Chemosensory proteins of the eastern honeybee, Apis cerana: Identification, tissue distribution and olfactory related functional characterization

The antennal transcriptome analysis and characterizations of odorant-binding proteins in Megachile saussurei (Hymenoptera, Megachilidae)

BACKGROUND

Odorant-binding proteins (OBPs) are essential in insect's daily behaviors mediated by olfactory perception. Megachile saussurei Radoszkowski (Hymenoptera, Megachilidae) is a principal insect pollinating alfalfa (Medicago sativa) in Northwestern China. The olfactory function have been less conducted, which provides a lot of possibilities for our research.

RESULTS

Our results showed that 20 OBPs were identified in total. Multiple sequence alignment analysis indicated MsauOBPs were highly conserved with a 6-cysteine motif pattern and all belonged to the classic subfamily, coding 113-196 amino acids and sharing 41.32%-99.12% amino acid identity with known OBPs of other bees. Phylogenetic analysis indicated there were certain homologies existed among MsauOBPs and most sequences were clustered with that of Osmia cornuta (Hymenoptera, Megachilidae). Expression analysis showed the identified OBPs were mostly enriched in antennae instead of other four body parts, especially the MsauOBP2, MsauOBP3, MsauOBP4, MsauOBP8, MsauOBP11 and MsauOBP17, in which the MsauOBP2, MsauOBP4 and MsauOBP8 presented obvious tissue-biased expression pattern. Molecular docking results indicated MsauOBP4 might be the most significant protein in recognizing alfalfa flower volatile 3-Octanone, while MsauOBP13 might be the most crucial protein identifying (Z)-3-hexenyl acetate. It was also found the lysine was a momentous hydrophilic amino acid in docking simulations.

CONCLUSION

In this study, we identified and analyzed 20 OBPs of M. saussurei. The certain homology existed among these OBPs, while some degree of divergence could also be noticed, indicating the complex functions that different MsauOBPs performed. Besides, the M. saussurei and Osmia cornuta were very likely to share similar physiological functions as most of their OBPs were clustered together. MsauOBP4 might be the key protein in recognizing 3-Octanone, while MsauOBP13 might be the key protein in binding (Z)-3-hexenyl acetate. These two proteins might contribute to the alfalfa-locating during the pollination process. The relevant results may help determine the highly specific and effective attractants for M. saussurei in alfalfa pollination and reveal the molecular mechanism of odor-evoked pollinating behavior between these two species.

Characterization of CrufCSP1 and Its Potential Involvement in Host Location by Cotesia ruficrus (Hymenoptera: Braconidae), an Indigenous Parasitoid of Spodoptera frugiperda (Lepidoptera: Noctuidae) in China

Chemosensory proteins (CSPs) are a class of soluble proteins that facilitate the recognition of chemical signals in insects. While CSP genes have been identified in many insect species, studies investigating their function remain limited. Cotesia ruficrus (Hymenoptera: Braconidae) holds promise as an indigenous biological control agent for managing the invasive pest Spodoptera frugiperda (Lepidoptera: Noctuidae) in China. This study aimed to shed light on the gene expression, ligand binding, and molecular docking of CrufCSP1 in C. ruficrus. A RT-qPCR analysis revealed that the expression of CrufCSP1 was higher in the wings, with male adults exhibiting significantly higher relative expression levels than other developmental stages. A fluorescence competitive binding analysis further demonstrated that CrufCSP1 has a high binding ability with several host-related volatiles, with trans-2-hexenal, octanal, and benzaldehyde showing the strongest affinity to CrufCSP1. A molecular docking analysis indicated that specific amino acid residues (Phe24, Asp25, Thr53, and Lys81) of CrufCSP1 can bind to these specific ligands. Together, these findings suggest that CrufCSP1 may play a crucial role in the process of C. ruficrus locating hosts. This knowledge can contribute to the development of more efficient and eco-friendly strategies for protecting crops and managing pests.

