Identification and tissue distribution of chemosensory protein and odorant binding protein genes in Tropidothorax elegans Distant (Hemiptera: Lygaeidae)

Identification and expression profiles of olfactory-related genes in the antennal transcriptome of Graphosoma rubrolineatum (Hemiptera: Pentatomidae)

Graphosoma rubrolineatum (Hemiptera: Pentatomidae) is an important pest of vegetables and herbs (e.g., Umbelliferae and Cruciferae) in China, Siberia, Korea, and Japan. Insects are highly dependent on their olfactory system to detect odorants. However, no molecular-mediated olfactory genes in G. rubrolineatum have yet been identified. In this study, we first established the antennal transcriptome of G. rubrolineatum and identified 189 candidate olfactory genes, including 31 odorant-binding proteins (OBPs), 15 chemosensory proteins (CSPs), four sensory neuron membrane proteins (SNMPs),94 odorant receptors (ORs), 23 ionotropic receptors (IRs), and 22 gustatory receptors (GRs). Additionally, phylogenetic trees were constructed for olfactory genes between G. rubrolineatum and other hemipteran insects. We also detected the expression profiles of ten OBPs, five CSPs, two SNMPs, five ORs, four IRs, and four GRs by real-time quantitative PCR. The results revealed that most genes (GrubOBP1/11/31, GrubCSP3/8, GrubSNMP1a/1b, GrubOrco/OR9/11/13, GrubGR1/4/22, GrubIR25/75h/76b/GluR1) were highly expressed in the antennae, GrubOBP13/31 and GrubCSP4/11/12 were highly expressed in the legs, while GrubOBP20 and GrubGR19 were highly expressed in the wings. Our results will enrich the gene inventory of G. rubrolineatum and provide further insight into the molecular chemosensory mechanisms of G. rubrolineatum.

Different binding properties of odorant-binding protein 8 to insecticides in Orius sauteri

Chemical sensing systems are vital in the growth and development of insects. Orius sauteri (Poppius) (Hemiptera: Anthocoridae) is an important natural enemy of many pests. The molecular mechanism of odorant binding proteins (OBPs) binding with common insecticides is still unknow in O. sauteri. In this study, we expressed in vitro OsauOBP8 and conducted fluorescence competition binding assay to investigate the function of OsauOBP8 to insecticides. The results showed that OsauOBP8 could bind with four common insecticides (phoxim, fenitrothion, chlorpyrifos, deltamethrin). Subsequently, we used molecular docking to predict and obtained candidate six amino acid residues (K4, K6, K13, R31, K49, K55) and then mutated. The result showed that three key residues (K4, K6, R31) play important role in OsauOBP8 bound to insecticides. Our study identified the key binding sites of OsauOBP8 to insecticides and help to better understand the molecular mechanism of OBPs to insecticides in O. sauteri.

Activity of Single Insect Olfactory Receptors Triggered by Airborne Compounds Recorded in Self-Assembled Tethered Lipid Bilayer Nanoarchitectures

Membrane proteins are among the most difficult to study as they are embedded in the cellular membrane, a complex and fragile environment with limited experimental accessibility. To study membrane proteins outside of these environments, model systems are required that replicate the fundamental properties of the cellular membrane without its complexity. We show here a self-assembled lipid bilayer nanoarchitecture on a solid support that is stable for several days at room temperature and allows the measurement of insect olfactory receptors at the single-channel level. Using an odorant binding protein, we capture airborne ligands and transfer them to an olfactory receptor from Drosophila melanogaster (OR22a) complex embedded in the lipid membrane, reproducing the complete olfaction process in which a ligand is captured from air and transported across an aqueous reservoir by an odorant binding protein and finally triggers a ligand-gated ion channel embedded in a lipid bilayer, providing direct evidence for ligand capture and olfactory receptor triggering facilitated by odorant binding proteins. This model system presents a significantly more user-friendly and robust platform to exploit the extraordinary sensitivity of insect olfaction for biosensing. At the same time, the platform offers a new opportunity for label-free studies of the olfactory signaling pathways of insects, which still have many unanswered questions.

