Expression pattern analysis of odorant-binding proteins in the pea aphid Acyrthosiphon pisum

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.

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.

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.

Effects of Aqueous Extracts of Endophyte-Infected Grass Achnatherum inebrians on Growth and Development of Pea Aphid Acyrthosiphon pisum

Simple Summary

Drunken horse grass Achnatherum inebrians is a widely distributed perennial poisonous grass on the grasslands of Northern and Northwestern China. The present study focused on contact toxicity of aqueous extracts of endophyte-infected (E+) and endophyte-free (E−) plants of A. inebrians in different growth periods on pea aphids. These results show that extracts from endophyte-containing plants may contain compounds that may be used to control insects and lay a foundation for deeper chemical analysis to identify insecticidal compounds.

Abstract

The pea aphid Acyrthosiphon pisum has a worldwide distribution and causes serious losses for agricultural production. Drunken horse grass Achnatherum inebrians is a widely distributed perennial poisonous grass on the grasslands of Northern and Northwestern China. The present study focused on contact toxicity activity of aqueous extracts of endophyte-infected (E+) and endophyte-free (E−) A. inebrians in different growth periods of pea aphids, and the growth and development of two color morphs of F1 generation nymphs. Both of the color morphs had development durations in E+ treatments that tended to be longer at 1st, 2nd, 3rd, and 4th instars than E− and control (CK). The E+ treated aphids also showed decreased weights at maturity with over all lower mean relative growth rates (MRGR). Aphid survival of E+ treated aphids was lower than that of E− and CK at all growth periods. Seeding stage E+ extracts showed a greater propensity for negatively affecting aphids than did E+ extract at maturity and the yellowing stage. These results show that extracts from endophyte-containing plants may contain compounds that may be used to control insects.

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.

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.

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.

Characterization of Antennal Sensilla and Immunolocalization of Odorant-Binding Proteins on Spotted Alfalfa Aphid, Therioaphis trifolii (Monell)

The spotted alfalfa aphid [Therioaphis trifolii (Monell), Homoptera, Drepanosiphidae] is a well-known destructive pest that can significantly reduce alfalfa yields. Herein, the morphology of antennal sensilla of T. trifolii has been examined by using scanning electron microscopy and the ultrastructure of sensilla stellate and placoidea was described by transmission electron microscopy. Stellate sensilla, placoid sensilla, and coeloconic sensilla were found on the 6th segment, and a single sensillum placoidea was located on the 5th segment. Placoid sensilla were also present on the 3rd antennal segment of alate and apterous aphids, and the number was similar between two morphs. Two types of trichoid sensilla and coeloconic sensilla were found on the antennae, respectively. The results of ultrastructure showed that stellate sensilla are innervated by three neurons, while placoid sensilla present three groups of neurons, equipped with 2–3 dendrites in each neuron group. Immunocytochemical localization of odorant-binding proteins (OBPs) was performed on ultrathin sections of sensilla stellate and placoidea, and we observed that the antiserum against OBP6 intensively labeled all placoid sensilla from both primary and secondary rhinaria. OBP7 and OBP8 could also be detected in placoid sensilla, but less strongly than OBP6. In addition, OBP6, OBP7, and OBP8 were densely labeled in stellate sensilla, suggesting OBP6, OBP7, and OBP8 may sense alarm pheromone germacrene A in T. trifolii.

