RETRACTED ARTICLE: Involvement of a Specific Chemosensory Protein from Bactrocera dorsalis in Perceiving Host Plant Volatiles

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

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

The Crystal Structure of the Spodoptera litura Chemosensory Protein CSP8

Spodoptera litura F. is a generalist herbivore and one of the most important economic pests feeding on about 300 host plants in many Asian countries. Specific insect behaviors can be stimulated after recognizing chemicals in the external environment through conserved chemosensory proteins (CSPs) in chemoreceptive organs, which are critical components of the olfactory systems. To explore its structural basis for ligand-recognizing capability, we solved the 2.3 Å crystal structure of the apoprotein of S. litura CSP8 (SlCSP8). The SlCSP8 protein displays a conserved spherical shape with a negatively charged surface. Our binding assays showed that SlCSP8 bound several candidate ligands with differential affinities, with rhodojaponin III being the most tightly bound ligand. Our crystallographic and biochemical studies provide important insight into the molecular recognition mechanism of the sensory protein SlCSP8 and the CSP family in general, and they suggest that CSP8 is critical for insects to identify rhodojaponin III, which may aid in the CSP-based rational drug design in the future.

Chemosensory proteins participate in insecticide susceptibility in Rhopalosiphum padi, a serious pest on wheat crops

Rhopalosiphum padi is a worldwide agricultural pest. Chemosensory proteins (CSPs) are considered to be a type of transporters which can bind chemicals from external environments. Previous research showed that the expression of some insect CSPs were significantly increased after exposure to insecticides, and CSPs were involved in insecticide resistance or susceptibility. However, the role of CSPs in the susceptibility and response of R. padi to insecticides is still unknown. In this study, we identified eight CSP (RpCSP) from R. padi by genome-wide investigation. Seven RpCSP genes had two exons, while RpCSP7 had three exons. qPCR analyses showed that the mRNA levels of the eight RpCSP genes were significantly affected by imidacloprid and beta-cypermethrin in different post-treatment periods. Molecular docking predicted that there were hydrogen bonding sites which played key roles in binding of RpCSP4, RpCSP5, RpCSP6, RpCSP7 and RpCSP10 with imidacloprid and beta-cypermethrin. Knockdown of RpCSP4, RpCSP5, RpCSP6 and RpCSP10 by RNA interference significantly increased the aphid mortality under two sublethal concentrations of imidacloprid. Mortalities under two sublethal concentrations of beta-cypermethrin conditions were significantly higher after injection of R. padi with dsCSP4 and dsCSP6. The results indicate that some RpCSP genes are involved in the insecticide susceptibility of R. padi.

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.

Three Chemosensory Proteins Involved in Chemoreception of Oedaleus asiaticus (Orthopera: Acridoidea)

Chemosensory proteins (CSPs) are thought to play roles in the insect olfactory system by binding and carrying hydrophobic odorants across the aqueous sensillar lymph. The band-winged grasshopper, Oedaleus asiaticus Bei-Bienko, is one of the most important grasshopper pests in northern China, but there is little information about its olfactory system. In order to investigate the olfactory functions of CSPs in this pest, three CSP genes (OasiCSP4, OasiCSP11 and OasiCSP12) were expressed in Escherichia coli, and the binding affinities of the three recombinant CSP proteins were measured for 16 volatiles from the host plant (Stipa krylovii), fecal material and body of live adult O. asiaticus using fluorescence competitive binding assays. To further verify their olfactory functions, RNA interference (RNAi) and electrophysiological recording were conducted. The three recombinant proteins displayed different degrees of binding to various volatiles in ligand-binding assays, with OasiCSP12 having higher binding affinities for more volatiles than OasiCSP4 and OasiCSP11. OasiCSP12 exhibited strong binding affinities (Ki < 20 μΜ) for five host plant volatiles and one volatile from the live body of adult O. asiaticus. The transcript levels of the three OasiCSP genes were significantly lower after silencing the individual genes by RNAi, which in turn reduced the EAG responses in adults of both sexes to most tested compounds. Our study indicates that these three OasiCSPs are involved in the detection of volatile semiochemicals, and may play important roles in finding host plants and in aggregation in O. asiaticus.