Identification and ligand binding of a chemosensory protein from sea louse Caligus rogercresseyi (Crustacea: Copepoda)

Caligus rogercresseyi is an ectoparasitic copepod that negatively affects the salmon farming industry, causing economic losses. To use phytochemicals as feed additives, or other chemicals that could elicit behavioral responses in C. rogercresseyi, the chemosensory recognition process is crucial. Therefore, to establish how C. rogercresseyi recognizes glucosinolates and their derivates isothiocyanates, a chemosensory protein (CSP) described as specific carrier of these chemicals in sea louse (CrogCSP) was identified in this study. The recombinant CSP and its selectivity against different chemical compounds was tested by fluorescence binding assays. Phylogenetic analysis revealed a close relationship among CrogCSP and other reported CSPs. Our results indicate that phenyl isothiocyanate and isophorone exhibited dissociation constants of 4.17 and 4.28 μM of K_i, respectively, indicating affinity over other chemicals, such as fatty acids and sinigrin. Structural findings suggest a unique binding site capable of accept several types of chemicals, similar to what has been reported for crystallized insect CSPs. Finally, this study lays the foundation for a deeper understanding of CSPs in crustaceans and especially in C. rogercresseyi. Likewise, the identification of chemosensory proteins could serve as the first step towards novel semiochemicals discovery to being applied in the sea louse controlling.

Insights Into Chemosensory Proteins From Non-Model Insects: Advances and Perspectives in the Context of Pest Management

Nowadays, insect chemosensation represents a key aspect of integrated pest management in the Anthropocene epoch. Olfaction-related proteins have been the focus of studies due to their function in vital processes, such ashost finding and reproduction behavior. Hence, most research has been based on the study of model insects, namely Drosophila melanogaster, Bombyx mori or Tribolium castaneum. Over the passage of time and the advance of new molecular techniques, insects considered non-models have been studied, contributing greatly to the knowledge of insect olfactory systems and enhanced pest control methods. In this review, a reference point for non-model insects is proposed and the concept of model and non-model insects is discussed. Likewise, it summarizes and discusses the progress and contribution in the olfaction field of both model and non-model insects considered pests in agriculture.

AlepPBP2, but not AlepPBP3, may involve in the recognition of sex pheromones and maize volatiles in Athetis lepigone

Athetis lepigone Möschler (Lepidoptera, Noctuidae) is a common maize pest in Europe and Asia. However, there is no long-term effective management strategy is available yet to suppress its population. Adults rely heavily on olfactory cues to locate their optimal host plants and oviposition sites. Pheromone-binding proteins (PBPs) are believed to be responsible for recognizing and transporting different odorant molecules to interact with receptor membrane proteins. In this study, the ligand-binding specificities of two AlepPBPs (AlepPBP2 and AlepPBP3) for sex pheromone components and host plant (maize) volatiles were measured by fluorescence ligand-binding assay. The results demonstrated that AlepPBP2 had a high affinity with two pheromones [(Z)-7-dodecenyl acetate, Ki = 1.11 ± 0.1 μM, (Z)-9-tetradecenyl acetate, Ki = 1.32 ± 0.15 μM] and ten plant volatiles, including (-)-limonene, α-pinene, myrcene, linalool, benzaldehyde, nonanal, 2-hexanone, 3-hexanone, 2-heptanone and 6-methyl-5-hepten-2-one. In contrast, we found that none of these chemicals could bind to AlepPBP3. Our results clearly show no significant differences in the functional characterization of the binding properties between AlepPBP2 and AlepPBP3 to sex pheromones and host plant volatiles. Furthermore, molecular docking was employed for further detail on some crucial amino acid residues involved in the ligand-binding of AlepPBP2. These findings will provide valuable information about the potential protein binding sites necessary for protein-ligand interactions which appear as attractive targets for the development of novel technologies and management strategies for insect pests.