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 of Candidate Olfactory Genes in the Antennal Transcriptome of the Stink Bug Halyomorpha halys

The brown marmorated stink bug, Halyomorpha halys (Hemiptera: Pentatomidae), is a serious agricultural and urban pest that has become an invasive species in many parts of the world. Olfaction plays an indispensable role in regulating insect behaviors, such as host plant location, partners searching, and avoidance of predators. In this study, we sequenced and analyzed the antennal transcriptomes of both male and female adults of H. halys to better understand the olfactory mechanisms in this species. A total of 241 candidate chemosensory genes were identified, including 138 odorant receptors (ORs), 24 ionotropic receptors (IRs), 15 gustatory receptors (GRs), 44 odorant-binding proteins (OBPs), 17 chemosensory proteins (CSPs), and three sensory neuron membrane proteins (SNMPs). The results of semi-quantitative reverse transcription PCR (RT-PCR) assays showed that some HhalOBP and HhalCSP genes have tissue-specific and sex-biased expression patterns. Our results provide an insight into the molecular mechanisms of the olfactory system in H. halys and identify potential novel targets for pest control strategies.

Bioactivities of synthetic salicylate-substituted carboxyl (E)-β-Farnesene derivatives as ecofriendly agrochemicals and their binding mechanism with potential targets in aphid olfactory system

BACKGROUND

The aphid alarm pheromone, (E)-β-farnesene (EβF), is a natural product secreted from the aphid cornicle as a signal to warn companions of danger. Odorant binding proteins (OBPs) are the vital targets in insect signal transduction pathways. To improve bioactivity of EβF as more economic and stable aphid control agents, EβF derivatives containing an active substructure, salicylic acid moiety, were designed, synthesized, and evaluated for their bioactivities in a structure-function study under laboratory conditions.

RESULTS

EβF derivatives, (E)-3,7-dimethylocta-2,6-dien-1-yl-2-hydroxy-3-methylbenzoate and (E)-3,7-dimethylocta-2,6-dien-1-yl-2-hydroxy-3-methoxybenzoate showed outstanding aphid-repellent activity at a dose of 5 μg against Acyrthosiphon pisum (repellency proportions of 67.3% and 71.2%, respectively) and Myzus persicae (repellency proportions of 80.0% and 74.4%, respectively) in laboratory. EβF and most of its derivatives bound strongly to ApisOBP9 with a higher affinity than those of the reported potential targets AphisOBP3 and ApisOBP7. The binding affinities to these three ApisOBPs were generally consistent with the in vivo aphid-repellent activity. A molecular docking study suggested that the hydrophobic effect was crucial for the interactions between the derivatives and the OBPs.

CONCLUSION

New EβF derivatives containing salicylic acid moiety and their repellent activity, binding mechanism with three potential OBPs are presented. A new OBP, ApisOBP9, was characterized as a potential EβF and EβF derivatives binding protein for the first time. The hydrophobic nature of these analogues is responsible for their activity. Two analogues 3b and 3e with outstanding aphid-repellent activity could be new leads for aphid control agents.

The role of chemosensory protein 10 in the detection of behaviorally active compounds in brown planthopper, Nilaparvata lugens

Chemosensory proteins (CSPs) play important roles in insects' chemoreception, although their specific functional roles have not been fully elucidated. In this study, we conducted the developmental expression patterns and competitive binding assay as well as knock-down assay by RNA interference both in vitro and in vivo to reveal the function of NlugCSP10 from the brown planthopper (BPH), Nilaparvata lugens (Stål), a major pest in rice plants. The results showed that NlugCSP10 messenger RNA was significantly higher in males than in females and correlated to gender, development and wing forms. The fluorescence binding assays revealed that NlugCSP10 exhibited the highest binding affinity with cis-3-hexenyl acetate, eicosane, and (+)-β-pinene. Behavioral assay revealed that eicosane displayed attractant activity, while cis-3-hexenyl acetate, similar to (+)-β-pinene significantly repelled N. lugens adults. Silencing of NlugCSP10, which is responsible for cis-3-hexenyl acetate binding, significantly disrupted cis-3-hexenyl acetate communication. Overall, findings of the present study showed that NlugCSP10 could selectively interrelate with numerous volatiles emitted from host plants and these ligands could be designated to develop slow-release mediators that attract/repel N. lugens and subsequently improve the exploration of plans to control this insect pest.