Evidence for the Participation of Chemosensory Proteins in Response to Insecticide Challenge in Conopomorpha sinensis

Chemosensory proteins (CSPs) are a type of efficient transporters that can bind various hydrophobic compounds. Previous research has shown that the expression levels of some insect CSPs were significantly increased after insecticide treatment. However, the role of CSPs in response to insecticide challenge is unclear. Conopomorpha sinensis is the most destructive borer pest of litchi (Litchi chinensis) and longan (Euphoria longan) in the Asia-Pacific region. Here, we studied the expression patterns and potential functions of 12 CSP genes (CsCSPs) from C. sinensis in response to λ-cyhalothrin exposure. The spatiotemporal distribution of CsCSPs suggested that they were predominantly expressed in the female abdomen, female legs, and male legs. The expression levels of CsCSPs were affected in a time-dependent manner after λ-cyhalothrin treatment in both sexes of C. sinensis adults. Compared to the control group, the expression levels of CsCSP1, CsCSP2, CsCSP9, and CsCSP12 in females were significantly increased by 2-4 times, while only one CsCSP, three CsCSPs, and two CsCSPs were significantly upregulated in males at three time points post-treatment. The sex-biased variance of CSP expression may be related to sex-specific detoxification enzymatic activities and survival rates of C. sinensis in response to insecticide challenge. Homology modeling and molecular docking analyses showed that the binding energy value of CsCSP1-12 to λ-cyhalothrin was negative and the binding energy between CsCSP9 and λ-cyhalothrin was the lowest (-11.35 kJ/mol). Combined with expression alterations of CsCSP1-12, the results indicate that CsCSP1, CsCSP2, CsCSP9, and CsCSP12 were involved in binding and ferrying of λ-cyhalothrin in C. sinensis.

Sex-Biased Expression of Olfaction-Related Genes in the Antennae of Apis cerana (Hymenoptera: Apidae)

The olfactory system is essential for honeybees to adapt to complex and ever-changing environments and maintain cohesiveness. The Eastern honeybee Apis cerana is native to Asia and has a long history of managed beekeeping in China. In this study, we analysed the antennal transcriptomes of A. cerana workers and drones using Illumina sequencing. A total of 5262 differentially expressed genes (DEGs) (fold change > 2) were identified between these two castes, with 2359 upregulated and 2903 downregulated in drones compared with workers. We identified 242 candidate olfaction-related genes, including 15 odourant-binding proteins (OBPs), 5 chemosensory proteins (CSPs), 110 odourant receptors (ORs), 9 gustatory receptors (GRs), 8 ionotropic receptors (IRs), 2 sensory neuron membrane proteins (SNMPs) and 93 putative odourant-degrading enzymes (ODEs). More olfaction-related genes have worker-biased expression than drone-biased expression, with 26 genes being highly expressed in workers’ antennae and only 8 genes being highly expressed in drones’ antennae (FPKM > 30). Using real-time quantitative PCR (RT-qPCR), we verified the reliability of differential genes inferred by transcriptomics and compared the expression profiles of 6 ORs (AcOR10, AcOR11, AcOR13, AcOR18, AcOR79 and AcOR170) between workers and drones. These ORs were expressed at significantly higher levels in the antennae than in other tissues (p < 0.01). There were clear variations in the expression levels of all 6 ORs between differently aged workers and drones. The relative expression levels of AcOR10, AcOR11, AcOR13, AcOR18 and AcOR79 reached a high peak in 15-day-old drones. These results will contribute to future research on the olfaction mechanism of A. cerana and will help to better reveal the odourant reception variations between different biological castes of honeybees.

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.

Identification and functional analysis of a chemosensory protein from Bactrocera minax (Diptera: Tephritidae)

BACKGROUND

Olfaction has an indispensable role in insect behavior, enabling location of suitable host plants and oviposition sites, finding mates and evasion of natural enemies. Chemosensory proteins (CSPs) function to screen external odorants and transport them to olfactory receptor neurons, thereby increasing the sensitivity of the olfactory system. At present, CSP genes have been identified in many insect species, but there are relatively few studies on the function of CSP, especially in Tephritidae.