Role of Genes in Regulating Host Plants Expansion in Tephritid Fruit Flies (Diptera) and Potential for RNAi-Based Control

Host plant expansion is an important survival strategy for tephritids as they expand their range. Successful host expansion requires tephritids to adapt to the chemical and nonchemical properties of a novel host fruit, such as fruit color, phenology, and phytochemicals. These plant properties trigger a series of processes in tephritids, with each process having its own genetic basis, which means that various genes are involved in regulating host plant expansion by tephritids. This review summarizes current knowledge on the categories and roles of genes involved in host plant expansion in several important tephritid species, including genes related to chemoreception (olfactory and gustation), vision, digestion, detoxification, development, ribosomal and energy metabolism. Chemoreception- and detoxification- and digestion-related genes are stimulated by volatile chemicals and secondary chemicals of different hosts, respectively, which are involved in the regulation of nervous signal transduction that triggers behavioral, physical, and chemical responses to the novel host fruit. Vision-, nerve-, and development-related genes and metabolism-associated genes are activated in response to nonchemical stimuli from different hosts, such as color and phenology, to regulate a comprehensive adaptation of the extending host for tephritids. The chemical and nonchemical signals of hosts activate ribosomal and energy-related genes that result in the basic regulation of many processes of host expansion, including detoxification and development. These genes do not regulate novel host use individually, but multiple genes regulate multilevel adaptation to novel host fruits via multiple mechanisms. These genes may also be potential target genes for RNAi-based control of tephritid pests.

Antennal Transcriptome Analysis and Identification of Olfactory Genes in Glenea cantor Fabricius (Cerambycidae: Lamiinae)

Simple Summary

In this study, we conducted antennal transcriptome analysis in Glenea cantor (Cerambycidae: Lamiinae) and identified 76 olfactory-related genes, including 29 odorant binding proteins (OBPs), 14 chemosensory proteins (CSPs), 13 odorant receptors (ORs), 18 ionotropic receptors (IRs) and 2 sensory neuron membrane proteins (SNMPs). We also verified the reliability of transcriptome differential genes by qRT-PCR, which indicated the reliability of the transcriptome. Based on the relative expression of 30 d adults, GcanOBP22 and GcanOBP25 were highly expressed not only in the antennae, but also in the wings and legs. In addition, GcanCSP4 was the highest expression on the female antennae at 12 d. These findings laid the foundation for further research on the mechanism of G. cantor olfactory mechanism at the molecular level.

Abstract

Glenea cantor Fabricius (Cerambycidae: Lamiinae) is a pest that devastates urban landscapes and causes ecological loss in southern China and Southeast Asian countries where its main host kapok trees are planted. The olfactory system plays a vital role in mating, foraging, and spawning in G. cantor as an ideal target for pest control. However, the olfactory mechanism of G. cantor is poorly understood at the molecular level. In this study, we first established the antennal transcriptome of G. cantor and identified 76 olfactory-related genes, including 29 odorant binding proteins (OBPs), 14 chemosensory proteins (CSPs), 13 odorant receptors (ORs), 18 ionotropic receptors (IRs) and 2 sensory neuron membrane proteins (SNMPs). Furthermore, the phylogenetic trees of olfactory genes were constructed to study the homology with other species of insects. We also verified the reliability of transcriptome differential genes by qRT-PCR, which indicated the reliability of the transcriptome. Based on the relative expression of 30 d adults, GcanOBP22 and GcanOBP25 were highly expressed not only in the antennae, but also in the wings and legs. In addition, GcanCSP4 was the highest expression on the female antennae at 12 d. These findings laid the foundation for further research on the mechanism of G. cantor olfactory mechanism at the molecular level.