Functional Analysis of Two Odorant-Binding Proteins, MaltOBP9 and MaltOBP10, in Monochamus alternatus Hope

Odorant-binding proteins (OBPs) are important for the perception of chemical signals by insects. Effective pest management strategies can be developed by understanding the host location mechanism and the physiological functions of OBPs in olfactory detection. In this study, we cloned two OBPs from Monochamus alternatus, where MaltOBP9 was highly expressed in multiple insect tissues and MaltOBP10 was highly expressed in the female antenna according to the results of qRT-PCR. The recombinant proteins were successfully purified in vitro. Immunocytochemistry indicated the high expression of MaltOBP9 and MaltOBP10 in the sensillum lymph of sensilla basiconica, sensilla trichodea, sensilla auricillica, and sensilla chaetica, thereby demonstrating their broad participation in semiochemical detection. Both proteins were localized in the inner cavity of mechanoreceptors and they exhibited broad binding abilities with volatiles from pine bark according to fluorescence competitive binding assays. Due to its broad binding ability and distribution, MaltOBP9 may be involved in various physiological processes as well as olfactory detection. MaltOBP10 appears to play a role in the fundamental olfactory recognition process of female adults according to its broad binding ability. These findings suggest that OBPs may have various physiological functions in insects, thereby providing novel insights into the olfactory receptive mechanism.

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.

Integrative transcriptomic and genomic analysis of odorant binding proteins and chemosensory proteins in aphids

Odorant binding proteins (OBPs) and chemosensory proteins (CSPs) play essential roles in insect chemosensory recognition. Here, we identified nine OBPs and nine CSPs from the Myzus persicae transcriptome and genome. Genomic structure analysis showed that the number and length of the introns are much higher, and this appears to be a unique feature of aphid OBP genes. Three M. persicae OBP genes (OBP3/7/8) as well as CSP1/4/6, CSP2/9 and CSP5/8 are tandem arrayed in the genome. Phylogenetic analyses of five different aphid species suggest that aphid OBPs and CSPs are conserved in single copy across all aphids (with occasional losses), indicating that each OBP and CSP class evolved from a single gene in the common ancestor of aphids without subsequent duplication. Motif pattern analysis revealed that aphid OBP and CSP motifs are highly conserved, and this could suggest the conserved functions of aphid OBPs and CSPs. Three OBPs (MperOBP6/7/10) are expressed antennae specifically, and five OBPs (MperOBP2/4/5/8/9) are expressed antennae enriched, consistent with their putative olfactory roles. M. persicae CSPs showed much broader expression profiles in nonsensory organs than OBPs. None of the nine MperCSPs were found to be antennae specific, but five of them (MperCSP1/2/4/5/6) showed higher expression levels in the legs than in other tissues. MperCSP10 mainly expressed in the antennae and legs. The broad and diverse expression patterns of M. persicae CSPs suggest their multifunctions in olfactory perception, development and other processes.

Identification and functional characterization of an odorant receptor in pea aphid, Acyrthosiphon pisum

The sensitive olfactory system is necessary for survival of insects. Odorant receptors (ORs) are located on the dendrites of olfactory receptor neurons and play a critical role in odor detection. Insect ORs are functionally analyzed via heterologous expression in a Xenopus oocyte system using a two-electrode voltage-clamp (TEVC) electrophysiological recording. Here, we have identified a novel OR in the pea aphid, Acyrthosiphon pisum, then we cloned and named it ApisOR4. We analyzed the ApisOR4 tissue expression patterns and found expression only in antennae tissues. Further functional analysis using TEVC revealed that ApisOR4 is broadly tuned to eight volatiles, which elicit electrophysiological response in pea aphid antennae. This study provides an initial functional analysis of aphid ORs and identifies candidate volatiles to be used in developing new strategies for aphid control.

The identification and expression analysis of candidate chemosensory genes in the bird cherry-oat aphid Rhopalosiphum padi (L.)

