Insect olfaction: receptors, signal transduction, and behavior

The Odorant-Binding Protein 1 Mediates the Foraging Behavior of Grapholita molesta Larvae

Since eggs are laid directly on fruit skin, it is typically believed that food odor has little impact on the foraging of Grapholita molesta larvae. It is crucial to note that larvae that hatch on twigs and leaves could need some sort of identification system when foraging. Here, 22 GmolOBP genes were identified from the G. molesta larval transcriptome via the comparison of conserved domain and homology in the protein level. GmolOBP1 had strong affinities for important pear-fruit volatiles, which caused larvae strong behavioral responses. However, after GmolOBP1 silencing, the larvae lost their attraction to methyl salicylate, α-farnesene, butyl acetate, ethyl butanoate, and ethyl hexanoate, and the effects of larvae seeking various pears were significantly reduced. Consequently, GmolOBP1 was required for the reception of pear volatiles and was involved in mediating how G. molesta larvae foraged. Our research revealed the GmolOBP1 foraging signal recognition mechanism as well as potential molecular targets for field pest management.

Ligand-binding properties of XaffOBP9, a Minus-C odorant-binding protein from Xyleborus affinis (Coleoptera: Curculionidae: Scolytinae)

Xyleborus affinis, one of the most important pests of rubber trees, has caused severe damage to the natural rubber industry in Hainan province. The ability to detect host plants through a sensitive and specific olfactory system is crucial for Xyleborus affinis. Odorant binding proteins (OBPs) are believed to bind and carry hydrophobic active compounds from the environment to the surface of olfactory receptor neurons. To investigate the potential functional role of the highly expressed XaffOBP9 in binding with semiochemicals, we cloned and analyzed the cDNA sequence of XaffOBP9. The results showed that XaffOBP9 contains a 411bp open reading frame that encodes 136 amino acids. Then XaffOBP9 was expressed in Escherichia coli. The binding affinity of the recombinant OBP to 15 different ligands (14 host plant volatiles and 1 aggregation pheromone) was then examined using a fluorescence competitive binding approach. The results demonstrated that XaffOBP9 exhibited broad binding capabilities and strong affinities for 14 ligands. The structure of XaffOBP9 and its interactions with fourteen ligands were further analyzed by modeling and molecular docking, respectively. Based on the docking result, we found hydrophobic interactions are important between XaffOBP9 to these ligands and three amino acid residues (L71, Y106, and L114) were highly overlapped and contributed to the interaction with ligands. Mutation functional assays confirmed that the mutant L114A showed significantly reduced binding capacity to these ligands. This study suggested that XaffOBP9 may be involved in the chemoreception of semiochemicals and that it is helpful for the integrated management of X. affinis.

Characterisation of Antennal Sensilla and Electroantennography Responses of the Dung Beetles Bubas bison, Onitis aygulus and Geotrupes spiniger (Coleoptera: Scarabaeoidea) to Dung Volatile Organic Compounds

Locating sporadically distributed food resources and mate finding are strongly aided by volatile cues for most insects, including dung beetles. However, there is limited information on the olfactory ecology of dung beetles. We conducted a scanning electron microscopy study on the morphology and distribution of the antennal sensilla of three introduced dung beetle species in Australia: Geotrupes spiniger (Coleoptera: Geotrupidae), Bubas bison and Onitis aygulus (Coleoptera: Scarabaeidae). Three main morphological types of antennal sensilla were identified: sensilla trichodea (ST), sensilla basiconica (SB) and sensilla chaetica (SCh). Distinct variations of SB distribution were observed in B. bison and G. spiniger and on different lamellar surfaces in both sexes of all three species. Sexual dimorphism in antennal sensilla distribution or their abundance was not evident. To complement the morphological characterisation of sensilla, electroantennography (EAG) was carried out to construct EAG response profiles of the three species to selected dung volatiles. An initial study revealed that antennae of all species were sensitive to a mix of phenol, skatole, indole, p-cresol, butanone and butyric acid, common components of livestock dung headspace. In addition to these six compounds, dimethyl sulfide, dimethyl disulfide, eucalyptol and toluene were tested for antennal activity. All compounds evoked measurable EAG responses, confirming antennal sensitivity. Geotrupes spiniger exhibited significant responses to all the compounds compared to the control, whereas B. bison and O. aygulus only responded to a subset of compounds. A comparison of relative EAG amplitudes revealed highly significant responses to p-cresol in G. spiniger and to skatole in B. bison. Geotrupes spiniger displayed differential responses to all the compounds. Pooled EAG data suggest highly significant differences in responses among the three species and among compounds. Our findings suggest that a blend of volatiles may offer potential for the trapping of dung beetles, thereby avoiding the use of dung baits that are inconvenient, inconsistent and may pose a threat to farm biosecurity.

Identification and expression analyses of the olfactory-related genes in different tissues' transcriptome of a predacious soldier beetle, Podabrus annulatus (Coleoptera, Cantharidae)

We sequenced and analyzed the transcriptomes from different tissues of the soldier beetle, Podabrus annulatus (Coleoptera: Cantharidae), and obtained 75.74 Gb clean reads which were assembled into 95,274 unigenes. Among these transcripts, 25,484 unigenes of highly quality were annotated. Based on annotation and tBLASTn results, we identified a total of 101 candidate olfactory-related genes for the first time, including 11 putative odorant-binding proteins (OBPs), 6 chemosensory proteins (CSP), 50 olfactory receptors (ORs), 25 gustatory receptors (GRs), 6 ionotropic receptors (IRs), and 3 sensory neuron membrane proteins (SNMPs). BLASTX best-hit results indicated that these chemosensory genes were most identical to their respective orthologs from Photinus pyralis. Phylogenetic analyses also revealed that the ORs, GRs, and IRs of Podabrus annulatus are closely related to those of Photinus pyralis. The fragment per kilobase per million mapped fragments (FPKM) values showed that the PannOBP2, PannOBP3, and PannOBP10 were predominantly expressed in the antennae, PannOBP1 in the abdomen-thorax, while others were not identified to be tissue-specific. These olfactory-related differentially expressed genes (DEGs) demonstrated different roles in the olfactory system of Podabrus annulatus. This study establishes the groundwork for future research into the molecular mechanism of olfactory recognition in Podabrus annulatus.

Odorant-binding Protein 10 From Bradysia odoriphaga (Diptera: Sciaridae) Binds Volatile Host Plant Compounds

Bradysia odoriphaga (Diptera: Sciaridae) is a major insect pest of seven plant families including 30 commercial crops in Asia. The long-term use of chemical pesticides leads to problems such as insect resistance, environmental issues, and food contamination. Against this background, a novel pest control method should be developed. In insects, odorant-binding proteins (OBPs) transport odor molecules, including pheromones and plant volatiles, to olfactory receptors. Here, we expressed and characterized the recombinant B. odoriphaga OBP BodoOBP10, observing that it could bind the sulfur-containing compounds diallyl disulfide and methyl allyl disulfide with Ki values of 8.01 μM and 7.00 μM, respectively. Homology modeling showed that the BodoOBP10 3D structure was similar to that of a typical OBP. Both diallyl disulfide and methyl allyl disulfide bound to the same site on BodoOBP10, mediated by interactions with six hydrophobic residues Met70, Ile75, Thr89, Met90, Leu93, and Leu94, and one aromatic residue, Phe143. Furthermore, silencing BodoOBP10 expression via RNAi significantly reduced the electroantennogram (EAG) response to diallyl disulfide and methyl allyl disulfide. These findings suggest that BodoOBP10 should be involved in the recognition and localization of host plants.