OcomCSP12, a Chemosensory Protein Expressed Specifically by Ovary, Mediates Reproduction in Ophraella communa (Coleoptera: Chrysomelidae)

Chemosensory proteins (CSPs) are considered to be the transporter linking odorant chemicals and receptors on sensory neurons. However, the extensive expression patterns of CSPs in insects suggest that CSPs are also involved in other physiological processes; the range of their functions, however, remains uncertain. In this study, we successfully characterized and cloned the CSP12 of Ophraella communa (OcomCSP12). The open reading frame of OcomCSP12 encodes 131 amino acids, with four conserved cysteine residues. The expression patterns of OcomCSP12 validated by quantitative real-time PCR (qRT-PCR) showed that OcomCSP12 is specifically expressed in female ovary. Furthermore, compared with the control treatment, silencing OcomCSP12 resulted in significantly reduced oviposition in females. Surprisingly, the knock-down rate of OcomCSP12 exceeded 95% and remained depressed for more than 15 days, indicating that RNA interference (RNAi) was a suitable method for exploring the function of CSP12 in O. communa. These findings increase our understanding of the expression profile and function of the CSP gene family in insects.

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.

Binding properties of four antennae-expressed chemosensory proteins (CSPs) with insecticides indicates the adaption of Spodoptera litura to environment

Insects receive a variety of chemical signals from the environment. Chemosensory protein (CSP) is one of the olfactory proteins that can accommodate a variety of small molecules and have the ability to bind to lipophilic compounds, transmitting nonvolatile odor molecules and chemical stimuli to target cells. To understand the correlation between the insect olfactory system and environment, we identified four antennae-expressed SlituCSP genes and investigated their expression profiles after treatment with different temperatures, starvation and three commonly used pesticides: chlorpyrifos, emamectin benzoate and fipronil. The transcriptions of four SlituCSP genes are affected by pesticide treatment and less affected by starvation and different temperatures. To further understand the molecular function of CSPs and their correlation with pesticides, we expressed and purified four SlituCSPs and assayed their binding ability with pesticides. The binding of four SlituCSPs with three pesticides were determined using a fluorescence competitive binding assay. We found direct binding between CSPs and pesticides, especially between SlituCSP18 and chlorpyrifos/fipronil and between SlituCSP6 and all three pesticides. The high binding affinity with pesticides and the significant down-regulation of SlituCSP18 by chlorpyrifos suggests that SlituCSP18 is more sensitive to pesticide treatment and may play an important role in mediating the interaction of the olfactory system and the pesticide. This study can help us understand the role of CSP proteins in the adaption of S. litura to the environment.

Identification of Genes Involved in Chemoreception in Plutella xyllostella by Antennal Transcriptome Analysis

Perception of environmental and habitat cues is of significance for insect survival and reproduction. Odor detection in insects is mediated by a number of proteins in antennae such as odorant receptors (ORs), ionotropic receptors (IRs), odorant binding proteins (OBPs), chemosensory proteins (CSPs), sensory neuron membrane proteins (SNMPs) and odorant degrading enzymes. In this study, we sequenced and assembled the adult male and female antennal transcriptomes of a destructive agricultural pest, the diamondback moth Plutella xyllostella. In these transcriptomes, we identified transcripts belonging to 6 chemoreception gene families related to ordor detection, including 54 ORs, 16 IRs, 7 gustatory receptors (GRs), 15 CSPs, 24 OBPs and 2 SNMPs. Semi-quantitative reverse transcription PCR analysis of expression patterns indicated that some of these ORs and IRs have clear sex-biased and tissue-specific expression patterns. Our results lay the foundation for future characterization of the functions of these P. xyllostella chemosensory receptors at the molecular level and development of novel semiochemicals for integrated control of this agricultural pest.