The bird cherry-oat aphid Rhopalosiphum padi (L.) is one of the most important wheat pests with polyphagia and autumn migrants. And, chemosensory genes were thought to play a key role in insect searching their hosts, food and mate. However, a systematic identification of the chemosensory genes in this pest has not been reported. Thus, in this study, we identified 14 odorant-binding proteins, nine chemosensory proteins, one sensory neuron membrane protein, 15 odorant receptors, 19 gustatory receptors and 16 ionotropic receptors from R. padi transcriptomes with a significantly similarity (E-value < 10-5) to known chemosensory genes in Acyrthosiphon pisum and Aphis gossypii. In addition, real-time quantitative polymerase chain reaction (RT-qPCR) was employed to determine the expression profiles of obtained genes. Among these obtained genes, we selected 23 chemosensory genes to analyze their expression patterns in different tissues, wing morphs and host plants. We found that except RpOBP1, RpOBP3, RpOBP4 and RpOBP5, the rest of the selected genes were highly expressed in the head with antennae compared with body without head and antennae. Besides that, the stimulation and depression of chemosensory genes by plant switch indicated that chemosensory genes might be involved in the plant suitability assessment. These results not only provide insights for the potential roles of chemosensory genes in plant search and perception of R. padi but also provide initial background information for the further research on the molecular mechanism of the polyphagia and autumn migrants of it. Furthermore, these chemosensory genes are also the candidate targets for pest management control in future.

iTRAQ-Based Comparative Proteomic Analysis Reveals Molecular Mechanisms Underlying Wing Dimorphism of the Pea Aphid, Acyrthosiphon pisum

Wing dimorphism is a widespread phenomenon in insects with an associated trade-off between flight capability and fecundity. Despite the molecular underpinnings of phenotypic plasticity that has already been elucidated, it is still not fully understood. In this study, we focused on the differential proteomics profiles between alate and apterous morphs of the pea aphid, Acyrthosiphon pisum at the fourth instar nymph and adult stages, using isobaric tags for relative and absolute quantitation (iTRAQ) in a proteomic-based approach. A total of 5,116 protein groups were identified and quantified in the three biological replicates, of which 836 were differentially expressed between alate and apterous morphs. A bioinformatics analysis of differentially expressed protein groups (DEPGs) was performed based on gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). KEGG enrichment analysis showed that DEPGs mainly participated in energy metabolism, amino acid biosynthesis and metabolism, and signal sensing and transduction. To verify the reliability of proteomics data, the transcriptional expression of 29 candidates of differentially expressed proteins were analyzed by quantitative real-time PCR (qRT-PCR), showing that 26 genes were consistent with those at proteomic levels. In addition, differentially expressed proteins between winged and wingless morphs that were linked to olfactory sense were investigated. Quantitative real-time PCR revealed the tissue- and morph-biased expression profiles. These results suggested that olfactory sense plays a key role in wing dimorphism of aphids. The comparative proteomic analysis between alate and apterous morphs of the pea aphid provides a novel insight into wing development and dimorphism in aphids and will help facilitate our understanding of these concepts at molecular levels.

Functional Analysis of the Chemosensory Protein MsepCSP8 From the Oriental Armyworm Mythimna separata

Chemosensory proteins (CSPs) play important roles in chemosensation in insects, but their exact physiological functions remain elusive. In order to investigate the functions of CSPs in the oriental armyworm Mythimna separata, in the present study we explored expression patterns and binding characteristics of the CSP, MsepCSP8. The distinctive functions of MsepCSP8 were also validated by RNAi. The results showed that MsepCSP8 shares high sequence similarity with CSPs of other insect family members, including the characteristic four-cysteine signature motif. MsepCSP8 mRNA was specifically expressed in antennae of females at levels well above those in other tissues. Competitive binding assays confirmed that 20 out of 56 ligands bound more strongly to MsepCSP8 at pH 7.4 than at pH 5.0. Protein structure modeling and molecular docking analyses identified amino acid residues involved in binding volatile compounds, and behavioral response experiments showed that M. separata elicited significant responses to five volatiles from compounds displaying high binding affinity to MsepCSP8. MsepCSP8 transcript abundance was decreased by dsMsepCSP8 injection, which affected the behavioral responses of M. separata to representative semiochemicals. Our findings demonstrate that MsepCSP8 likely contributes to mediating responses of M. separata adults to plant volatiles.