Comparative analyses of the venom components in the salivary gland transcriptomes and saliva proteomes of some heteropteran insects

Salivary gland-specific transcriptomes of nine heteropteran insects with distinct feeding strategies (predaceous, hematophagous, and phytophagous) were analyzed and annotated to compare and identify the venom components as well as their expression profiles. The transcriptional abundance of venom genes was verified via quantitative real-time PCR. Hierarchical clustering of 30 representative differentially expressed venom genes from the nine heteropteran species revealed unique groups of salivary gland-specific genes depending on their feeding strategy. The commonly transcribed genes included a paralytic neurotoxin (arginine kinase), digestive enzymes (cathepsin and serine protease), an anti-inflammatory protein (cystatin), hexamerin, and an odorant binding protein. Both predaceous and hematophagous (bed bug) heteropteran species showed relatively higher transcription levels of genes encoding proteins involved in proteolysis and cytolysis, whereas phytophagous heteropterans exhibited little or no expression of these genes, but had a high expression of vitellogenin, a multifunctional allergen. Saliva proteomes from four representative species were also analyzed. All venom proteins identified via saliva proteome analysis were annotated using salivary gland transcriptome data. The proteomic expression profiles of venom proteins were in good agreement with the salivary gland-specific transcriptomic profiles. Our results indicate that profiling of the salivary gland transcriptome provides important information on the composition and evolutionary features of venoms depending on their feeding strategy.

Identification and expression profile of odorant-binding proteins in the parasitic wasp Microplitis pallidipes using PacBio long-read sequencing

Microplitis pallidipes Szépligeti (Hymenoptera: Braconidae) is an important parasitic wasp of second and third-instar noctuid larvae such as the insect pests Spodoptera exigua, Spodoptera litura, and Spodoptera frugiperda. As in other insects, M. pallidipes has a chemosensory recognition system that is critical to foraging, mating, oviposition, and other behaviors. Odorant-binding proteins (OBPs) are important to the system, but those of M. pallidipes have not been determined. This study used PacBio long-read sequencing to identify 170,980 M. pallidipes unigenes and predicted 129,381 proteins. Following retrieval of possible OBP sequences, we removed those that were redundant or non-full-length and eventually cloned five OBP sequences: MpOBP2, MpOBP3, MpOBP8, MpOBP10, and MpPBP 429, 429, 459, 420, and 429 bp in size, respectively. Each M. pallidipes OBP had six conserved cysteine residues. Phylogenetic analysis revealed that the five OBPs were located at different branches of the phylogenetic tree. Additionally, tissue expression profiles indicated that MpOBP2 and MpPBP were mainly expressed in the antennae of male wasps, while MpOBP3, MpOBP8, and MpOBP10 were mainly expressed in the antennae of female wasps. MpOBP3 was also highly expressed in the legs of female wasps. Temporal profiles revealed that the expression of each M. pallidipes OBP peaked at different days after emergence to adulthood. In conclusion, we identified five novel odorant-binding proteins of M. pallidipes and demonstrated biologically relevant differences in expression patterns.

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.

Functional characteristics of a novel odorant binding protein in the legume pod borer, Maruca vitrata

Insect olfaction system plays a key role in the foraging food, pollination, mating, oviposition, reproduction and other insect physiological behavior. Odorant binding protein are widely found in the various olfactory sensilla of different insect antennae and involved in chemical signals discrimination from natural environment. In this study, a novel OBP gene, MvitOBP3 is identified from the legume pod borer, Maruca vitrata, which it mainly harms important legume vegetables including cowpea, soybean and lablab bean. Real-time PCR results demonstrated that MvitOBP3 gene was abundantly expressed in the antennal tissue of M. vitrata, while low levels were distributed in the head, thorax, abdomen, leg and wing of adult moths. The recombinant OBP3 protein was purified using the prokaryotic expression and affinity chromatography system. Fluorescence competitive binding experiments indicated that that MvitOBP3 protein exhibited greater binding affinities with host-plant flower volatiles including Butanoic acid butyl ester, Limonene, 1H-indol-4-ol and 2-methyl-3-phenylpropanal, highlighting they may have attractant activities for the oviposition of female moths on the legume vegetables. Moreover, protein homology modeling and molecular docking analysis revealed that there are six amino acid sites of MvitOBP3 involved in the binding of the host-plant volatiles. These findings will further promote to understand the key role of odorant binding protein during host perception and oviposition of M. vitrata moths, which improve the efficiency of semiochemical-based prevention and monitoring for this pest in the legume vegetables field.