RESULTS

In this study, we sequenced and analyzed 12 transcriptomes of Bactrocera minax and identified five CSP genes. The results of polymerase chain reactions with reverse transcription showed that BminCSP3 was highly expressed only in antennae. Results from competitive binding experiments showed that BminCSP3 has good binding ability to citral compared with 23 other volatile organic compounds. The docking model with citral showed hydrogen bond formation with residues (ARG97); however, no hydrogen bonds were formed in the docking of five other ligands (furfuryl alcohol, linalool, cis-3-hexenyl acetate, (R)-(+)-limonene and (+)-carvone). Electroantennogram (EAG) analyses revealed that citral was active in B. minax at the antennal level, and the EAG response value of female adults was significantly higher than that of male adults. Furthermore, the results of behavioral bioassays showed that females were significantly attracted to citral.

CONCLUSION

Our results suggest that BminCSP3 plays an important role in the recognition of citral by B. minax adults. © 2022 Society of Chemical Industry.

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.

Phosphoproteomic basis of neuroplasticity in the antennal lobes influences the olfactory differences between A. mellifera and A. cerana honeybees

The honeybee species A. mellifera and A. cerana have evolved substantial differences in olfactory-driven behaviors and in peripheral olfactory systems. Knowledge of the central nervous system regulating these olfaction differences is limited, however. We compared the phosphoproteome of the antennal lobes (ALs, the primary olfactory neuropil) of A. mellifera and A. cerana, and identified a total of 2812 phosphopeptides carrying 2971 phosphosites from 1265 phosphoproteins. Of these, 76% of the phosphoproteins were shared by both species, which were mainly presynapse and cytoskeleton components, and were involved in signal transduction and neurotransmitter secretion. This finding indicates the fundamental role of protein phosphorylation in regulating signal transduction in the ALs. The mTOR signaling pathway, the phagosome pathway, and the autophagy pathway, which are important in protein metabolism, were enriched, suggesting glomeruli plasticity and olfactory processing are intensively modulated by phosphorylation via these pathways. Compared with A. mellifera, 107 phosphoproteins associated with protein metabolism and transport were uniquely expressed in A. cerana, indicating the protein synthesis-dependent synaptic plasticity is enhanced in A. cerana to facilitate the processing of more complex floral odor clues in mountain foraging areas. This finding is further supported by the significantly upregulated key phosphoproteins of the mTOR signaling pathway in A. cerana ALs. These results provide insights into the phosphoproteomic basis of neuroplasticity that is coupled with the divergent evolution of bees in different environments. SIGNIFICANCE: To adapt to their own ecological niche, the two major honeybee species, A. mellifera and A. cerana, have developed significant difference in olfactory-driven behaviors, but our understanding of the underlying regulation of the central nervous system is still limitate. Here we performed the first comprehensive phosphoproteomic comparison of antennal lobes (Als) between A. mellifera and A. cerena. A large proportion of the identified phosphosites and phosphoproteins were shared between the two species to serve as a core network in the regulation of signal transduction and glomeruli plasticity of ALs. However, compared with A. mellifera, 107 phosphoproteins associated with protein metabolism and transport were uniquely identified in A. cerana ALs, and also several key phosphoproteins in mTOR signaling pathway were found upregulated in A. cerana. These findings indicate protein phosphorylation enhanced the protein synthesis-dependent synaptic plasticity in A. cerana to facilitate the processing of more complex floral odor clues in mountain foraging areas. Our data provide a valuable insight into phosphoproteome-driven cerebral regulation of honeybee olfactory behaviors, which is potentially useful for further neurobiological investigation in both honeybees and other insects.

Use of waggle dance information in honey bees is linked to gene expression in the antennae, but not in the brain

Communication is essential for social animals, but deciding how to utilize information provided by conspecifics is a complex process that depends on environmental and intrinsic factors. Honey bees use a unique form of communication, the waggle dance, to inform nestmates about the location of food sources. However, as in many other animals, experienced individuals often ignore this social information and prefer to rely on prior experiences, i.e., private information. The neurosensory factors that drive the decision to use social information are not yet understood. Here we test whether the decision to use social dance information or private information is linked to gene expression differences in different parts of the nervous system. We trained bees to collect food from sugar water feeders and observed whether they utilize social or private information when exposed to dances for a new food source. We performed transcriptome analysis of four brain parts (11-16 bees per tissue type) critical for cognition: the subesophageal ganglion, the central brain, the mushroom bodies, and the antennal lobes but, unexpectedly, detected no differences between social or private information users. In contrast, we found 413 differentially expressed genes in the antennae, suggesting that variation in sensory perception mediates the decision to use social information. Social information users were characterized by the upregulation of biogenic amine genes, while private information users upregulated several genes coding for odour perception. These results highlight that decision-making in honey bees might also depend on peripheral processes of perception rather than higher-order brain centres of information integration.