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

Chemoreception in insects is mediated by several components interacting at different levels and including odorant-binding proteins (OBPs). Although recent studies demonstrate that the function of OBPs cannot be restricted to an exclusively olfactory role, and that OBPs have been found also in organs generally not related to chemoreception, their feature of binding molecules remains undisputed. Studying the vetch aphid Megoura viciae (Buckton), we used a transcriptomic approach to identify ten OBPs in the antennae and we examined the ultrastructural morphology of sensilla and their distribution on the antennae, legs, mouthparts and cauda of wingless and winged adults by scanning electron microscopy (SEM). Three types of sensilla, trichoid, coeloconic and placoid, differently localized and distributed on antennae, mouthparts, legs and cauda, were described. The expression analysis of the ten OBPs was performed by RT-qPCR in the antennae and other body parts of the wingless adults and at different developmental stages and morphs. Five of the ten OBPs (MvicOBP1, MvicOBP3, MvicOBP6, MvicOBP7, and MvicOBP8), whose antibodies were already available, were selected for experiments of whole-mount immunolocalization on antennae, mouthparts, cornicles and cauda of adult aphids. Most of the ten OBPs were more expressed in antennae than in other body parts; MvicOBP1, MvicOBP3, MvicOBP6, MvicOBP7 were also immunolocalized in the sensilla on the antennae, suggesting a possible involvement of these proteins in chemoreception. MvicOBP6, MvicOBP7, MvicOBP8, MvicOBP9 were highly expressed in the heads and three of them (MvicOBP6, MvicOBP7, MvicOBP8) were immunolocalized in the sensilla on the mouthparts, supporting the hypothesis that also mouthparts may be involved in chemoreception. MvicOBP2, MvicOBP3, MvicOBP5, MvicOBP8 were highly expressed in the cornicles-cauda and two of them (MvicOBP3, MvicOBP8) were immunolocalized in cornicles and in cauda, suggesting a possible new function not related to chemoreception. Moreover, the response of M. viciae to different components of the alarm pheromone was assessed by behavioral assays on wingless adult morph; (-)-α-pinene and (+)-limonene were found to be the components mainly eliciting an alarm response. Taken together, our results represent a road map for subsequent in-depth analyses of the OBPs involved in several physiological functions in M. viciae, including chemoreception.

Identification and expression profiles of novel odorant binding proteins and functional analysis of OBP99a in Bactrocera dorsalis

Odorant-binding proteins (OBPs) in insects are essential for mating and oviposition host selection. How these OBPs respond to different hosts at the mRNA level and their effects on behavior remain poorly characterized. The oriental fruit fly Bactrocera dorsalis is a highly invasive agricultural pest with an extremely broad host range and high fecundity. Based on our previously constructed B. dorsalis transcriptome, six OBPs that were differentially expressed during three different physiological adult stages were identified. A phylogenetic tree was constructed to illustrate the relationships of these six OBPs with OBP sequences from three other dipteran species (Drosophila melanogaster, Anopheles gambiae, and Ceratitis capitata). The spatiotemporal expression profiles of the six OBPs were analyzed using quantitative real-time PCR. Our results revealed that OBP19c, OBP44a, OBP99a, and OBP99d were abundantly expressed from the prepupa stage to the adult stage, and most of the OBPs were mainly expressed in the head, wings, and antennae. The expression levels of these OBPs were upregulated when female flies were exposed to their preferred hosts. Silencing OBP99a resulted fewer eggs being laid compared with the control group when the females were exposed to their preferred host, that is, banana, whereas more eggs were laid when a non-preferred host, that is, tomato, was used. Furthermore, silencing OBP99a led to sexually dimorphic mating behavior. dsOBP99a-injected males dramatically reduced courtship, whereas enhanced courtship was observed in the treated females. These data indicate that OBPs may participate in different biological processes of B. dorsalis. Our study will provide insight into the molecular mechanism of chemoreception and help develop ecologically friendly pest-control strategies.