Proteomic analysis of the kissing bug Rhodnius prolixus antenna

Reception of odorants is essential in insects' life since the chemical signals in the environment (=semiochemicals) convey information about availability of hosts for a blood meal, mates for reproduction, sites for oviposition and other relevant information for fitness in the environment. Once they reach the antennae, these semiochemicals bind to odorant-binding proteins and are transported through the sensillar lymph until reach the odorant receptors. Such perireceptor events, particularly the interactions with transport proteins, are the liaison between the external environment and the entire neuroethological system and, therefore, a potential target to disrupt insect chemical communication. In this study, a proteomic profile of female and male antennae of Rhodnius prolixus, a vector of Chagas disease, was obtained in an attempt to unravel the entire repertoire of olfactory proteins involved in perireceptor events. Using shotgun proteomics and two-dimensional gel electrophoresis approaches followed by nano liquid chromatography coupled with tandem LTQ Velos Orbitrap mass spectrometry, we have identified 581 unique proteins. Putative olfactory proteins, including 17 odorant binding proteins, 6 chemosensory proteins, 2 odorant receptors, 3 transient receptor channels and 1 gustatory receptor were identified. Proteins involved in general cellular functions such as generation of precursor metabolites, energy generation and catabolism were expressed at high levels. Additionally, proteins that take part in signal transduction, ion binding, and stress response, kinase and oxidoreductase activity were frequent in antennae from both sexes. This proteome strategy unraveled for the first time the complex nature of perireceptor and other olfactory events that occur in R. prolixus antennae, including evidence for phosphorylation of odorant-binding and chemosensory proteins. These findings not only increase our understanding of the olfactory process in triatomine species, but also identify potential molecular targets to be explored for population control of such insect vectors.

Identification of odorant binding proteins in Carpomya vesuviana and their binding affinity to the male-borne semiochemicals and host plant volatiles

Carpomya vesuviana (Costa; Diptera: Tephritidae) is an agricultural pest that causes serious damage to jujube fruits. However, the mechanism of olfaction, which is critical for host identification, is not well understood in this pest. In this study, we have identified for the first time five protein types involved in the olfactory signal transduction of C. vesuviana by using transcriptome sequencing. These include 6 odorant-binding proteins (OBPs), 15 odorant receptors (ORs), 22 gustatory receptors (GRs), 2 chemosensory proteins (CSPs), and 2 sensory neuron membrane proteins (SNMPs). Amino acids alignment and phylogenetic analysis showed that all 6 OBPs have a signal peptide at their respective N-termini with four OBPs belonging with the classic OBPs, and OBP2 and OBP5 belonging to the Minus-C family. OBP3 clustered with the OBP83a/83b clade, which comprised pheromone binding protein related proteins (PBPRPs). Moreover, volatiles from C. vesuviana adults and its host plants were collected and identified by using solid phase microextraction (SPME) and gas-chromatography/mass spectrometry (GC/MS). The results indicated that male adults emitted nonanal, and five other compounds, caryophyllene, chamigrene, camphene, (Z)-3-hexen-1-ol acetate, and ocimene were identified in the fruits of jujubes. Electroantennogram (EAG) assays revealed that adult C. vesuviana responded to all six compounds along with two additional pheromones (geranyl acetate and α-farnesene) from other tephritids and the values ranged from 0.50mV to 1.26mV. To further explore the interaction between OBPs and volatiles, competitive binding assays were carried out. The results showed that only CvesOBP2 had binding affinity to (Z)-3-hexen-1-ol acetate. OBP5 and OBP6 exhibited broad spectrum binding to compounds with relatively low molecular weights, and OBP1 and OBP4 had some affinity to caryophyllene and chamigrene. However, OBP3 exhibited relatively high binding affinity to α-farnesene. The findings of this study provide insights into the olfactory mechanisms and the potential functions of OBPs in the olfactory reception pathway in C. vesuviana. The OBPs identified in this study could be used as potential targets to develop attractants to monitor this insect pest for effective pest control.