Characterization of Two Aldehyde Oxidases from the Greater Wax Moth, Galleria mellonella Linnaeus. (Lepidoptera: Pyralidae) with Potential Role as Odorant-Degrading Enzymes

Odorant-degrading enzymes (ODEs) are proposed to degrade/inactivate volatile organic compounds (VOCs) on a millisecond timescale. Thus, ODEs play an important role in the insect olfactory system as a reset mechanism. The inhibition of these enzymes could incapacitate the olfactory system and, consequently, disrupt chemical communication, promoting and complementing the integrated pest management strategies. Here, we report two novel aldehyde oxidases, AOX-encoding genes GmelAOX2 and GmelAOX3, though transcriptomic analysis in the greater wax moth, Galleria mellonella. GmelAOX2 was clustered in a clade with ODE function, according to phylogenetic analysis. Likewise, to unravel the profile of volatiles that G. mellonella might face besides the sex pheromone blend, VOCs were trapped from honeycombs and the identification was made by gas chromatography-mass spectrometry. Semi-quantitative RT-PCR showed that GmelAXO2 has a sex-biased expression, and qRT-PCR indicated that both GmelAOX2 and GmelAOX3 have a higher relative expression in male antennae rather than female antennae. A functional assay revealed that antennal extracts had the strongest enzymatic activity against undecanal (4-fold) compared to benzaldehyde (control). Our data suggest that these enzymes have a crucial role in metabolizing sex pheromone compounds as well as plant-derived aldehydes, which are related to honeycombs and the life cycle of G. mellonella.

Transcriptome Analysis and Identification of Chemosensory Genes in Baryscapus dioryctriae (Hymenoptera: Eulophidae)

Baryscapus dioryctriae is a pupal endoparasitoid of many Pyralidae pests and has been used as a biocontrol agent against insect pests that heavily damage the cone and seed of the Korean pine. The olfactory system of wasps plays an essential role in sensing the chemical signals during their foraging, mating, host location, etc., and the chemosensory genes are involved in detecting and transducing these signals. Many chemosensory genes have been identified from the antennae of Hymenoptera; however, there are few reports on the chemosensory genes of Eulophidae wasps. In this study, the transcriptome databases based on ten different tissues of B. dioryctriae were first constructed, and 274 putative chemosensory genes, consisting of 27 OBPs, 9 CSPs, 3 NPC2s, 155 ORs, 49 GRs, 23 IRs and 8 SNMPs genes, were identified based on the transcriptomes and manual annotation. Phylogenetic trees of the chemosensory genes were constructed to investigate the orthologs between B. dioryctriae and other insect species. Additionally, twenty-eight chemosensory genes showed female antennae- and ovipositor-biased expression, which was validated by RT-qPCR. These findings not only built a molecular basis for further research on the processes of chemosensory perception in B. dioryctriae, but also enriched the identification of chemosensory genes from various tissues of Eulophidae wasps.

Insect-Microorganism Interaction Has Implicates on Insect Olfactory Systems

Olfaction plays an essential role in various insect behaviors, including habitat selection, access to food, avoidance of predators, inter-species communication, aggregation, and reproduction. The olfactory process involves integrating multiple signals from external conditions and internal physiological states, including living environments, age, physiological conditions, and circadian rhythms. As microorganisms and insects form tight interactions, the behaviors of insects are constantly challenged by versatile microorganisms via olfactory cues. To better understand the microbial influences on insect behaviors via olfactory cues, this paper summarizes three different ways in which microorganisms modulate insect behaviors. Here, we deciphered three interesting aspects of microorganisms-contributed olfaction: (1) How do volatiles emitted by microorganisms affect the behaviors of insects? (2) How do microorganisms reshape the behaviors of insects by inducing changes in the synthesis of host volatiles? (3) How do symbiotic microorganisms act on insects by modulating behaviors?

Transcriptome Analysis of Antennal Chemosensory Genes in Curculio Dieckmanni Faust. (Coleoptera: Curculionidae)

The olfactory system plays a key role in regulating insect behaviors, such as locating host plants, spawning sites, and mating partners and avoiding predators. Chemosensory genes are required for olfactory recognition in insects. Curculio dieckmanni Faust. (Coleoptera: Curculionidae) damages hazelnuts and causes severe economic losses. There are no effective control measures, but understanding the olfaction mechanisms of this insect could lead to a new approach for population management. However, the genes that perform chemosensory functions in C. dieckmanni are still unclear. Using high-throughput sequencing, we assembled the antennal transcriptome of C. dieckmanni and annotated the major chemosensory gene families. Of the chemosensory gene families, we found 23 odorant-binding proteins, 15 chemosensory proteins, 2 sensory neuron membrane proteins, 15 odorant receptors, 23 ionotropic receptors, and nine gustatory receptors. Using Blast sequence alignment and phylogenetic analysis, the sequences of these proteins were identified. Male- and female-specific chemosensory genes involved in odorant detection and recognition were validated by qRT-PCR. Among the chemosensory genes, we found significant differences in the expression of CdieOBP8, CdieOBP9, CdieOBP19, CdieOBP20, CdieOBP21, CdieCSP15, CdieOR13, and CdieOR15 between adult male and female C. dieckmanni. A total of 87 expressed chemosensory proteins were found in C. dieckmanni. Investigating these proteins will help reveal the molecular mechanism of odorant recognition in C. dieckmanni and may aid the development of novel control strategies for this species.

A high-quality functional genome assembly of Delia radicum L. (Diptera: Anthomyiidae) annotated from egg to adult

Belowground herbivores are overseen and underestimated, even though they can cause significant economic losses in agriculture. The cabbage root fly Delia radicum (Anthomyiidae) is a common pest in Brassica species, including agriculturally important crops, such as oilseed rape. The damage is caused by the larvae, which feed specifically on the taproots of Brassica plants until they pupate. The adults are aboveground-living generalists feeding on pollen and nectar. Female flies are attracted by chemical cues in Brassica plants for oviposition. An assembled and annotated genome can elucidate which genetic mechanisms underlie the adaptation of D. radicum to its host plants and their specific chemical defenses, in particular isothiocyanates. Therefore, we assembled, annotated and analyzed the D. radicum genome using a combination of different Next Generation Sequencing and bioinformatic approaches. We assembled a chromosome-level D. radicum genome using PacBio and Hi-C Illumina sequence data. Combining Canu and 3D-DNA genome assembler, we constructed a 1.3 Gbp genome with an N50 of 242 Mbp and 6 pseudo-chromosomes. To annotate the assembled D. radicum genome, we combined homology-, transcriptome- and ab initio-prediction approaches. In total, we annotated 13,618 genes that were predicted by at least two approaches. We analyzed egg, larval, pupal and adult transcriptomes in relation to life-stage specific molecular functions. This high-quality annotated genome of D. radicum is a first step to understanding the genetic mechanisms underlying host plant adaptation. As such, it will be an important resource to find novel and sustainable approaches to reduce crop losses to these pests.