Pheromone Binding Protein EhipPBP1 Is Highly Enriched in the Male Antennae of the Seabuckthorn Carpenterworm and Is Binding to Sex Pheromone Components

The seabuckthorn carpenterworm moth Eogystia hippophaecolus is a major threat to seabuckthorn plantations, causing considerable ecological and economic losses in China. Transcriptomic analysis of E. hippophaecolus previously identified 137 olfactory proteins, including three pheromone-binding proteins (PBPs). We investigated the function of E. hippophaecolus PBP1 by studying its mRNA and protein expression profiles and its binding ability with different compounds. The highest levels of expression were in the antennae, particularly in males, with much lower levels of expression in the legs and external genitals. Recombinant PBP1 showed strong binding to sex-pheromone components, suggesting that antennal EhipPBP1 is involved in binding sex-pheromone components during pheromone communication.

Identification and Expression Profiling of Odorant-Binding Proteins and Chemosensory Proteins of Daktulosphaira vitifoliae (Hemiptera: Phylloxeridae)

In insects, odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) are primary peripheral olfactory proteins playing critical roles in odorant detection. In this study, we present the first identification of OBPs and CSPs from the transcriptome of grape phylloxera Daktulosphaira vitifoliae Fitch, an important pest that damages both roots and leaves of grapes. The OBPs contained six conserved cysteine residues and the CSPs contained four conserved cysteine residues in this insect. Phylogenetic analysis showed that most of the olfactory proteins were closely related to OBPs and CSPs from other aphids. However, DviOBP7 and DviCSP9 were different because they were classified into different independent branches, respectively. Real-time polymerase chain reaction (RT-PCR) was used to examine the tissue expression of these transcripts. DviOBP1, DviOBP6, and DviOBP7 were uniquely or primarily expressed in antennae and not in the body. DviOBP2 was more abundantly expressed in the body than in the antennae. The expression levels of OBPs and CSPs of phylloxera varied depending upon where they were expressed in different body tissues.

Molecular Basis of Alarm Pheromone Detection in Aphids

The sesquiterpene (E)-β-farnesene (EBF) is the alarm pheromone for many species of aphids [1]. When released from aphids attacked by parasitoids or predators, it alerts nearby conspecifics to escape by walking away and dropping off the host plant [2, 3]. The reception of alarm pheromone in aphids is accomplished through a highly sensitive chemosensory system. Although olfaction-related gene families including odorant receptors (ORs) and odorant-binding proteins (OBPs) have recently been identified from aphid genomes [4-6], the cellular and molecular mechanisms of EBF reception are still largely unknown. Here we demonstrate that ApisOR5, a member of the large superfamily of odorant receptors, is expressed in large placoid sensillum neurons on the sixth antennal segment and confers response to EBF when co-expressed with Orco, an obligate odorant receptor co-receptor, in parallel heterologous expression systems. In addition, the repellent behavior of Acyrthosiphon pisum to EBF disappears after knocking down ApisOR5 by RNAi as well as two A. pisum odorant-binding proteins known to bind EBF (ApisOBP3 and ApisOBP7). Furthermore, other odorants that can also activate ApisOR5, such as geranyl acetate, significantly repel A. pisum, as does EBF. Taken together, these data allow us to conclude that ApisOR5 is essential to EBF reception in A. pisum. The characterization of the EBF receptor allows high-throughput screening of aphid repellents, providing the necessary information to develop new strategies for aphid control.

