBP7 and OBP9) are E-β-farnesene (EBF)-binding proteins. Winged aphids are generally more sensitive than wingless aphids to VOCs, thus, wing presence is a phenotypic correlate of olfaction sensitivity. Here, we investigate the detailed temporal expression of these EBF-binding proteins and two other OBPs (OBP6 and OBP10), in the grain aphid Sitobion miscanthi 0 h, 2 h, 1 day, 3 days, 10 days, and 20 days after adult emergence. Both winged and wingless aphids were examined to further uncover phenotypic specification. Then, the expression patterns before and after EBF induction were analyzed. Throughout adulthood, only OBP7 had significantly higher antennal expression in winged aphids; however, there was no significant difference in the antennal expression of OBP3 between wing morphs at most time points. Except it was lower in newly emerged winged aphids but increased rapidly to the same level in wingless aphids at 1 day. OBP9 did not differ in expression between the morphs and was the only OBP that did not exhibit an expression trough at the beginning of the adult stage (0 h). The expression of OBP9 remained relatively stable and high throughout the adult stage in both phenotypes, showing the highest level among the three EBF-binding proteins. After EBF induction, its expression was further up-regulated in both morphs. Therefore, this protein may be an important molecule for EBF recognition in aphids. OBP7 strongly responded to EBF but only in winged aphids, suggesting that this protein is important in the more sensitive EBF recognition process of winged aphids. In addition, the antennal expression level of OBP3 did not respond to EBF induction. These findings revealed a temporal expression pattern of OBPs in aphids and showed that figuring out the pattern is critical for correctly selecting morphs and sampling times, which will support the discovery of reliable findings and allow solid conclusions to be drawn. Our findings also inspire on the interaction mode of the three EBF-binding proteins in relation to EBF perception in aphids.

Hermetia illucens (L.) (Diptera: Stratiomyidae) Odorant Binding Proteins and Their Interactions with Selected Volatile Organic Compounds: An In Silico Approach

The black soldier fly (BSF), Hermetia illucens (Diptera: Stratiomyidae), has considerable global interest due to its outstanding capacity in bioconverting organic waste to insect biomass, which can be used for livestock, poultry, and aquaculture feed. Mass production of this insect in colonies requires the development of methods concentrating oviposition in specific collection devices, while the mass production of larvae and disposing of waste may require substrates that are more palatable and more attractive to the insects. In insects, chemoreception plays an essential role throughout their life cycle, responding to an array of chemical, biological and environmental signals to locate and select food, mates, oviposition sites and avoid predators. To interpret these signals, insects use an arsenal of molecular components, including small proteins called odorant binding proteins (OBPs). Next generation sequencing was used to identify genes involved in chemoreception during the larval and adult stage of BSF, with particular attention to OBPs. The analysis of the de novo adult and larval transcriptome led to the identification of 27 and 31 OBPs for adults and larvae, respectively. Among these OBPs, 15 were common in larval and adult transcriptomes and the tertiary structures of 8 selected OBPs were modelled. In silico docking of ligands confirms the potential interaction with VOCs of interest. Starting from the information about the growth performance of H. illucens on different organic substrates from the agri-food sector, the present work demonstrates a possible correlation between a pool of selected VOCs, emitted by those substrates that are attractive for H. illucens females when searching for oviposition sites, as well as phagostimulants for larvae. The binding affinities between OBPs and selected ligands calculated by in silico modelling may indicate a correlation among OBPs, VOCs and behavioural preferences that will be the basis for further analysis.

Identification and Behavioral Assays of Alarm Pheromone in the Vetch Aphid Megoura viciae