Identification and Expression Profile of Chemosensory Genes in the Small Hive Beetle Aethina tumida

Aethina tumida is a parasite and predator of honeybee causing severe loss to the bee industry. No effective and environmentally friendly methods are available to control this pest at present. Chemosensory genes play key roles in insect behavior which can potentially be used as targets for developing environmentally friendly pest control agents. In this study, the putative chemosensory genes in antennae and forelegs of A. tumida involved in olfaction or contact chemical communication of adults were investigated using RNA transcriptome sequencing and PCR methods. Based on transcriptomic data, unigenes encoding 38 odorant receptors (ORs), 24 ionotropic receptors (IRs), 14 gustatory receptors (GRs), 3 sensory neuron membrane proteins (SNMPs), 29 odorant binding proteins (OBPs), and 22 chemosensory proteins (CSPs) were identified. The analyses of tissue expression profiles revealed that genes encoding 38 ORs, 13 antennal IRs, 11 GRs, 1 SNMP, 24 OBPs, and 12 CSPs were predominately expressed in antennae. No significant differences in expression levels of these genes were found between males and females. Genes encoding 5 non-NMDA iGluRs, 3 GRs, 2 SNMPs, 5 OBPs, and 12 CSPs were predominately expressed in forelegs. RT-PCR assays for SNMPs, OBPs and CSPs further revealed that 3 OBPs (AtumOBP3, 26 and 28) and 3 CSPs (AtumCSP7, 8 and 21) were highly expressed in antennae. Our results enrich the gene inventory of A. tumida and facilitate the discovery of potential novel targets for developing new pest control measures.

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

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

Comparison and Functional Analysis of Chemosensory Protein Genes From Eucryptorrhynchus scrobiculatus Motschulsky and Eucryptorrhynchus brandti Harold

The tree-of-heaven root weevil (Eucryptorrhynchus scrobiculatus) and the tree-of-heaven trunk weevil (Eucryptorrhynchus brandti) are closely related species that monophagously feed on the same host plant, the Ailanthus altissima (Mill) Swingle, at different locations. However, the mechanisms of how they select different parts of the host tree are unclear. As chemosensory systems play important roles in host location and oviposition, we screened candidate chemosensory protein genes from the transcriptomes of the two weevils at different developmental stages. In this study, we identified 12 candidate chemosensory proteins (CSPs) of E. scrobiculatus and E. brandti, three EscrCSPs, and one EbraCSPs, respectively, were newly identified. The qRT-PCR results showed that EscrCSP7/8a/9 and EbraCSP7/8/9 were significantly expressed in adult antennae, while EscrCSP8a and EbraCSP8 shared low sequence identity, suggesting that they may respond to different odorant molecule binding. Additionally, EbraCSP6 and EscrCSP6 were mainly expressed in antennae and proboscises and likely participate in the process of chemoreception. The binding simulation of nine volatile compounds of the host plant to EscrCSP8a and EbraCSP8 indicated that (1R)-(+)-alpha-pinene, (-)-beta-caryophyllene, and beta-elemen have higher binding affinities with EscrCSP8a and lower affinities with EbraCSP8. In addition, there were seven, two, and one EbraCSPs mainly expressed in pupae, larvae, and eggs, respectively, indicating possible developmental-related roles in E. brandti. We screened out several olfactory-related possible CSP genes in E. brandti and E. scrobiculatus and simulated the binding model of CSPs with different compounds, providing a basis for explaining the niche differentiation of the two weevils.

Characterizing the Role of Orco Gene in Detecting Aggregation Pheromone and Food Resources in Protaetia brevitarsis Leiws (Coleoptera: Scarabaeidae)

An accurate olfactory system for recognizing semiochemicals and environmental chemical signals plays crucial roles in survival and reproduction of insects. Among all olfaction-related proteins, olfactory receptors (ORs) contribute to the conversion of chemical stimuli to electric signals and thereby are vital in odorant recognition. Olfactory receptor co-receptor (Orco), one of the most conserved ORs, is extremely essential in recognizing odorants through forming a ligand-gated ion channel complex with conventional ligand-binding odorant receptors. We have previously identified aggregation pheromone in Protaetia brevitarsis (Coleoptera: Scarabaeidae), a native agricultural and horticultural pest in East-Asia. However, to our best knowledge, its olfaction recognition mechanisms are still veiled. To illustrate how P. brevitarsis recognize aggregation pheromone and host plants, in the present study we cloned and sequenced the full-length Orco gene from P. brevitarsis antennae (named PbreOrco) and found that PbreOrco is highly conserved and similar to Orcos from other Coleoptera insects. Our real-time quantitative PCR (qRT-PCR) results showed that PbreOrco is mainly expressed in antenna. We also demonstrated that silencing PbreOrco using RNA interference through injecting dsOrco fragment significantly inhibited PbreOrco expression in comparison with injecting control dsGFP and subsequently revealed using electroantennogram and behavioral bioassays that decreasing PbreOrco transcript abundance significantly impaired the responses of P. brevitarsis to intraspecific aggregation pheromone and prolonged the time of P. brevitarsis spending on food seeking. Overall, our results demonstrated that PbreOrco is crucial in mediating odorant perception in P. brevitarsis.

An Overview of Antennal Esterases in Lepidoptera

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

Network mechanism for insect olfaction

Early olfactory pathway responses to the presentation of an odor exhibit remarkably similar dynamical behavior across phyla from insects to mammals, and frequently involve transitions among quiescence, collective network oscillations, and asynchronous firing. We hypothesize that the time scales of fast excitation and fast and slow inhibition present in these networks may be the essential element underlying this similar behavior, and design an idealized, conductance-based integrate-and-fire model to verify this hypothesis via numerical simulations. To better understand the mathematical structure underlying the common dynamical behavior across species, we derive a firing-rate model and use it to extract a slow passage through a saddle-node-on-an-invariant-circle bifurcation structure. We expect this bifurcation structure to provide new insights into the understanding of the dynamical behavior of neuronal assemblies and that a similar structure can be found in other sensory systems.

Analysis of Chemosensory Genes in Full and Hungry Adults of Arma chinensis (Pentatomidae) Through Antennal Transcriptome

The predatory insect Arma chinensis (Hemiptera: Pentatomidae) is widely distributed in China, where it is also used to control many agricultural and forest pests. The chemosensory genes expressed in its antennae play crucial roles in food-seeking and mating behaviors. To better understand the olfaction of A. chinensis antennae, we identified the genes related to food-seeking and mating. Sequencing of the antennal transcriptomes of full and hungry male and female A. chinensis revealed 38 odorant-binding proteins (OBPs), 1 chemosensory protein (CSP), 1 Niemann-Pick C2 protein (NPC2), 3 odorant receptors (ORs), 12 ionotropic receptors (IRs), 2 gustatory receptors (GRs), and 3 sensory neuron membrane proteins (SNMPs). These results were used to construct phylogenetic trees. A quantitative real-time PCR (qRT-PCR) analysis showed that the relative transcript levels of AchiGR1, AchiGR2, and AchiOBP28 were higher in female than in male antennae in both full and hungry insects, but that the expression of AchiOBP13 and AchiOBP16 was higher only in full A. chinensis females. Thus, the latter genes may encode proteins involved in oviposition selection behavior. AchiGRs (1 and 2), AchiIR6, and AchiOBPs (6-8, 12, 20-22, 28, and 34) were highly expressed only in the antennae of full males, indicating the participation of these genes in mate-searching or male pheromone recognition. The expression of AchiOBP31 in the antennae of starved males, AchiOBPs (15, 18, and 29) in the antennae of starved females, and AchiOBPs (3, 4, and 24) in the antennae of starved males and females suggested that these genes encode food-seeking functions. Our identification of chemosensory genes in A. chinensis antennae and their differential expression in full and hungry insects provides the basis for further functional studies on the chemoreception system of A. chinensis and the sex hormones of predatory insects.