OBP14 (Odorant-Binding Protein) Sensing in Adelphocoris lineolatus Based on Peptide Nucleic Acid and Graphene Oxide

OBPs play a crucial role in the recognition of ligands and are involved in the initial steps of semiochemical perception. The diverse expression of OBP genes allows them to participate in different physiological functions in insects. In contrast to classic OBPs with typical olfactory roles in A. lineolatus, the physiological functions of Plus-C OBPs remain largely unknown. In addition, detection of the expression of insect OBP genes by conventional methods is difficult in vitro. Here, we focused on AlinOBP14, a Plus-C OBP from A. lineolatus, and we developed a PNA-GO-based mRNA biosensor to detect the expression of AlinOBP14. The results demonstrated that AlinOBP14 plays dual roles in A. lineolatus. The AlinOBP14 is expressed beneath the epidermis of the vertex and gena in heads of A. lineolatus, and it functions as a carrier for three terpenoids, while AlinOBP14 is also expressed in the peripheral antennal lobe and functions as a carrier for endogenous compounds such as precursors for juvenile hormone (JH) and JHⅢ. Our investigation provides a new method to detect the expression of OBP genes in insects, and the technique will facilitate the use of these genes as potential targets for novel insect behavioral regulation strategies against the pest.

Identification of candidate chemosensory receptors in the antennal transcriptome of Tropidothorax elegans

Chemosensory receptors in the dendritic membrane of olfactory cells are critical for the molecular recognition and discrimination of odorants. Tropidothorax elegans is a major pest of agricultural, ornamental, and medicinal plants. However, very little is known about olfactory genes in T. elegans. The purpose of this study was to obtain chemosensory receptor genes by sequencing the antennal transcriptome of T. elegans using Illumina sequencing technology. We identified 153 candidate chemosensory receptors, including 121 olfactory receptors (including one olfactory receptor co-receptor), 10 ionotropic receptors (including one IR8a and one IR25a), and 22 gustatory receptors (GRs). TeleOR76, 104 and 112 displayed more highly expression level than TeleOrco. Other TeleGR genes were expressed at very low levels except TeleGR1 and 20. TeleIR76b was the most highly expressed among TeleIR genes. Our results provide valuable biological information for studies of the olfactory communication system of T. elegans.

An Overview of Antennal Esterases in Lepidoptera

Lepidoptera are used as a model for the study of insect olfactory proteins. Among them, odorant degrading enzymes (ODEs), that degrade odorant molecules to maintain the sensitivity of antennae, have received less attention. In particular, antennal esterases (AEs; responsible for ester degradation) are crucial for intraspecific communication in Lepidoptera. Currently, transcriptomic and genomic studies have provided AEs in several species. However, efforts in gene annotation, classification, and functional assignment are still lacking. Therefore, we propose to combine evidence at evolutionary, structural, and functional level to update ODEs as well as key information into an easier classification, particularly of AEs. Finally, the kinetic parameters for putative inhibition of ODEs are discussed in terms of its role in future integrated pest management (IPM) strategies.

Identification and motif analyses of candidate nonreceptor olfactory genes of Dendroctonus adjunctus Blandford (Coleoptera: Curculionidae) from the head transcriptome

The round-headed pine beetle Dendroctonus adjunctus, whose dispersion and colonization behaviors are linked to a communication system mediated by semiochemicals, is one of the five most critical primary pests in forest ecosystems in Mexico. This study provides the first head transcriptome analysis of D. adjunctus and the identification of the nonreceptor olfactory genes involved in the perception of odors. De novo assembly yielded 44,420 unigenes, and GO annotations were similar to those of antennal transcriptomes of other beetle species, which reflect metabolic processes related to smell and signal transduction. A total of 36 new transcripts of nonreceptor olfactory genes were identified, of which 27 encode OBPs, 7 encode CSPs, and 2 encode SNMP candidates, which were subsequently compared to homologous proteins from other bark beetles and Coleoptera species by searching for sequence motifs and performing phylogenetic analyses. Our study provides information on genes encoding nonreceptor proteins in D. adjunctus and broadens the knowledge of olfactory genes in Coleoptera and bark beetle species, and will help to understand colonization and aggregation behaviors for the development of tools that complement management strategies.