Aphids are destructive pests, and alarm pheromones play a key role in their chemical ecology. Here, we conducted a detailed analysis of terpenoids in the vetch aphid, Megoura viciae, and its host plant Pisum sativum using gas chromatography/mass spectrometry. Four major components, (-)-β-pinene (49.74%), (E)-β-farnesene (32.64%), (-)-α-pinene (9.42%) and ( +)-limonene (5.24%), along with trace amounts of ( +)-sabinene, camphene and α-terpineol) (3.14%) were found in the aphid. In contrast, few terpenoids were found in the host plant, consisting mainly of squalene (66.13%) and its analog 2,3-epoxysqualene (31.59%). Quantitative analysis of the four major terpenes in different developmental stages of the aphid showed that amounts of the monoterpenes increased with increasing stage, while the sesquiterpene amount peaked in the 3rd instar. (-)-β-Pinene was the most abundant terpene at all developmental stages. Behavioral assays using a three-compartment olfactometer revealed that the repellency of single compounds varied in a concentration-dependent manner, but two mixtures [(-)-α-pinene: (-)-β-pinene: (E)-β-farnesene: ( +)-limonene = 1:44.4:6.5:2.2 or 1:18.4:1.3:0.8], were repellent at all concentrations tested. Our results suggest that (-)-α-pinene and (-)-β-pinene are the major active components of the alarm pheromone of M. viciae, but that mixtures play a key role in the alarm response. Our study contributes to the understanding of the chemical ecology of aphids and may help design new control strategies against this aphid pest.

A Detailed Spatial Expression Analysis of Wing Phenotypes Reveals Novel Patterns of Odorant Binding Proteins in the Soybean Aphid, Aphis glycines

The wide range of insect niches has led to a rapid expansion of chemosensory gene families as well as their relatively independent evolution and a high variation. Previous studies have revealed some functions for odorant-binding proteins (OBPs) in processes beyond olfaction, such as gustation and reproduction. In this study, a comparative transcriptomic analysis strategy was applied for the soybean aphid, Aphis glycines, focusing on various functional tissues and organs of winged aphids, including the antenna, head, leg, wing, thorax, cauda, and cornicle. Detailed spatial OBP expression patterns in winged and wingless parthenogenetic aphids were detected by RT-qPCR. Twelve OBPs were identified, and three new OBPs in A. glycines are first reported. All OBPs showed comparatively higher expression in sensory organs and tissues, such as the antenna, head, or leg. Additionally, we found some novel expression patterns for aphid OBPs (Beckendorf et al., 2008). Five OBPs exhibited high-expression levels in the cauda and four in the cornicle (Biasio et al., 2015). Three genes (OBP2/3/15) were highly expressed in the wing (Calvello et al., 2003). Two (OBP3/15) were significantly more highly expressed in the wingless thorax than in the winged thorax with the wings removed, and these transcripts were significantly enriched in the removed wings. More details regarding OBP spatial expression were revealed under our strategy. These findings supported the existence of carrier transport functions other than for foreign chemicals and therefore broader ligand ranges of aphid OBPs. It is important for understanding how insect OBPs function in chemical perception as well as their other potential physiological functions.

Identification and expression profiles analysis of odorant-binding proteins in soybean aphid, Aphis glycines (Hemiptera: Aphididae)

The soybean aphid, Aphis glycines, is an extreme specialist and an important invasive pest that relies on olfaction for behaviors such as feeding, mating, and foraging. Odorant-binding proteins (OBPs) play a vital role in olfaction by binding to volatile compounds and by regulating insect sensing of the environment. In this work we used rapid amplification of complementary DNA ends technology to identify and characterize 10 genes encoding A. glycines OBPs (AglyOBPs) belonging to 3 subfamilies, including 4 classic OBPs, 5 Plus-C OBPs, and one Minus-C OBP. Quantitative real-time polymerase chain reaction demonstrated variable specific expression patterns for the 10 genes based on developmental stage and aphid tissue sampled. Expression levels of 7 AglyOBPs (2, 3, 4, 5, 7, 9, and 10) were highest in the 4th instar, indicating that the 4th nymphal instar is an important developmental period during which soybean aphids regulate feeding and search for host plants. Tissue-specific expression results demonstrated that AglyOBP2, 7, and 9 exhibited significantly higher expression levels in antennae. Meanwhile, ligand-binding analysis of 5 OBPs demonstrated binding of AglyOBP2 and AglyOBP3 to a broad spectrum of volatiles released by green leaf plants, with bias toward 6- to 8-carbon chain volatiles and strong binding of AglyOBP7 to trans-β-farnesene. Taken together, our findings build a foundation of knowledge for use in the study of molecular olfaction mechanisms and provide insights to guide future soybean aphid research.