Identification and Expression Profile Analysis of Chemosensory Genes From the Antennal Transcriptome of Bamboo Locust (Ceracris kiangsu)

Studies of chemosensory genes are key to a better understanding of intra- and interspecific communications between insects and their environment and provide opportunities for developing environmentally friendly pesticides to target pest species. The bamboo locust Ceracris kiangsu Tsai (Orthoptera: Acrididae) is one of the most important bamboo leaf-eating insects in southern China. However, the genes underlying olfactory sensation are lacking in the bamboo locust. In this study, the transcriptomes of male and female C. kiangsu antennae were sequenced and analyzed. A total of 125 chemosensory genes, including 91 odorant receptors (ORs), 13 ionotropic receptors (IRs), 13 odorant-binding proteins (OBPs), six chemosensory proteins (CSPs), and two sensory neuron membrane proteins, were identified based on sequence alignment and phylogenetic analyses. The expression patterns of all candidate genes on the antennae of males and females, maxillary palps, tarsi, wings, and thoraxes-abdomens were confirmed by real-time quantitative PCR. The analyses demonstrated that most genes are highly expressed in the antennae, and 35 ORs, 7 IRs, 10 OBPs, and 1 CSP exhibit significantly male-biased expression patterns, indicating their potential functions in mating behavior and the recognition of female sex pheromones. In addition to the antennal-predominant genes, some were abundant in the maxillary palps and some in the non-olfactory tissues, suggesting their different functions in the olfactory system of C. kiangsu. Our research offers an extensive resource for investigating the chemoreception mechanism of C. kiangsu. Further studies of olfactory function will provide comprehensive methods and original strategies for integrated pest management.

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.

Identification and Expression Profile of Olfactory Receptor Genes Based on Apriona germari (Hope) Antennal Transcriptome

Insects’ olfactory receptor plays a central role in detecting chemosensory information from the environment. Odorant receptors (ORs) and ionotropic receptors (IRs) are two types of olfactory receptors, and they are essential for the recognition of ligands at peripheral neurons. Apriona germari (Hope) (Coleoptera: Cerambycidae) is one of the most serious insect pests that cause damage to economic trees and landscaping trees, resulting in massive environmental damages and economic losses. Olfactory-based management strategy has been suggested as a promising strategy to control this wood-boring beetle. However, the olfactory perception mechanism in A. germari is now almost unknown. In the present study, RNA sequencing analysis was used to determine the transcriptomes of adult A. germari antennae. Among 36,834 unigenes derived from the antennal assembly, we identified 42 AgerORs and three AgerIRs. Based on the tissue expression pattern analysis, 27 AgerORs displayed a female-biased expression. Notably, AgerOR3, 5, 13, 33, and 40 showed a significant female-biased expression and were clustered with the pheromone receptors of Megacyllene caryae in the phylogenetic tree, suggesting that these AgerORs could be potential pheromone receptors for sensing male-produced sex pheromones in A. germari. The AgerIRs expression profile demonstrated that AgerIR2 had high expression levels in male labial palps, suggesting that this receptor may function to detect female-deposited trail-sex pheromone blend of A. germari. In addition, the phylogenetic tree showed that the Orco gene of five cerambycidae species was highly conservative. These results provide a foundation for further studies on the molecular mechanisms of olfactory chemoreception in A. germari apart from suggesting novel targets for the control of this pest in the future.

Antennal Lobe Atlas of an Emerging Corn Pest, Athetis dissimilis

Moths develop sophisticated olfactory systems to sense the airborne chemical cues from the environment. Understanding the structural basis in the neuronal center is a fundamental neuroethological step. Little is known about the emerging crop pest Athetis dissimilis with regard to its morphology or its neuronal organizations. Through antibody staining and digital 3D modeling, we re-constructed the primary olfactory center-the antennal lobe of A. dissimilis. In the antennal lobes 68.8 ± 3.1 male glomeruli and 70.8 ± 1.0 female glomeruli were identified with obvious sexual dimorphism. In particular, male adults of A. dissimilis contain a macroglomerular complex (MGC) that consists of three subunits, while the female lobe has four relatively enlarged glomeruli at the entrance of the antennal nerve. Glomeruli were later clustered with deviation and variance, and referring to reported olfactory related receptor family genes in seven different moth species, we found that glomerular counts of these insects are better related to the sum of odorant receptor and ionotropic receptor numbers, suggesting olfactory receptors and ionotropic receptors may both involved in olfaction of Noctuidae moths.

Identification of candidate chemosensory genes of Ophraella communa LeSage (Coleoptera: Chrysomelidae) based on antennal transcriptome analysis

Antennal olfaction plays a key role in insect survival, which mediates important behaviors like host search, mate choice, and oviposition site selection. As an oligophagous insect, olfaction is extremely important for Ophraella communa to locate host plants. However, information on the olfactory genes has been lacking in O. communa. Using next generation sequencing, we assembled the antennal transcriptome of O. communa and first reported the major chemosensory genes necessary for olfaction in this species. In this study, a total 105 candidate chemosensory genes were identified in O. communa antennae, including 25 odorant-binding proteins (OBPs), 11 chemosensory proteins (CSPs), four sensory neuron membrane proteins (SNMPs), 30 odorant receptors (ORs), 18 ionotropic receptors (IRs), and 17 gustatory receptors (GRs). We also identified full-length sequences of the highly conserved ORco and IR8a/25a family in O. communa. In addition, the expression profile of 15 ORs and four OBPs were validated by quantitative real-time polymerase chain reaction (qPCR). We found that OcomOR2/4/19 and OcomOBP19/20 had a biased expression in male antennae, and OcomOR8 had a biased expression in the female antennae. This large number of chemosensory genes handled by homology analysis and qPCR results will provide the first insights into molecular basis for the olfactory systems of O. communa as well as advance our understanding of olfactory mechanisms in Coleoptera.

Functional Analysis of the Chemosensory Protein GmolCSP8 From the Oriental Fruit Moth, Grapholita molesta (Busck) (Lepidoptera: Tortricidae)

Chemosensory proteins (CSPs) belong to a family of small water-soluble proteins that can selectively bind and transport odorant molecules for olfactory communication in insects. To date, their definite physiological functions in olfaction remain controversial when compared with odorant binding proteins (OBPs). To investigate the functions of CSPs in the oriental fruit moth Grapholita molesta, we determined the tissue expression patterns and binding properties of the CSP, GmolCSP8. The key binding sites of GmolCSP8 with a representative ligand were evaluated using molecular flexible docking, site-directed mutagenesis and ligand-binding experiments. Multiple sequence alignment and phylogenetic analysis showed that GmolCSP8 possesses a typical conserved four cysteines motif and shares high sequence identity with some CSP members of other Lepidopteran insects. GmolCSP8 was predominantly expressed in the wings and antennae of both male and female adults and may be involve in contact chemoreception. Recombinant GmolCSP8 (rGmolCSP8) exhibited specific-binding affinities to small aliphatic alcohols (C4–12) and had the strongest binding affinity to 1-hexanol. The three-dimensional structure of GmolCSP8 was constructed using the structure of sgCSP4 as a template. Site-directed mutagenesis and ligand-binding experiments confirmed that Thr27 is the key binding site in GmolCSP8 for 1-hexanol binding, because this residue can form hydrogen bond with the oxygen atom of the hydroxyl group in 1-hexanol, and Leu30 may play an important role in binding to 1-hexanol. We found that pH significantly affected the binding affinities of rGmolCSP8 to ligand, revealing that ligand-binding and -release by this protein is related to a pH-dependent conformational transition. Based on these results, we infer that GmolCSP8 may participate in the recognition and transportation of 1-hexanol and other small aliphatic alcohols.