Evidence for the Involvement of the Chemosensory Protein AgosCSP5 in Resistance to Insecticides in the Cotton Aphid, Aphis gossypii

Simple Summary

Insect chemosensory proteins (CSPs) are potential targets for insect pest control strategies and are proposed to function in insect chemoreception, because they play a role in crop host location by binding and transporting odorant molecules. They are also thought to have other functions, for example, in tissue regeneration and in insecticide resistance, because they also express in nonolfactory tissues and are capable of binding insecticides. However, there are few reports that provide direct evidence for this proposal. In this study, we discovered gene gain-and-loss among aphid populations, possibly associated with different insecticide resistance, and then identified and cloned a CSP gene responsive to insecticide treatments. The introduction of such gene in Drosophila fruit flies made the transgenic flies less sensitive to the treatment of different insecticides. Our study advances the research of insect CSP functions and offers valuable new information to target CSPs for pest management.

Abstract

It has been speculated that insect chemosensory proteins (CSPs) may have additional roles beyond olfaction. In this study, the phylogenetic and genomic analyses of the CSPs of the cotton aphid, Aphis gossypii, revealed the presence of gene gain-and-loss among different aphid field populations. Differential expressions of eight CSP genes were demonstrated after treatments with insecticides of different modes of action. The expression of AgosCSP5 was significantly upregulated by the insecticide treatments in a dose-dependent manner. The Drosophila flies overexpressing AgosCSP5 were significantly less susceptible to the insecticides, omethoate, imidacloprid and cypermethrin but not to deltamethrin and tau-fluvalinate, compared with control flies. The transgenic Drosophila flies exhibited an LC₅₀ resistance ratio of 2.6 to omethoate, compared with control flies. Likewise, the mortality of the transgenic flies to imidacloprid and cypermethrin was significantly lower than that of the control flies (p < 0.01). Homology modelling, molecular docking and dynamic simulation supported the interactions and revealed a higher stability of AgosCSP5/insecticide complexes than AgosCSP5/semiochemical complexes. Our study demonstrates for first time the in vivo evidence for the involvement of CSP genes in insecticide resistance of crop insect pests and provides new insights of the newly discovered CSP-mediated insect resistance mechanism to insecticides.

Red Imported Fire Ant (Solenopsis invicta) Chemosensory Proteins Are Expressed in Tissue, Developmental, and Caste-Specific Patterns

The red imported fire ant, Solenopsis invicta, is a eusocial invasive insect that has spread worldwide. Chemosensory proteins (CSPs) are ligand-binding proteins that participate in a diverse range of physiological processes that include olfaction and chemical transport. Here, we performed a systematic survey of the expression of the 21 gene S. invicta CSP family that includes at least two groups of apparent S. invicta-specific gene expansions. These data revealed caste, tissue, and developmental stage-specific differential expression of the SiCSPs. In general, moderate to high SiCSP expression was seen in worker antennae and abdomen tissues with lower expression in head/thorax regions. Male and female alates showed high antennal expression of fewer SiCSPs, with the female alate thorax showing comparatively high SiCSP expression. SiCSP expression was lower in male alates tissues compared to workers and female alates, albeit with some highly expressed SiCSPs. SiCSP expression was low during development including in eggs, larvae (early and late instars), and pupae. Global analyses revealed examples of conserved, divergent, and convergent SiCSP expression patterns linked to phylogenetic relationships. The developmental and caste-specific variation seen in SiCSP expression patterns suggests specific functional diversification of CSPs that may translate into differential chemical recognition and communication among individuals and/or reflect other cellular roles of CSPs. Our results support a model for CSPs acting as general ligand carriers involved in a wide range of physiological processes beyond olfaction. As compared to the expression patterns of the S. invicta odorant binding proteins (OBPs), an inverse correlation between SiOBP and SiCSP expression was seen, suggesting potential complementary and/or compensatory functions between these two classes of ligand carriers.