Citronellal perception and transmission by Anopheles gambiae s.s. (Diptera: Culicidae) females

Anopheles gambiae s.s. is a key vector of Plasmodium parasites. Repellents, which may be a promising alternative to pesticides used to control malaria mosquitoes. Although citronellal is a known mosquito repellent, its repellency characteristics are largely unknown. Determining the specific odorant-binding proteins (OBPs) and odorant receptors (ORs) that detect and transfer the citronellal molecule in A. gambiae s.s. will help to define the mode of action of this compound. In this research, we assessed the repellent activity of citronellal in A. gambiae s.s. using a Y-tube olfactory meter, screened candidate citronellal-binding OBPs and ORs using reverse molecular docking, clarified the binding properties of predicted proteins for citronellal using fluorescence competition binding assay. Results showed that citronellal had a dosage effect on repelling A. gambiae s.s.. The 50% repellent rate was determined to be 4.02 nmol. Results of simulated molecular docking showed that the only proteins that bound tightly with citronellal were AgamOBP4 and AgamORC7. Fluorescence competitive binding assays confirmed the simulations. This research determined that citronellal was captured by AgamOBP4 and transmitted to AgamORC7 in A. gambiae s.s.. Our study will be beneficial in the further understanding the repellent mechanism of citronellal against A. gambiae s.s..

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.

Identification and preliminary characterization of chemosensory-related proteins in the gall fly, Procecidochares utilis by transcriptomic analysis

Chemoreception is critical for insect behaviors such as foraging, host searching and oviposition. The process of chemoreception is mediated by a series of proteins, including odorant-binding proteins (OBPs), gustatory receptors (GRs), odorant receptors (ORs), ionotropic receptors (IRs), chemosensory proteins (CSPs) and sensory neuron membrane proteins (SNMPs). The tephritid stem gall fly, Procecidochares utilis Stone, is a type of egg parasitic insect, which is an effective biological control agent for the invasive weed Ageratina adenophora in many countries. However, the study of molecular components related to the olfactory system of P. utilis has not been investigated. Here, we conducted the developmental transcriptome (egg, first-third instar larva, pupa, female and male adult) of P. utilis using next-generation sequencing technology and identified a total of 133 chemosensory genes, including 40 OBPs, 29 GRs, 24 ORs, 28 IRs, 6 CSPs, and 6 SNMPs. The sequences of these candidate chemosensory genes were confirmed by BLAST, and phylogenetic analysis was performed. Quantitative real-time PCR (qRT-PCR) confirmed that the expression levels of the candidate OBPs varied at the different developmental stages of P. utilis with most OBPs expressed mainly in the pupae, female and male adults but scarcely in eggs and larvae, which was consistent with the differentially expressed genes (DEGs) analysis using the fragments per kilobase per million fragments (FPKM) value. Our results provide a significant contribution towards the knowledge of the set of chemosensory proteins and help advance the use of P. utilis as biological control agents.

Molecular cloning and comparative analysis of transcripts encoding chemosensory proteins from two plant bugs, Lygus lineolaris and Lygus hesperus

Chemosensory proteins (CSPs) are soluble carrier proteins typically characterized by a six-helix bundle structure joined by two disulfide bridges and a conserved Cys spacing pattern (C1-X_6-8 -C2-X_16-21 -C3-X₂ -C4). CSPs are functionally diverse with reported roles in chemosensation, immunity, development, and resistance. To expand our molecular understanding of CSP function in plant bugs, we used recently developed transcriptomic resources for Lygus lineolaris and Lygus hesperus to identify 17 and 14 CSP-like sequences, respectively. The Lygus CSPs are orthologous and share significant sequence identity with previously annotated CSPs. Three of the CSPs are predicted to deviate from the typical CSP structure with either five or seven helical segments rather than six. The seven helix CSP is further differentiated by an atypical C3-X₃ -C4 Cys spacing motif. Reverse transcriptase PCR-based profiling of CSP transcript abundance in adult L. lineolaris tissues revealed broad expression for most of the CSPs with antenna specific expression limited to a subset of the CSPs. Comparative sequence analyses and homology modeling suggest that variations in the amino acids that comprise the Lygus CSP binding pockets affect the size and nature of the ligands accommodated.