Degradation of sex pheromone and plant volatile components by an antennal glutathione S-transferase in the oriental fruit moth,Grapholita molesta Busck (Lepidoptera: Tortricidae)

Insect antennae have a primary function of perceiving and discerning odorant molecules including sex pheromones and host plant volatiles. The assumption that genes highly expressed in the antennae may have an olfactory-related role associated with signal transduction. Here, one delta subfamily glutathione S-transferase (GST) gene (GmolGSTD1) was obtained from an antennal transcriptome of Grapholita molesta. Quantitative real-time polymerase chain reaction results revealed that GmolGSTD1 was mainly expressed in antennae and the expression levels were significantly higher in female antennae than in male antennae. The recombinant GmolGSTD1 (rGmolGSTD1) showed glutathione-conjugating activity toward 1-chloro-2,4-dinitrobenzene (CDNB) as substrates. The pH range for optimal rGmolGSTD1 enzyme activity was 6.0-6.5, and rGmolGSTD1 enzyme activity had maximal peaks at 35-40°C. Spectrophotometric analysis indicated that insecticides had weak inhibitory effects on the activity of rGmolGSTD1 with the inhibitory rates of 28.82% for chlorpyrifos, 22.27% for lambda-cyhalothrin, 18.07% for bifenthrin, 20.42% for acetamiprid, 17.57% for thiamethoxam, 25.67% for metaflumizone, 27.43% for abamectin, and 7.24% for chlorbenzuron. rGmolGSTD1 exhibited high degradation activity to the sex pheromone component (Z)-8-dodecenyl alcohol and the host plant volatile butyl hexanoate with the degradation efficiency of 75.01% and 48.54%, respectively. We speculate that GmolGSTD1 works in inactivating odorant molecules and maintaining sensitivity to olfactory communication of G. molesta.

Comparative Genomics Reveals Thousands of Novel Chemosensory Genes and Massive Changes in Chemoreceptor Repertories across Chelicerates

Chemoreception is a widespread biological function that is essential for the survival, reproduction, and social communication of animals. Though the molecular mechanisms underlying chemoreception are relatively well known in insects, they are poorly studied in the other major arthropod lineages. Current availability of a number of chelicerate genomes constitutes a great opportunity to better characterize gene families involved in this important function in a lineage that emerged and colonized land independently of insects. At the same time, that offers new opportunities and challenges for the study of this interesting animal branch in many translational research areas. Here, we have performed a comprehensive comparative genomics study that explicitly considers the high fragmentation of available draft genomes and that for the first time included complete genome data that cover most of the chelicerate diversity. Our exhaustive searches exposed thousands of previously uncharacterized chemosensory sequences, most of them encoding members of the gustatory and ionotropic receptor families. The phylogenetic and gene turnover analyses of these sequences indicated that the whole-genome duplication events proposed for this subphylum would not explain the differences in the number of chemoreceptors observed across species. A constant and prolonged gene birth and death process, altered by episodic bursts of gene duplication yielding lineage-specific expansions, has contributed significantly to the extant chemosensory diversity in this group of animals. This study also provides valuable insights into the origin and functional diversification of other relevant chemosensory gene families different from receptors, such as odorant-binding proteins and other related molecules.

The Effects of High Fat Diet-Induced Stress on Olfactory Sensitivity, Behaviors, and Transcriptional Profiling in Drosophila melanogaster

High-fat diet (HFD) often causes obesity and it has detrimental effects on the sensory system. In particular, sensory-mediated responses are crucial for maintaining energy balance, as they are involved in a metabolic regulation; however, there is still no clear explanation about the relationship between HFD-induced stress and sensory system. To gain insight on how HFD-induced stress affects olfactory sensitivity and behavioral responses, we have used a Drosophila melanogaster model for olfactory and nutrient-related signaling and accessed physiological, behavioral, and transcriptional changes. We demonstrated that lifespan and climbing ability in HFD-treated flies decreased and that olfactory sensitivity and behavioral responses to odorants were changed. Olfactory sensitivity to eight of ten odorants after 14 days on HFD treatment were reduced, while behavioral attraction was increased to benzaldehyde in flies that were treated with HFD. This behavioral and physiological modification in HFD-treated flies for 14 days was accompanied by a significant decrease in DmOrco gene expression in a peripheral olfactory organ, suggesting that is could be involved in the action of metabolic and sensory signal. Gene expression profiles of antennae showed significant differences on the olfactory receptors, odorant-binding proteins, and insulin signaling. Our results suggested that olfactory sensitivity and behavioral responses to HFD-induced stress are mediated through olfactory and nutrient-related signaling pathways.

Antennal transcriptome analysis of the chemosensory gene families in Carposina sasakii (Lepidoptera: Carposinidae)

BACKGROUND

The peach fruit moth, Carposina sasakii Matsumura (Lepidoptera: Carposinidae), poses a serious threat to a variety of fruits and causes significant economic loss owing to difficulties in its prevention and control. The olfactory sense is generally acknowledged to be a novel target for pest control. However, a systematic study of the olfactory genes expressed in C. sasakii has not been reported yet. Here, we reported the antennal transcriptome of C. sasakii using high-throughput sequencing and annotated the main chemosensory multi-gene families.

RESULTS

In the chemosensory gene families, 29 odorant-binding proteins, 13 chemosensory proteins, 1 sensory neuron membrane protein, 52 odorant receptors, 8 ionotropic receptors and 11 gustatory receptors were annotated in the C. sasakii antennal transcriptome. The number of olfactory genes obtained in our transcriptome was consistent with that identified in other lepidopteran insects, confirming that we basically accomplished the annotation of the chemosensory genes of C. sasakii in the adult antennal transcriptome. All sequences were annotated and analyzed by BLAST (basic local alignment search tool), and some chemosensory genes with specific functions were named according to the BLAST results and phylogenetic trees. Based on the expression profile in the transcriptome and phylogenetic analysis, differentially expressed genes (DEGs) were analyzed in both male and female adults. Finally, fluorescence quantitative real-time PCR was used to identify the male-specific or female-specific chemosensory genes that were putatively related to odor detection and recognition. Moreover, expression levels of OR33 and PBP2 were significantly higher in males than in females, indicating that these genes may interact with sex pheromones. We found some conserved antennal IRs and GRs involved in detecting sugar compounds (GR2, GR5, GR6, GR8) and carbon dioxide (GR1), which were also identified based on phylogenetic analysis.

CONCLUSIONS

There are 114 putative chemosensory proteins expressed in C. sasakii identified in this study. The identification of these proteins will make the molecular mechanism of odor recognition accessible.

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.

Identification and expression analysis of chemosensory receptor genes in an aphid endoparasitoid Aphidius gifuensis

Olfaction and gustation play critical roles during the host-location search process of insects. Several chemosensory receptor genes are thought to be involved in providing specificity to the olfactory sensory neuron responses. The aphid endoparasitoid, Aphidius gifuensis, has been used as a biological control agent against a variety of aphid species; this parasitoid is able to detect its target host(s) effectively during the parasitic process. To understand the mechanism of host detection in A. gifuensis, we assembled specific antennal transcriptomes of each sex through next generation sequencing technology to identify the major chemosensory receptor genes. Using a bioinformatics screen, we identified 100 olfactory receptors candidates (62 odorant receptors, 15 gustatory receptors, and 23 ionotropic receptors) from the sex-specific antennal transcriptome. In addition, combining with the demonstrated functions of chemosensory genes in other insects, the sex-, tissue-, and host-specific expression profile of chemosensory genes potentially revealed the candidate physiological functions. The identification and expression profile of chemosensory receptor genes in A. gifuensis provide valuable information for understanding and investigating the intraspecific or interspecific chemical communications in the solitary parasitic wasps.