Analysis of the Antennal Transcriptome and Insights into Olfactory Genes in Hyphantria cunea (Drury)

Hyphantria cunea (Drury) (Lepidoptera: Arctiidae) is an invasive insect pest which, in China, causes unprecedented damage and economic losses due to its extreme fecundity and wide host range, including forest and shade trees, and even crops. Compared to the better known lepidopteran species which use Type-I pheromones, little is known at the molecular level about the olfactory mechanisms of host location and mate choice in H. cunea, a species using Type-II lepidopteran pheromones. In the present study, the H. cunea antennal transcriptome was constructed by Illumina Hiseq 2500^TM sequencing, with the aim of discovering olfaction-related genes. We obtained 64,020,776 clean reads, and 59,243 unigenes from the analysis of the transcriptome, and the putative gene functions were annotated using gene ontology (GO) annotation. We further identified 124 putative chemosensory unigenes based on homology searches and phylogenetic analysis, including 30 odorant binding proteins (OBPs), 17 chemosensory proteins (CSPs), 52 odorant receptors (ORs), 14 ionotropic receptors (IRs), nine gustatory receptors (GRs) and two sensory neuron membrane proteins (SNMPs). We also found many conserved motif patterns of OBPs and CSPs using a MEME system. Moreover, we systematically analyzed expression patterns of OBPs and CSPs based on reverse transcription PCR and quantitative real time PCR (RT-qPCR) with RNA extracted from different tissues and life stages of both sexes in H. cunea. The antennae-biased expression may provide a deeper further understanding of olfactory processing in H. cunea. The first ever identification of olfactory genes in H. cunea may provide new leads for control of this major pest.

The genomewide transcriptional response underlying the pea aphid wing polyphenism

Phenotypic plasticity is a key life history strategy used by many plants and animals living in heterogeneous environments. A multitude of studies have investigated the costs and limits of plasticity, as well as the conditions under which it evolves. Much less well understood are the molecular genetic mechanisms that enable an organism to sense its environment and respond in a plastic manner. The pea aphid wing polyphenism is a compelling laboratory model to study these mechanisms. In this polyphenism, environmental stressors like high density cause asexual, viviparous adult female aphids to change the development of their embryos from wingless to winged morphs. The life history trade-offs between the two morphs have been intensively studied, but the molecular mechanisms underlying this process remain largely unknown. We therefore performed a genomewide study of the maternal transcriptome at two time points with and without a crowding stress to discover the maternal molecular changes that lead to the development of winged vs. wingless offspring. We observed significant transcriptional changes in genes associated with odorant binding, neurotransmitter transport, hormonal activity and chromatin remodelling in the maternal transcriptome. We also found that titres of serotonin, dopamine and octopamine were higher in solitary compared to crowded aphids. We use these results to posit a model for how maternal signals inform a developing embryo to be winged or wingless. Our findings add significant insights into the identity of the molecular mechanisms that underlie environmentally induced morph determination and suggest a possible role for biogenic amine regulation in polyphenisms generally.

Identification and Expression Analysis of Candidate Odorant-Binding Protein and Chemosensory Protein Genes by Antennal Transcriptome of Sitobion avenae

Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) of aphids are thought to be responsible for the initial molecular interactions during olfaction that mediate detection of chemical signals. Analysis of the diversity of proteins involved comprises critical basic research work that will facilitate the development of sustainable pest control strategies. To help us better understand differences in the olfactory system between winged and wingless grain aphids, we constructed an antennal transcriptome from winged and wingless Sitobion avenae (Fabricius), one of the most serious pests of cereal fields worldwide. Among the 133,331 unigenes in the antennal assembly, 13 OBP and 5 CSP putative transcripts were identified with 6 OBP and 3 CSP sequences representing new S. avenae annotations. We used qPCR to examine the expression profile of these genes sets across S. avenae development and in various tissues. We found 7 SaveOBPs and 1 SaveCSP were specifically or significantly elevated in antennae compared with other tissues, and that some transcripts (SaveOBP8, SaveCSP2 and SaveCSP5) were abundantly expressed in the legs of winged or wingless aphids. The expression levels of the SaveOBPs and SaveCSPs varied depending on the developmental stage. Possible physiological functions of these genes are discussed. Further molecular and functional studies of these olfactory related genes will explore their potential as novel targets for controlling S. avenae.

Antennal transcriptome analysis and expression profiles of odorant binding proteins in Eogystia hippophaecolus (Lepidoptera: Cossidae)

Background

Eogystia hippophaecolus (Hua et al.) (Lepidoptera: Cossidae) is the major threat to seabuckthorn plantations in China. Specific and highly efficient artificial sex pheromone traps was developed and used to control it. However, the molecular basis for the pheromone recognition is not known. So we established the antennal transcriptome of E. hippophaecolus and characterized the expression profiles of odorant binding proteins. These results establish and improve the basis knowledge of the olfactory receptive system, furthermore provide a theoretical basis for the development of new pest control method.

Results

We identified 29 transcripts encoding putative odorant-binding proteins (OBPs), 18 putative chemosensory proteins (CSPs), 63 odorant receptors (ORs), 13 gustatory receptors (GRs), 12 ionotropic receptors (IRs), and two sensory neuron membrane proteins (SNMPs). Based on phylogenetic analysis, we found one Orco and three pheromone receptors of E. hippophaecolus and found that EhipGR13 detects sugar, EhipGR11 and EhipGR3 detect bitter. Nine OBPs expression profile indicated that most were the highest expression in antennae, consistent with functions of OBPs in binding and transporting odors during the antennal recognition process. OBP6 was external expressed in male genital-biased in, and this locus may be responsible for pheromone binding and recognition as well as mating. OBP1 was the highest and biased expressed in the foot and may function as identification of host plant volatiles.

Conclusions

One hundred thirty-seven chemosensory proteins were identified and the accurate functions and groups of part proteins were obtained by phylogenetic analysis. The most OBPs were antenna-biased expressed, which are involved in antennal recognition. However, few OBP was detected biased expression in the foot and external genitalia, and these loci may function in pheromone recognition, mating, and the recognition of plant volatiles.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3008-4) contains supplementary material, which is available to authorized users.

Crystal Structures and Binding Dynamics of Odorant-Binding Protein 3 from two aphid species Megoura viciae and Nasonovia ribisnigri

Aphids use chemical cues to locate hosts and find mates. The vetch aphid Megoura viciae feeds exclusively on the Fabaceae, whereas the currant-lettuce aphid Nasonovia ribisnigri alternates hosts between the Grossulariaceae and Asteraceae. Both species use alarm pheromones to warn of dangers. For N. ribisnigri this pheromone is a single component (E)-β-farnesene but M. viciae uses a mixture of (E)-β-farnesene, (-)-α-pinene, β-pinene, and limonene. Odorant-binding proteins (OBP) are believed to capture and transport such semiochemicals to their receptors. Here, we report the first aphid OBP crystal structures and examine their molecular interactions with the alarm pheromone components. Our study reveals some unique structural features: 1) the lack of an internal ligand binding site; 2) a striking groove in the surface of the proteins as a putative binding site; 3) the N-terminus rather than the C-terminus occupies the site closing off the conventional OBP pocket. The results from fluorescent binding assays, molecular docking and dynamics demonstrate that OBP3 from M. viciae can bind to all four alarm pheromone components and the differential ligand binding between these very similar OBP3s from the two aphid species is determined mainly by the direct π-π interactions between ligands and the aromatic residues of OBP3s in the binding pocket.