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.

Contact Chemosensory Genes Identified in Leg Transcriptome of Apis cerana cerana (Hymenoptera: Apidae)

Correct gustatory recognition and selection of foods both within and outside the hive by honey bee workers are fundamental to the maintenance of colonies. The tarsal chemosensilla located on the legs of workers are sensitive to nonvolatile compounds and proposed to be involved in gustatory detection. However, little is known about the molecular mechanisms underlying the gustatory recognition of foods in honey bees. In the present study, RNA-seq was performed with RNA samples extracted from the legs of 1-, 10-, and 20-d-old workers of Apis cerana cerana Fabricius, a dominant indigenous crop pollinator with a keen perception ability for phytochemicals. A total of 124 candidate chemosensory proteins (CSPs), including 15 odorant-binding proteins (OBPs), 5 CSPs, 7 gustatory receptors (GRs), 2 sensory neuron membrane proteins (SNMPs), and 95 odorant receptors (ORs), were identified from the assembled leg transcriptome. In silico analysis of expression showed that 36 of them were differentially expressed among the three different ages of A. c. cerana workers. Overall, the genes encoding OBPs and CSPs had great but extremely variable FPKM values and thus were highly expressed in the legs of workers, whereas the genes encoding ORs, GRs, and SNMPs (except SNMP2) were expressed at low levels. Tissue-specific expression patterns indicated that two upregulated genes, AcerOBP15 and AcerCSP3, were predominately expressed in the legs of 20-d-old foragers, suggesting they may play an essential role in gustatory recognition and selection of plant nectars and pollens. This study lays a foundation for further research on the feeding preferences of honey bees.

Tools in the Investigation of Volatile Semiochemicals on Insects: From Sampling to Statistical Analysis

The recognition of volatile organic compounds (VOCs) involved in insect interactions with plants or other organisms is essential for constructing a holistic comprehension of their role in ecology, from which the implementation of new strategies for pest and disease vector control as well as the systematic exploitation of pollinators and natural enemies can be developed. In the present paper, some of the general methods employed in this field are examined, focusing on their available technologies. An important part of the investigations conducted in this context begin with VOC collection directly from host organisms, using classical extraction methods, by the employment of adsorption materials used in solid-phase micro extraction (SPME) and direct-contact sorptive extraction (DCSE) and, subsequently, analysis through instrumental analysis techniques such as gas chromatography (GC), nuclear magnetic resonance (NMR) and mass spectrometry (MS), which provide crucial information for determining the chemical identity of volatile metabolites. Behavioral experiments, electroantennography (EAG), and biosensors are then carried out to define the semiochemicals with the best potential for performing relevant functions in ecological relationships. Chemical synthesis of biologically-active VOCs is alternatively performed to scale up the amount to be used in different purposes such as laboratory or field evaluations. Finally, the application of statistical analysis provides tools for drawing conclusions about the type of correlations existing between the diverse experimental variables and data matrices, thus generating models that simplify the interpretation of the biological roles of VOCs.

Hydrocarbons mediate seed dispersal: a new mechanism of vespicochory

Vespicochory, seed dispersal by hornets, is an uncommon seed dispersal pattern in angiosperms. To date, this phenomenon has been recorded in only four families. Because of its rarity, the causes and consequences of vespicochory remain unclear. Hence, this seed dispersal syndrome is often regarded as anecdotal. Through field investigations, chemical analyses, electrophysiological tests, identification of chemosensory proteins from the antennae of hornets, and behavioral assays, we investigated whether olfactory and/or visual cues of the diaspores of Stemona tuberosa mediate the behavior of the social hornets and maintain their mutualism. This study demonstrated that the elaiosome of S. tuberosa emits hydrocarbons, which are attractive to hornets. However, these compounds, which induce responses in the antennae of naive hornets, are ubiquitous substances on insect cuticle surfaces. Innate preference and experienced foraging behavior of hornets can increase their seed dispersal efficiency. This is the first example in which hydrocarbons have been identified as a diaspore odour involved in the attraction of hornets. Given that the ubiquity of hornets, and the communication function of hydrocarbons in insects, we predict that this rare seed dispersal mechanism may be an overlooked mechanism of insect-plant mutualism.