Sensory Receptors Associated with the Labial Tip and Precibarium of Philaenus spumarius L. (Hemiptera: Aphrophoride)

The meadow spittlebug, Philaenus spumarius (Linnaeus) (Hemiptera: Aphrophoridae), is an important vector for the xylem-limited bacterium Xylella fastidiosa (Wells, Raju, Hung, Weisburg, Mandelco-Paul, and Brenner), which is associated with olive quick decline syndrome in southern Italy. The mouthparts of Hemiptera have important roles in host plant selection, feeding behavior and for vectoring pathogens that cause plant diseases. In this study, the functional morphology of the sensory structures located on the labium tip and precibarium of P. spumarius was investigated using scanning and transmission electron microscopy. The labium tip is composed of two symmetrical sensory complexes, each with five different types of sensilla: aporous sensilla trichodea type 1 and 2; uniporous sensilla chaetica type 1 and 2; and multiporous sensilla basiconica. The precibarium of P. spumarius has two kinds of sensory structures: bulbous sensilla and papillae sensilla. In particular, two groups of sensilla are located on the epipharynx: a distal group that consists of ten papillae sensilla and a proximal group composed of six papillae sensilla and two bulbous sensilla, while the hypopharynx has only two papillae sensilla. The involvement of these sensory structures in the context of feeding behavior and pathogen transmission is discussed.

Evolution and functional analysis of odorant-binding proteins in three rice planthoppers: Nilaparvata lugens, Sogatella furcifera, and Laodelphax striatellus

BACKGROUND

The white-backed planthopper (WBPH) Sogatella furcifera, the brown planthopper (BPH) Nilaparvata lugens, and the small brown planthopper (SBPH) Laodelphax striatellus are three notorious rice pests that cause annual losses in rice yield through sap-sucking and virus transmission. Odorant-binding proteins (OBPs) are crucial olfactory genes involved in host-seeking behavior.

RESULTS

We discovered the presence of 12, 12, and 16 OBPs in WBPH, BPH, and SBPH, respectively, including two novel OBPs in BPH and seven novel OBPs in SBPH. Phylogenetic analysis indicated that most of these OBPs have homologous genes, and one group (SfurOBP11, NlugOBP8, and LstrOBP2) show a slower evolution rate and are more conserved. Further, in vitro binding studies demonstrated that the three OBPs have similar binding affinities for some rice plant volatiles. Finally, RNA interference (RNAi) successfully inhibited the mRNA expression of the three OBPs, and in vivo behavioral tests showed that the OBP-deficient rice planthoppers were partly anosmic and lost some of their ability to locate rice plants.

CONCLUSION

These results demonstrate the crucial role of the rice planthopper OBP genes in seeking rice plants. This information complements the current genetic resources for the development of RNAi-based transgenic rice and other pest management technologies. © 2018 Society of Chemical Industry.

The chemoreceptors and odorant binding proteins of the soybean and pea aphids

We examined the genome of the soybean aphid, Aphis glycines, and an updated genome assembly of the pea aphid, Acyrthosiphon pisum, for members of the three major families of chemoreceptors, the Odorant Receptors (ORs), Gustatory Receptors (GRs) and Ionotropic Receptors (IRs), as well as the Odorant Binding Proteins (OBPs). The soybean aphid has 47 ORs, 61 GRs, 19 IRs, and 10 OBPs, compared with 87 ORs, 78 Grs, 19 IRs, and 18 OBPs in the pea aphid, with variable numbers of pseudogenes in the OR and GR families. Phylogenetic analysis reveals that while all of the IRs are simple orthologs between these two species, the OR, GR, and OBP families in the pea aphid have experienced major expansions of particular gene lineages and fewer losses of gene lineages. This imbalance in birth-and-death of chemosensory genes has led to the larger pea aphid gene repertoire, which might be related to the broader host range of pea aphids versus the specialization of soybean aphids on a single summer host plant. Examination of the expression levels of these chemosensory genes in parthenogenetic and sexual females and males of pea aphids revealed multiple genes that are differentially expressed in sexual females or males and might be involved in reproductive biology. Examination of the soybean aphid genes in parthenogenetic females under multiple stressors revealed multiple genes whose expression levels changed with heat or starvation stress, the latter potentially important in finding new food sources.