Differential expression of chemosensory-protein genes in midguts in response to diet of Spodoptera litura

While it has been well characterized that chemosensory receptors in guts of mammals have great influence on food preference, much remains elusive in insects. Insect chemosensory proteins (CSPs) are soluble proteins that could deliver chemicals to olfactory and gustatory receptors. Recent studies have identified a number of CSPs expressed in midgut in Lepidoptera insects, which started to reveal their roles in chemical recognition and stimulating appetite in midgut. In this study, we examined expression patterns in midgut of 21 Spodoptera litura CSPs (SlitCSPs) characterized from a previously reported transcriptome, and three CSPs were identified to be expressed highly in midgut. The orthologous relationships between midgut expressed CSPs in S. litura and those in Bombyx mori and Plutella xylostella also suggest a conserved pattern of CSP expression in midgut. We further demonstrated that the expression of midgut-CSPs may change in response to different host plants, and SlitCSPs could bind typical chemicals from host plant in vitro. Overall, our results suggested midgut expressed SlitCSPs may have functional roles, likely contributing to specialization and adaption to different ecosystems. Better knowledge of this critical component of the chemsensation signaling pathways in midguts may improve our understanding of food preference processes in a new perspective.

Functional analysis of five trypsin-like protease genes in the oriental fruit fly, Bactrocera dorsalis (Diptera: Tephritidae)

Insect midgut proteases catalyze the release of free amino acids from dietary proteins and are essential for insect normal development. To date, digestive proteases as potential candidates have made great progress in pest control. To clarify the function of trypsin-like protease genes in the digestive system of Bactrocera dorsalis, a serious pest of a wide range of tropical and subtropical fruit and vegetable crops, five trypsin genes (BdTry1, BdTry2, BdTry3, BdTry4 and BdTry5) were identified from transcriptome dataset, and the effects of feeding condition on their expression levels were examined subsequently. RNA interference (RNAi) was applied to further explore their function on the growth of B. dorsalis. The results showed that all the BdTrys in starving midgut expressed at a minimal level but up-regulated upon feeding (except BdTry3). Besides, RNAi by feeding dsRNAs to larvae proved to be an effective method to cause gene silencing and the mixed dsRNAs of the five BdTrys slowed larvae growth of B. dorsalis. The current data suggest that trypsin genes are actively involved in digestion process of B. dorsalis larvae and thereafter play crucial roles in their development.

Effect of sweeteners on the survival and behaviour of Bactrocera dorsalis (Hendel) (Diptera: Tephritidae)

BACKGROUND

The oriental fruit fly Bactrocera dorsalis (Hendel) (Diptera: Tephritidae) causes serious damage that affects fruit production. Chemical insecticides have been widely used for the prevention and control of this destructive pest. However, the resistance of B. dorsalis to these compounds has become a serious problem. This study tested six sweeteners for their toxicity to B. dorsalis.

RESULTS

B. dorsalis fed on erythritol, aspartame and saccharin exhibited significantly higher mortality than those fed on sucrose. Flies fed on erythritol died faster than did the control flies (water). However, no dose-dependent effects were observed at the concentrations tested. These three sweeteners decreased the climbing ability of B. dorsalis. Notably, adults fed on saccharin exhibited significantly decreased climbing ability after 12 h compared with those fed on sucrose. Additionally, these three sweeteners had a negative effect on the frequency and duration of the flies' behaviour patterns (flying, walking, grooming and inactivity). Saccharin not only induced a marked reduction in the frequency of flights and walks but also induced decreases in the time spent flying and walking and increases in inactivity compared with sucrose. Erythritol induced a reduction in movement and increased the time spent inactive compared with the control and other treatments.

CONCLUSION

Three sweeteners had significant negative effects on the survival of B. dorsalis. Erythritol was toxic to B. dorsalis. Aspartame and saccharin also decreased the survival and behaviour of adult flies and may be toxic to (or contribute to poor nutrition in) B. dorsalis. These sweeteners could therefore be developed as additive ingredients in baits.