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.

Combinatorial multispectral, thermodynamics, docking and site-directed mutagenesis reveal the cognitive characteristics of honey bee chemosensory protein to plant semiochemical

In the chemoreceptive system of insects, there are always some soluble binding proteins, such as some antennal-specific chemosensory proteins (CSPs), which are abundantly distributed in the chemosensory sensillar lymph. The antennal-specific CSPs usually have strong capability to bind diverse semiochemicals, while the detailed interaction between CSPs and the semiochemicals remain unclear. Here, by means of the combinatorial multispectral, thermodynamics, docking and site-directed mutagenesis, we detailedly interpreted a binding interaction between a plant semiochemical β-ionone and antennal-specific CSP1 from the worker honey bee. Thermodynamic parameters (ΔH < 0, ΔS > 0) indicate that the interaction is mainly driven by hydrophobic forces and electrostatic interactions. Docking prediction results showed that there are two key amino acids, Phe44 and Gln63, may be involved in the interacting process of CSP1 to β-ionone. In order to confirm the two key amino acids, site-directed mutagenesis were performed and the binding constant (K_A) for two CSP1 mutant proteins was reduced by 60.82% and 46.80% compared to wild-type CSP1. The thermodynamic analysis of mutant proteins furtherly verified that Phe44 maintained an electrostatic interaction and Gln63 contributes hydrophobic and electrostatic forces. Our investigation initially elucidates the physicochemical mechanism of the interaction between antennal-special CSPs in insects including bees to plant semiochemicals, as well as the development of twice thermodynamic analysis (wild type and mutant proteins) combined with multispectral and site-directed mutagenesis methods.

Silencing of Chemosensory Protein Gene NlugCSP8 by RNAi Induces Declining Behavioral Responses of Nilaparvata lugens

Chemosensory proteins (CSPs) play imperative functions in chemical and biochemical signaling of insects, as they distinguish and transfer ecological chemical indications to a sensory system in order to initiate behavioral responses. The brown planthopper (BPH), Nilaparvata lugens Stål (Hemiptera: Delphacidae), has emerged as the most destructive pest, causing serious damage to rice in extensive areas throughout Asia. Biotic characteristics like monophagy, dual wing forms, and annual long-distance migration imply a critical role of chemoreception in N. lugens. In this study, we cloned the full-length CSP8 gene from N. lugens. Protein sequence analysis indicated that NlugCSP8 shared high sequence resemblance with the CSPs of other insect family members and had the typical four-cysteine signature. Analysis of gene expression indicated that NlugCSP8 mRNA was specifically expressed in the wings of mated 3-day brachypterous females with a 175-fold difference compare to unmated 3-day brachypterous females. The NlugCSP8 mRNA was also highly expressed in the abdomen of unmated 5-day brachypterous males and correlated to the age, gender, adult wing form, and mating status. A competitive ligand-binding assay demonstrated that ligands with long chain carbon atoms, nerolidol, hexanal, and trans-2-hexenal were able to bind to NlugCSP8 in declining order of affinity. By using bioinformatics techniques, three-dimensional protein structure modeling and molecular docking, the binding sites of NlugCSP8 to the volatiles which had high binding affinity were predicted. In addition, behavioral experiments using the compounds displaying the high binding affinity for the NlugCSP8, revealed four compounds able to elicit significant behavioral responses from N. lugens. The in vivo functions of NlugCSP8 were further confirmed through the testing of RNAi and post-RNAi behavioral experiments. The results revealed that reduction in NlugCSP8 transcript abundance caused a decrease in behavioral response to representative attractants. An enhanced understanding of the NlugCSP8 is expected to contribute in the improvement of more effective and eco-friendly control strategies of BPH.