Identification of candidate chemosensory genes by transcriptome analysis in Loxostege sticticalis Linnaeus

Loxostege sticticalis Linnaeus is an economically important agricultural pest, and the larvae cause great damage to crops, especially in Northern China. However, effective and environmentally friendly chemical methods for controlling this pest have not been discovered to date. In the present study, we performed HiSeq2500 sequencing of transcriptomes of the male and female adult antennae, adult legs and third instar larvae, and we identified 54 candidate odorant receptors (ORs), including 1 odorant receptor coreceptor (Orco) and 5 pheromone receptors (PRs), 18 ionotropic receptors (IRs), 13 gustatory receptors (GRs), 34 odorant binding proteins (OBPs), including 1 general odorant binding protein (GOBP1) and 3 pheromone binding proteins (PBPs), 10 chemosensory proteins (CSPs) and 2 sensory neuron membrane proteins (SNMPs). The results of RNA-Seq and RT-qPCR analyses showed the expression levels of most genes in the antennae were higher than that in the legs and larvae. Furthermore, PR4, OR1-4, 7–11, 13–15, 23, 29–32, 34, 41, 43, 47/IR7d.2/GR5b, 45, 7/PBP2-3, GOBP1, OBP3, 8 showed female antennae-biased expression, while PR1/OBP2, 7/IR75d/CSP2 showed male antennae-biased expression. However, IR1, 7d.3, 68a/OBP11, 20–22, 28/CSP9 had larvae enriched expression, and OBP15, 17, 25, 29/CSP5 were mainly expressed in the legs. The results shown above indicated that these genes might play a key role in foraging, seeking mates and host recognition in the L. sticticalis. Our findings will provide the basic knowledge for further studies on the molecular mechanisms of the olfactory system of L. sticticalis and potential novel targets for pest control strategies.

Gene set of chemosensory receptors in the polyembryonic endoparasitoid Macrocentrus cingulum

Insects are extremely successful animals whose odor perception is very prominent due to their sophisticated olfactory system. The main chemosensory organ, antennae play a critical role in detecting odor in ambient environment before initiating appropriate behavioral responses. The antennal chemosensory receptor genes families have been suggested to be involved in olfactory signal transduction pathway as a sensory neuron response. The Macrocentrus cingulum is deployed successfully as a biological control agent for corn pest insects from the Lepidopteran genus Ostrinia. In this research, we assembled antennal transcriptomes of M. cingulum by using next generation sequencing to identify the major chemosensory receptors gene families. In total, 112 olfactory receptors candidates (79 odorant receptors, 20 gustatory receptors, and 13 ionotropic receptors) have been identified from the male and female antennal transcriptome. The sequences of all of these transcripts were confirmed by RT-PCR, and direct DNA sequencing. Expression profiles of gustatory receptors in olfactory and non-olfactory tissues were measured by RT-qPCR. The sex-specific and sex-biased chemoreceptors expression patterns suggested that they may have important functions in sense detection which behaviorally relevant to odor molecules. This reported result provides a comprehensive resource of the foundation in semiochemicals driven behaviors at molecular level in polyembryonic endoparasitoid.

Carbon dioxide receptor genes and their expression profile in Diabrotica virgifera virgifera

Background

Diabrotica virgifera virgifera, western corn rootworm, is one of the most devastating species in North America. D. v. virgifera neonates crawl through the soil to locate the roots on which they feed. Carbon dioxide (CO₂) is one of the important volatile cues that attract D. v. virgifera larvae to roots.

Results

In this study, we identified three putative D. v. virgifera gustatory receptor genes (Dvv_Gr1, Dvv_Gr2, and Dvv_Gr3). Phylogenetic analyses confirmed their orthologous relationships with known insect CO₂ receptor genes from Drosophila, mosquitoes, and Tribolium. The phylogenetic reconstruction of insect CO₂ receptor proteins and the gene expression profiles were analyzed. Quantitative analysis of gene expression indicated that the patterns of expression of these three candidate genes vary among larval tissues (i.e., head, integument, fat body, and midgut) and different development stages (i.e., egg, three larval stages, adult male and female).

Conclusion

The Dvv_Gr2 gene exhibited highest expression in heads and neonates, suggesting its importance in allowing neonate larvae to orient to its host plant. Similar expression patterns across tissues and developmental stages for Dvv_Gr1 and Dvv_Gr3 suggest a potentially different role. Findings from this study will allow further exploration of the functional role of specific CO₂ receptor proteins in D. v. virgifera.

Identification of Putative Olfactory Genes from the Oriental Fruit Moth Grapholita molesta via an Antennal Transcriptome Analysis

BACKGROUND

The oriental fruit moth, Grapholita molesta, is an extremely important oligophagous pest species of stone and pome fruits throughout the world. As a host-switching species, adult moths, especially females, depend on olfactory cues to a large extent in locating host plants, finding mates, and selecting oviposition sites. The identification of olfactory genes can facilitate investigation on mechanisms for chemical communications.

METHODOLOGY/PRINCIPAL FINDING

We generated transcriptome of female antennae of G.molesta using the next-generation sequencing technique, and assembled transcripts from RNA-seq reads using Trinity, SOAPdenovo-trans and Abyss-trans assemblers. We identified 124 putative olfactory genes. Among the identified olfactory genes, 118 were novel to this species, including 28 transcripts encoding for odorant binding proteins, 17 chemosensory proteins, 48 odorant receptors, four gustatory receptors, 24 ionotropic receptors, two sensory neuron membrane proteins, and one odor degrading enzyme. The identified genes were further confirmed through semi-quantitative reverse transcription PCR for transcripts coding for 26 OBPs and 17 CSPs. OBP transcripts showed an obvious antenna bias, whereas CSP transcripts were detected in different tissues.

CONCLUSION

Antennal transcriptome data derived from the oriental fruit moth constituted an abundant molecular resource for the identification of genes potentially involved in the olfaction process of the species. This study provides a foundation for future research on the molecules involved in olfactory recognition of this insect pest, and in particular, the feasibility of using semiochemicals to control this pest.

Male- and Female-Biased Gene Expression of Olfactory-Related Genes in the Antennae of Asian Corn Borer, Ostrinia furnacalis (Guenée) (Lepidoptera: Crambidae)

The Asian corn borer (ACB), Ostrinia furnacalis (Guenée), is a destructive pest insect of cultivated corn crops, for which antennal-expressed receptors are important to detect olfactory cues for mate attraction and oviposition. Few olfactory related genes were reported in ACB, so we sequenced and characterized the transcriptome of male and female O. furnacalis antennae. Non-normalized male and female O. furnacalis antennal cDNA libraries were sequenced on the Illumina HiSeq 2000 and assembled into a reference transcriptome. Functional gene annotations identified putative olfactory-related genes; 56 odorant receptors (ORs), 23 odorant binding proteins (OBPs), and 10 CSPs. RNA-seq estimates of gene expression respectively showed up- and down-regulation of 79 and 30 genes in female compared to male antennae, which included up-regulation of 8 ORs and 1 PBP gene in male antennae as well as 3 ORs in female antennae. Quantitative real-time RT-PCR analyses validated strong male antennal-biased expression of OfurOR3, 4, 6, 7, 8, 11, 12, 13 and 14 transcripts, whereas OfurOR17 and 18 were specially expressed in female antennae. Sex-biases gene expression described here provides important insight in gene functionalization, and provides candidate genes putatively involved in environmental perception, host plant attraction, and mate recognition.