Identification of Male- and Female-Specific Olfaction Genes in Antennae of the Oriental Fruit Fly (Bactrocera dorsalis)

The oriental fruit fly (Bactrocera dorsalis) is a species of tephritid fruit fly, endemic to Southeast Asia but also introduced to many regions of the US, and it is one of the major pest species with a broad host range of cultivated and wild fruits. Although males of B. dorsalis respond strongly to methyl eugenol and this is used for monitoring and estimating populations, the molecular mechanism of the oriental fruit fly olfaction has not been elucidated yet. Therefore, in this project, using next generation sequencing technologies, we sequenced the transcriptome of the antennae of male and female adults of B. dorsalis. We identified a total of 20 candidate odorant binding proteins (OBPs), 5 candidate chemosensory proteins (CSPs), 35 candidate odorant receptors (ORs), 12 candidate ionotropic receptors (IRs) and 4 candidate sensory neuron membrane proteins (SNMPs). The sex-specific expression of these genes was determined and a subset of 9 OR genes was further characterized by qPCR with male and female antenna, head, thorax, abdomen, leg and wing samples. In the male antennae, 595 genes showed a higher expression, while 128 genes demonstrated a higher expression in the female antennae. Interestingly, 2 ORs (BdorOR13 and BdorOR14) were highly and specifically expressed in the antennae of males, and 4 ORs (BdorOR13, BdorOR16, BdorOR18 and BdorOR35) clustered with DmOR677, suggesting pheromone reception. We believe this study with these antennae-enriched OBPs, CSPs, ORs, IRs and SNMPs can play an important role in the detection of pheromones and general odorants, and so in turn our data improve our current understanding of insect olfaction at the molecular level and provide important information for disrupting the behavior of the oriental fruit fly using chemical communication methods.

Discovery of Chemosensory Genes in the Oriental Fruit Fly, Bactrocera dorsalis

The oriental fruit fly, Bactrocera dorsalis, is a devastating fruit fly pest in tropical and sub-tropical countries. Like other insects, this fly uses its chemosensory system to efficiently interact with its environment. However, our understanding of the molecular components comprising B. dorsalis chemosensory system is limited. Using next generation sequencing technologies, we sequenced the transcriptome of four B. dorsalis developmental stages: egg, larva, pupa and adult chemosensory tissues. A total of 31 candidate odorant binding proteins (OBPs), 4 candidate chemosensory proteins (CSPs), 23 candidate odorant receptors (ORs), 11 candidate ionotropic receptors (IRs), 6 candidate gustatory receptors (GRs) and 3 candidate sensory neuron membrane proteins (SNMPs) were identified. The tissue distributions of the OBP and CSP transcripts were determined by RT-PCR and a subset of nine genes were further characterized. The predicted proteins from these genes shared high sequence similarity to Drosophila melanogaster pheromone binding protein related proteins (PBPRPs). Interestingly, one OBP (BdorOBP19c) was exclusively expressed in the sex pheromone glands of mature females. RT-PCR was also used to compare the expression of the candidate genes in the antennae of male and female B. dorsalis adults. These antennae-enriched OBPs, CSPs, ORs, IRs and SNMPs could play a role in the detection of pheromones and general odorants and thus could be useful target genes for the integrated pest management of B. dorsalis and other agricultural pests.

Ligands binding and molecular simulation: the potential investigation of a biosensor based on an insect odorant binding protein

Based on mimicking biological olfaction, biosensors have been applied for the detection of various ligands in complex environment, which could represent one of the most promising research fields. In this study, the basic characters of one insect odorant binding protein (OBP) as a biosensor were explored. To explore the molecular recognition process, the tertiary structure of the protein was modeled and the protein-ligand interactions with 1,536,550 chemicals were investigated by the molecular docking. The availability of large amount of recombinant SlitOBP1 overcame the difficulty to obtain biological sensing material. After obtained the purified recombinant protein, the result of fluorescence binding assays proved the candidate protein has good affinities with the majority of the tested chemicals. With the aid of simulation docking, the key conserved amino acids within the binding site were identified and then mutated to alanine. After mutation, the protein-ligand binding characteristics were recorded, and the competitive binding assays were carried out to provide experimental verification. The detailed information on its structure and affinities investigated in this study could allow the design of specific mutants with desired characteristics, which provides a solid base for tailoring OBP for biosensor and provides a role model for screening the other elements in olfactory system for different applications.