Identification and expression analysis of odorant binding proteins and chemosensory proteins from dissected antennae and mouthparts of the rice bug Leptocorisa acuta

The rice bug, Leptocorisa acuta (Tunberg) (Hemiptera: Alydidae), is a notorious pest in Asia, and it is significantly attracted by the volatiles derived from host plants. However, it remains unknown how L. acuta recognizes host volatile compounds at the molecular level. Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) are thought to be responsible for the initial biochemical recognition during olfactory perception. Here, we followed the RNA Sequencing (RNA-Seq) approach to identify candidate genes encoding OBPs and CSPs from dissected antennae and mouthparts of L. acuta. In total, 26 unigenes were identified coding for OBPs (22 Classic OBPs and four Plus-C OBPs), and 17 unigenes coding for CSPs. Real-time quantitative PCR (RT-qPCR) revealed that 11 OBPs (LacuOBP1, 5, 6, 7, 9, 11, 13, 14, 17, 20 and 23) and nine CSPs (LacuCSP2, 3, 4, 5, 6, 8, 9, 10 and 12) were predominantly expressed in antennae, indicating that they might be essential for detection of general odorants and pheromones. Among these antennae-predominantly expressed genes, LacuOBP11 and LacuOBP13 showed male-biased expression and therefore may play crucial roles in the detection of sex pheromones. Seven LacuOBPs (LacuOBP4, 8, 10, 12, 21, 25 and 26) and two CSPs (LacuCSP7 and LacuOBP11) were predominantly expressed in mouthparts, suggesting that these genes might be involved in taste perception. Our work provides a starting point to facilitate functional study of these OBPs and CSPs in L. acuta at the molecular level in the future.

Genome-wide identification and expression profiling of odorant-binding proteins in the oriental fruit fly, Bactrocera dorsalis

Olfaction contributes to many crucial behaviors in insects, such as foraging, locating hosts, mating, and avoiding predators. In the first step of the olfaction process in insects, odorant-binding proteins (OBPs) bind with the odorants and transport hydrophobic odorants. OBPs are also believed to accelerate the termination of the odorant response. The oriental fruit fly, Bactrocera dorsalis, is one of the most destructive fruit-eating pests, causing enormous economic losses to the fruit and vegetable industry worldwide. However, information relating to the number, diversity, and expression patterns of OBPs still remains fragmented in this insect pest. Here, we attempted to identify the OBPs in B. dorsalis using genomic and transcriptomic information. In this study, we expanded the repository of B. dorsalis OBPs to 49. Phylogenetic analysis of BdorOBPs with other species revealed that these proteins grouped into four subfamilies. Furthermore, we determined the expression profiles in six body parts (namely, the legs, wings, antenna, cuticles of the head, thorax, and abdomen) and five internal tissues (namely, the fat body, midgut, Malpighian tubule, testis, and ovary). The results indicated that 21 BdorOBPs showed high expression levels in the antenna, legs, and head cuticles and may thus perform olfactory functions, which corroborates previous evidence. Two BdorOBPs were specifically expressed in the abdomen cuticles. Nineteen OBPs were highly expressed in the fat body, while four OBPs were highly expressed in the reproductive organs. This indicated that they may have physiological roles other than in chemoreception. In summary, our results contribute to the knowledge base of insect OBPs and provide a foundation for the further study of the molecular mechanisms of chemoreception in B. dorsalis.