Antennal transcriptome analysis and comparison of chemosensory gene families in two closely related noctuidae moths, Helicoverpa armigera and H. assulta

To better understand the olfactory mechanisms in the two lepidopteran pest model species, the Helicoverpa armigera and H. assulta, we conducted transcriptome analysis of the adult antennae using Illumina sequencing technology and compared the chemosensory genes between these two related species. Combined with the chemosensory genes we had identified previously in H. armigera by 454 sequencing, we identified 133 putative chemosensory unigenes in H. armigera including 60 odorant receptors (ORs), 19 ionotropic receptors (IRs), 34 odorant binding proteins (OBPs), 18 chemosensory proteins (CSPs), and 2 sensory neuron membrane proteins (SNMPs). Consistent with these results, 131 putative chemosensory genes including 64 ORs, 19 IRs, 29 OBPs, 17 CSPs, and 2 SNMPs were identified through male and female antennal transcriptome analysis in H. assulta. Reverse Transcription-PCR (RT-PCR) was conducted in H. assulta to examine the accuracy of the assembly and annotation of the transcriptome and the expression profile of these unigenes in different tissues. Most of the ORs, IRs and OBPs were enriched in adult antennae, while almost all the CSPs were expressed in antennae as well as legs. We compared the differences of the chemosensory genes between these two species in detail. Our work will surely provide valuable information for further functional studies of pheromones and host volatile recognition genes in these two related species.

Narrow tuning of an odorant receptor to plant volatiles in Spodoptera exigua (Hübner)

Olfaction plays an important role in insects in recognizing volatile compounds, which are used to find food and mates, as well as avoid danger, predators and pathogens. The key players in the detection of volatile compounds are olfactory receptor (OR) proteins, which are located within the dendritic membrane of sensory neurons and extend into the lymph of sensilla on insect antennae. In the present study, we identify an OR gene, named SexiOR3, which is expressed in adult antenna in both sexes, but with female bias, in the beet armyworm moth Spodoptera exigua. Further in situ hybridization analysis revealed that SexiOR3 was mainly located in short trichoid sensilla. In in vitro heterologous expression experiments, SexiOR3 was narrowly tuned to E-β-farnesene and several compounds of related structure, among 62 different compounds tested in this study. Furthermore, SexiOR3 responds to E-β-farnesene at a low concentration of 10(-9)  M, comparable to that of pheromone receptors (PRs) responding to the pheromones. This is a very interesting finding for a general OR, indicating that high specificity is a feature of at least one general OR and not only the PRs. The results suggest that the OR3 gene may play an important role in the moth olfactory system, and underpins important insect behaviour.

Identification of candidate olfactory genes in Chilo suppressalis by antennal transcriptome analysis

Antennal olfaction, which is extremely important for insect survival, mediates key behaviors such as host preference, mate choice, and oviposition site selection. In insects, odor detection is mediated by multiple proteins in the antenna, especially the odorant receptors (ORs) and ionotropic receptors (IRs), which ensure the specificity of the olfactory sensory neuron responses. In this study, we identified the olfactory gene repertoire of the rice stem borer, Chilo suppressalis, an economically important agricultural pest, which inflicts great damage to the rice yield in south and east part of Asia, especially in Southern China. By Illumina sequencing of male and female antennal transcriptomes, we identified 47 odorant receptors, 20 ionotropic receptors, 26 odorant binding proteins, 21 chemosensory proteins and 2 sensory neuron membrane proteins. Our findings make it possible for future research of the olfactory system of C. suppressalis at the molecular level.

Promising Aedes aegypti repellent chemotypes identified through integrated QSAR, virtual screening, synthesis, and bioassay

Molecular field topology analysis, scaffold hopping, and molecular docking were used as complementary computational tools for the design of repellents for Aedes aegypti, the insect vector for yellow fever, chikungunya, and dengue fever. A large number of analogues were evaluated by virtual screening with Glide molecular docking software. This produced several dozen hits that were either synthesized or procured from commercial sources. Analysis of these compounds by a repellent bioassay resulted in a few highly active chemicals (in terms of minimum effective dosage) as viable candidates for further hit-to-lead and lead optimization effort.

Antennal transcriptome analysis of the chemosensory gene families in the tree killing bark beetles, Ips typographus and Dendroctonus ponderosae (Coleoptera: Curculionidae: Scolytinae)

Background

The European spruce bark beetle, Ips typographus, and the North American mountain pine beetle, Dendroctonus ponderosae (Coleoptera: Curculionidae: Scolytinae), are severe pests of coniferous forests. Both bark beetle species utilize aggregation pheromones to coordinate mass-attacks on host trees, while odorants from host and non-host trees modulate the pheromone response. Thus, the bark beetle olfactory sense is of utmost importance for fitness. However, information on the genes underlying olfactory detection has been lacking in bark beetles and is limited in Coleoptera. We assembled antennal transcriptomes from next-generation sequencing of I. typographus and D. ponderosae to identify members of the major chemosensory multi-gene families.

Results

Gene ontology (GO) annotation indicated that the relative abundance of transcripts associated with specific GO terms was highly similar in the two species. Transcripts with terms related to olfactory function were found in both species. Focusing on the chemosensory gene families, we identified 15 putative odorant binding proteins (OBP), 6 chemosensory proteins (CSP), 3 sensory neuron membrane proteins (SNMP), 43 odorant receptors (OR), 6 gustatory receptors (GR), and 7 ionotropic receptors (IR) in I. typographus; and 31 putative OBPs, 11 CSPs, 3 SNMPs, 49 ORs, 2 GRs, and 15 IRs in D. ponderosae. Predicted protein sequences were compared with counterparts in the flour beetle, Tribolium castaneum, the cerambycid beetle, Megacyllene caryae, and the fruit fly, Drosophila melanogaster. The most notable result was found among the ORs, for which large bark beetle-specific expansions were found. However, some clades contained receptors from all four beetle species, indicating a degree of conservation among some coleopteran OR lineages. Putative GRs for carbon dioxide and orthologues for the conserved antennal IRs were included in the identified receptor sets.

Conclusions

The protein families important for chemoreception have now been identified in three coleopteran species (four species for the ORs). Thus, this study allows for improved evolutionary analyses of coleopteran olfaction. Identification of these proteins in two of the most destructive forest pests, sharing many semiochemicals, is especially important as they might represent novel targets for population control.

Identification of odor-processing genes in the emerald ash borer, Agrilus planipennis

Background

Insects rely on olfaction to locate food, mates, and suitable oviposition sites for successful completion of their life cycle. Agrilus planipennis Fairmaire (emerald ash borer) is a serious invasive insect pest that has killed tens of millions of North American ash (Fraxinus spp) trees and threatens the very existence of the genus Fraxinus. Adult A. planipennis are attracted to host volatiles and conspecifics; however, to date no molecular knowledge exists on olfaction in A. planipennis. Hence, we undertook an antennae-specific transcriptomic study to identify the repertoire of odor processing genes involved in A. planipennis olfaction.