Identification, Expression Patterns, and Functional Characterization of Chemosensory Proteins in Dendroctonus armandi (Coleoptera: Curculionidae: Scolytinae)

The Chinese white pine beetle, Dendroctonus armandi Tsai and Li (Coleoptera: Curculionidae: Scolytinae), is a serious pest of coniferous forests in China. Thus, there is considerable interest in developing eco-friendly pest-control methods, with the use of semiochemicals as a distinct possibility. Olfaction is extremely important for fitness of D. armandi because it is the primary mechanism through which the insect locates hosts and mates. Thus, here we characterized nine full-length genes encoding chemosensory proteins (CSPs) from D. armandi. The genes were ubiquitously and multiply expressed across different developmental stages and adult tissues, indicating various roles in developmental metamorphosis, olfaction, and gustation. Ligand-binding assays implied that DarmCSP2 may be the carrier of D. armandi pheromones and various plant host volatiles. These volatiles were identified through RNA interference of DarmCSP2 as: (+)-α-pinene, (+)-β-pinene, (−)-β-pinene, (+)-camphene, (+)-3-carene, and myrcene. The systematic chemosensory functional analysis of DarmCSP2 in this study clarified the molecular mechanisms underlying D. armandi olfaction and provided a theoretical foundation for eco-friendly pest control.

Identification of odorant-binding and chemosensory protein genes and the ligand affinity of two of the encoded proteins suggest a complex olfactory perception system in Periplaneta americana

The American cockroach (Periplaneta americana) is an urban pest with a precise chemosensory system that helps it achieve complex physiological behaviours, including locating food and mating. However, its chemosensory mechanisms have not been well studied. Here, we identified 71 putative odorant carrier protein genes in P. americana, including 57 new odorant-binding proteins (OBPs) and 11 chemosensory proteins (CSPs). To identify their physiological functions, we investigated their tissue expression patterns in antennae, mouthparts, legs, and the remainder of the body of both sexes, and determined that most of these genes were expressed in chemosensory organs. A phylogenetic tree showed that the putative pheromone-binding proteins of P. americana were in different clades from those of moths. Two genes, PameOBP24 and PameCSP7, were expressed equally in antennae of both sexes and highly expressed amongst the OBPs and CSPs. These genes were expressed in Escherichia coli and the resultant proteins were purified. The binding affinities of 74 common odorant compounds were tested with recombinant PameOBP24 and PameCSP7. Both proteins bound a variety of ligands. Our findings provide a foundation for future research into the chemosensory mechanisms of P. americana and help in identifying potential target genes for managing this pest.

Antennal Transcriptome and Differential Expression Analysis of Five Chemosensory Gene Families from the Asian Honeybee Apis cerana cerana

Chemosensory genes play a central role in sensing chemical signals and guiding insect behavior. The Chinese honeybee, Apis cerana cerana, is one of the most important insect species in China in terms of resource production, and providing high-quality products for human consumption, and also serves as an important pollinator. Communication and foraging behavior of worker bees is likely linked to a complex chemosensory system. Here, we used transcriptome sequencing on adult A. c. cerana workers of different ages to identify the major chemosensory gene families and the differentially expressed genes(DEGs), and to investigate their expression profiles. A total of 109 candidate chemosensory genes in five gene families were identified from the antennal transcriptome assemblies, including 17 OBPs, 6 CSPs, 74 ORs, 10 IRs, and 2SNMPs, in which nineteen DEGs were screened and their expression values at different developmental stages were determined in silico. No chemosensory transcript was specific to a certain developmental period. Thirteen DEGs were upregulated and 6were downregulated. We created extensive expression profiles in six major body tissues using qRT-PCR and found that most DEGs were exclusively or primarily expressed in antennae. Others were abundantly expressed in the other tissues, such as head, thorax, abdomen, legs, and wings. Interestingly, when a DEG was highly expressed in the thorax, it also had a high level of expression in legs, but showed a lowlevel in antennae. This study explored five chemoreceptor superfamily genes using RNA-Seq coupled with extensive expression profiling of DEGs. Our results provide new insights into the molecular mechanism of odorant detection in the Asian honeybee and also serve as an extensive novel resource for comparing and investigating olfactory functionality in hymenopterans.