Candidate genes coding for odorant binding proteins and chemosensory proteins identified from dissected antennae and mouthparts of the southern green stink bug Nezara viridula

The southern green stink bug Nezara viridula (Hemiptera: Pentatomidae) is a highly polyphagous pest that can significantly impact many major crops worldwide. Insect odorant binding proteins (OBPs) and chemosensory proteins (CSPs) transport chemicals and play critical roles in chemoreception. Studies on N. viridula OBPs and CSPs should increase our overall understandings on chemosensory systems and chemical ecology of stink bugs, which may lead to improved pest control. In this study, we identified candidate genes encoding putative OBPs and CSPs in N. viridula by generating transcriptomes from dissected antennae and mouthparts. In total, the 42 unigenes were identified coding for OBPs (34 Classic OBPs and eight Plus-C OBPs) and 13 unigenes coding for CSPs. Expression profiles of OBP- and CSP -encoding genes were compared between antennae and mouthparts based on FKPM values. Candidates for antenna-predominant OBPs and CSPs were selected for real-time quantitative PCR analyses. Analyses of tissue expression profiles revealed that 17 OBP-encoding genes, and four CSP genes were primarily expressed in antennae, suggesting their putative roles in perception of volatiles. The sex-biased expression patterns of these antenna-predominant genes suggested that they may have important functions in reproduction of the insect. This is a systematic analysis on OBPs and CSPs in a stink bug, providing a comprehensive resource for future functional studies not only for N. viridula, but also for other stink bugs as well.

Identification and expression analysis of putative chemoreception genes from Cyrtorhinus lividipennis (Hemiptera: Miridae) antennal transcriptome

Cyrtorhinus lividipennis Reuter (Hemiptera: Miridae) is an important egg predator of planthoppers which are destructive rice pests. The chemosensory genes in the mirid antennae play important roles in mating and prey-seeking behaviors. To gain a better understanding of the olfaction of C. lividipennis, we sequenced the antennal transcriptomes of the predator to identify the key olfaction genes. We identified 18 odorant binding proteins (OBPs), 12 chemosensory proteins (CSPs), 1 Niemann-Pick C2 protein (NPC2), 15 odorant receptors (ORs), 6 ionotropic receptors (IRs), 3 gustatory receptors (GRs) and 3 sensory neuron membrane proteins (SNMPs). Quantitative real-time PCR results showed that the relative transcript levels of three ClivORs (ClivOR6, 7 and 14) in the female antennae were 3 to 6 folds higher than that in the male antennae, indicating these genes were more related to oviposition site selection. The relative transcript levels of ClivCSP8 and ClivOR11 were 2.6 and 2.7 times higher in the male antennae than that of the female, respectively, indicating that these genes might be involved in mate searching. Moreover, the responses of dsorco treated predators to volatiles emitted from infested rice were significantly reduced, indicating these volatiles might serve as crucial cues in the host searching of C. lividipennis.

Odorant Receptors and Odorant-Binding Proteins as Insect Pest Control Targets: A Comparative Analysis

Recently, two alternative targets in insect periphery nerve system have been explored for environmentally-friendly approaches in insect pest management, namely odorant-binding proteins (OBPs) and odorant receptors (ORs). Located in insect antennae, OBPs are thought to be involved in the transport of odorants to ORs for the specific signal transduction of behaviorally active odorants. There is rich information on OBP binding affinity and molecular docking to bioactive compounds as well as ample 3D crystal structures due to feasible production of recombinant proteins. Although these provide excellent opportunities for them to be considered as pest control targets and a tool to design pest control agents, the debates on their binding specificity represent an obstacle. On the other hand, ORs have recently been functionally characterized with increasing evidence for their specificity, sensitivity and functional roles in pest behaviors. However, a major barrier to use ORs for semiochemical discovery is the lack of 3D crystal structures. Thus, OBPs and ORs have not been analyzed comparatively together so far for their feasibility as pest control targets. Here, we summarize the state of OBPs and ORs research in terms of its application in insect pest management. We discuss the suitability of both proteins as pest control targets and their selection toward the discovery of new potent semiochemicals. We argue that both proteins represent promising targets for pest control and can be used to identify new super-ligands likely present in nature and with reduced risk of resistance development than insect pesticides currently used in agriculture. We discuss that with the massive identification of OBPs through RNA-seq and improved binding affinity measurements, these proteins could be reconsidered as suitable targets for semiochemical discovery.