Methodology and Principal Findings

We acquired 139,085 Roche/454 GS FLX transcriptomic reads that were assembled into 30,615 high quality expressed sequence tags (ESTs), including 3,249 isotigs and 27,366 non-isotigs (contigs and singletons). Intriguingly, the majority of the A. planipennis antennal transcripts (59.72%) did not show similarity with sequences deposited in the non-redundant database of GenBank, potentially representing novel genes. Functional annotation and KEGG analysis revealed pathways associated with signaling and detoxification. Several odor processing genes (9 odorant binding proteins, 2 odorant receptors, 1 sensory neuron membrane protein and 134 odorant/xenobiotic degradation enzymes, including cytochrome P450s, glutathione-S-transferases; esterases, etc.) putatively involved in olfaction processes were identified. Quantitative PCR of candidate genes in male and female A. planipennis in different developmental stages revealed developmental- and sex-biased expression patterns.

Conclusions and Significance

The antennal ESTs derived from A. planipennis constitute a rich molecular resource for the identification of genes potentially involved in the olfaction process of A. planipennis. These findings should help in understanding the processing of antennally-active compounds (e.g. 7-epi-sesquithujene) previously identified in this serious invasive pest.

Candidate olfaction genes identified within the Helicoverpa armigera Antennal Transcriptome

Antennal olfaction is extremely important for insect survival, mediating key behaviors such as host preference, mate choice, and oviposition site selection. Multiple antennal proteins are involved in olfactory signal transduction pathways. Of these, odorant receptors (ORs) and ionotropic receptors (IRs) confer specificity on olfactory sensory neuron responses. In this study, we identified the olfactory gene repertoire of the economically important agricultural pest moth, Helicoverpa armigera, by assembling the adult male and female antennal transcriptomes. Within the male and female antennal transcriptomes we identified a total of 47 OR candidate genes containing 6 pheromone receptor candidates. Additionally, 12 IR genes as well as 26 odorant-binding proteins and 12 chemosensory proteins were annotated. Our results allow a systematic functional analysis across much of conventional ORs repertoire and newly reported IRs mediating the key olfaction-mediated behaviors of H. armigera.

Elemental and configural olfactory coding by antennal lobe neurons of the honeybee (Apis mellifera)

When smelling an odorant mixture, olfactory systems can be analytical (i.e. extract information about the mixture elements) or synthetic (i.e. creating a configural percept of the mixture). Here, we studied elemental and configural mixture coding in olfactory neurons of the honeybee antennal lobe, local neurons in particular. We conducted intracellular recordings and stimulated with monomolecular odorants and their coherent or incoherent binary mixtures to reproduce a temporally dynamic environment. We found that about half of the neurons responded as ‘elemental neurons’, i.e. responses evoked by mixtures reflected the underlying feature information from one of the components. The other half responded as ‘configural neurons’, i.e. responses to mixtures were clearly different from responses to their single components. Elemental neurons divided in late responders (above 60 ms) and early responder neurons (below 60 ms), whereas responses of configural coding neurons concentrated in-between these divisions. Latencies of neurons with configural responses express a tendency to be faster for coherent stimuli which implies employment in different processing circuits.

Transcriptome profiling of chemosensory appendages in the malaria vector Anopheles gambiae reveals tissue- and sex-specific signatures of odor coding

Background

Chemosensory signal transduction guides the behavior of many insects, including Anopheles gambiae, the major vector for human malaria in sub-Saharan Africa. To better understand the molecular basis of mosquito chemosensation we have used whole transcriptome RNA sequencing (RNA-seq) to compare transcript expression profiles between the two major chemosensory tissues, the antennae and maxillary palps, of adult female and male An. gambiae.

Results

We compared chemosensory tissue transcriptomes to whole body transcriptomes of each sex to identify chemosensory enhanced genes. In the six data sets analyzed, we detected expression of nearly all known chemosensory genes and found them to be highly enriched in both olfactory tissues of males and females. While the maxillary palps of both sexes demonstrated strict chemosensory gene expression overlap, we observed acute differences in sensory specialization between male and female antennae. The relatively high expression levels of chemosensory genes in the female antennae reveal its role as an organ predominately assigned to chemosensation. Remarkably, the expression of these genes was highly conserved in the male antennae, but at much lower relative levels. Alternatively, consistent with a role in mating, the male antennae displayed significant enhancement of genes involved in audition, while the female enhancement of these genes was observed, but to a lesser degree.

Conclusions

These findings suggest that the chemoreceptive spectrum, as defined by gene expression profiles, is largely similar in female and male An. gambiae. However, assuming sensory receptor expression levels are correlated with sensitivity in each case, we posit that male and female antennae are perceptive to the same stimuli, but possess inverse receptive prioritizations and sensitivities. Here we have demonstrated the use of RNA-seq to characterize the sensory specializations of an important disease vector and grounded future studies investigating chemosensory processes.

Sensory detection and responses to toxic gases: mechanisms, health effects, and countermeasures

The inhalation of reactive gases and vapors can lead to severe damage of the airways and lung, compromising the function of the respiratory system. Exposures to oxidizing, electrophilic, acidic, or basic gases frequently occur in occupational and ambient environments. Corrosive gases and vapors such as chlorine, phosgene, and chloropicrin were used as warfare agents and in terrorist acts. Chemical airway exposures are detected by the olfactory, gustatory, and nociceptive sensory systems that initiate protective physiological and behavioral responses. This review focuses on the role of airway nociceptive sensory neurons in chemical sensing and discusses the recent discovery of neuronal receptors for reactive chemicals. Using physiological, imaging, and genetic approaches, Transient Receptor Potential (TRP) ion channels in sensory neurons were shown to respond to a wide range of noxious chemical stimuli, initiating pain, respiratory depression, cough, glandular secretions, and other protective responses. TRPA1, a TRP ion channel expressed in chemosensory C-fibers, is activated by almost all oxidizing and electrophilic chemicals, including chlorine, acrolein, tear gas agents, and methyl isocyanate, the highly noxious chemical released in the Bhopal disaster. Chemicals likely activate TRPA1 through covalent protein modification. Animal studies using TRPA1 antagonists or TRPA1-deficient mice confirmed the role of TRPA1 in chemically induced respiratory reflexes, pain, and inflammation in vivo. New research shows that sensory neurons are not merely passive sensors of chemical exposures. Sensory channels such as TRPA1 are essential for maintenance of airway inflammation in asthma and may contribute to the progression of airway injury following high-level chemical exposures.

Olfactory receptors: G protein-coupled receptors and beyond

Sensing the chemical environment is critical for all organisms. Diverse animals from insects to mammals utilize highly organized olfactory system to detect, encode, and process chemostimuli that may carry important information critical for health, survival, social interactions and reproduction. Therefore, for animals to properly interpret and react to their environment it is imperative that the olfactory system recognizes chemical stimuli with appropriate selectivity and sensitivity. Because olfactory receptor proteins play such an essential role in the specific recognition of diverse stimuli, understanding how they interact with and transduce their cognate ligands is a high priority. In the nearly two decades since the discovery that the mammalian odorant receptor gene family constitutes the largest group of G protein-coupled receptor (GPCR) genes, much attention has been focused on the roles of GPCRs in vertebrate and invertebrate olfaction. However, is has become clear that the 'family' of olfactory receptors is highly diverse, with roles for enzymes and ligand-gated ion channels as well as GPCRs in the primary detection of olfactory stimuli.