Chemosensory receptor genes in the Oriental tobacco budworm Helicoverpa assulta

Transcuticular calcium imaging as a tool for the functional study of insect odorant receptors

The primary actors in the detection of olfactory information in insects are odorant receptors (ORs), transmembrane proteins expressed at the dendrites of olfactory sensory neurons (OSNs). In order to decode the insect olfactome, many studies focus on the deorphanization of ORs (i.e., identification of their ligand), using various approaches involving heterologous expression coupled to neurophysiological recordings. The "empty neuron system" of the fruit fly Drosophila melanogaster is an appreciable host for insect ORs, because it conserves the cellular environment of an OSN. Neural activity is usually recorded using labor-intensive electrophysiological approaches (single sensillum recordings, SSR). In this study, we establish a simple method for OR deorphanization using transcuticular calcium imaging (TCI) at the level of the fly antenna. As a proof of concept, we used two previously deorphanized ORs from the cotton leafworm Spodoptera littoralis, a specialist pheromone receptor and a generalist plant odor receptor. We demonstrate that by co-expressing the GCaMP6s/m calcium probes with the OR of interest, it is possible to measure robust odorant-induced responses under conventional microscopy conditions. The tuning breadth and sensitivity of ORs as revealed using TCI were similar to those measured using single sensillum recordings (SSR). We test and discuss the practical advantages of this method in terms of recording duration and the simultaneous testing of several insects.

Conserved Odorant Receptor, EcorOR4, Mediates Attraction of Mated Female Eupeodes corollae to 1-Octen-3-ol

Odorant receptors (ORs) in insects are crucial for the detection of chemical signals. However, the functions of the conserved OR genes among insect species are rarely studied. In this study, we analyzed a well-conserved OR clade in Diptera insects and cloned a gene from this clade, EcorOR4, in the hoverfly Eupeodes corollae. Real-time quantitative PCR showed that EcorOR4 was highly expressed in the antennae and upregulated in the mated females, and in vitro functional characterization showed that EcorOR4 was narrowly tuned to 1-octen-3-ol. Electroantennogram assays revealed that the antennal response of mated females to 1-octen-3-ol was significantly higher than that of mated males, but no significant differences were observed between male and female virgins. Finally, a Y-tube olfactometer bioassay showed that 1-octen-3-ol is an attractant for only mated female E. corollae adults. These results demonstrate that EcorOR4 is involved in the detection of 1-octen-3-ol and that this compound may affect the host-finding and oviposition behavior in female E. corollae.

Identification and expression profile analysis of chemosensory genes in pine needle gall midge, Thecodiplosis japonensis (Diptera: Cecidomyiidae)

Insects have highly specialized and sensitive olfactory systems involving several chemosensory genes to locate their mates and hosts or escape from predators. Pine needle gall midge, Thecodiplosis japonensis (Diptera: Cecidomyiidae), has invaded China since 2016 and caused serious damage. Till now, there is no environmentally friendly measure to control this gall midge. Screening molecules with high affinity to target odorant-binding protein to develop highly efficient attractants is a potential pest management method. However, the chemosensory genes in T. japonensis are still unclear. We identified 67 chemosensory-related genes in the transcriptomes of antennae, including 26 OBPs, 2 CSPs, 17 ORs, 3 SNMPs, 6 GRs, and 13 IRs, using high throughput sequencing. Phylogenetic analysis of these six chemosensory gene families among Dipteran was performed to classify and predict the functions. The expression profiles of OBPs, CSPs and ORs were validated by quantitative real-time PCR. 16 of the 26 OBPs were biased expressed in antennae. TjapORco and TjapOR5 were highly expressed in the antenna of unmated male and female adults. The functions of related OBPs and ORs genes were also discussed. These results provide a basis for the functional research on chemosensory genes at the molecular level.

Candidate chemosensory receptors in the antennae and maxillae of Spodoptera frugiperda (J. E. Smith) larvae

Although most of the damage caused by lepidopteran insects to plants is caused by the larval stage, chemosensory systems have been investigated much more frequently for lepidopteran adults than for larvae. The fall armyworm Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) is a polyphagous and worldwide pest. To understand the larval chemosensory system in S. frugiperda, we sequenced and assembled the antennae and maxillae transcriptome of larvae in the sixth instar (larval a-m) using the Illumina platform. A total of 30 putative chemosensory receptor genes were identified, and these receptors included 11 odorant receptors (ORs), 4 gustatory receptors (GRs), and 15 ionotropic receptors/ionotropic glutamate receptors (IRs/iGluRs). Phylogeny tests with the candidate receptors and homologs from other insect species revealed some specific genes, including a fructose receptor, a pheromone receptor, IR co-receptors, CO2 receptors, and the OR co-receptor. Comparison of the expression of annotated genes between S. frugiperda adults and larvae (larval a-m) using RT-qPCR showed that most of the annotated OR and GR genes were predominantly expressed in the adult stage, but that 2 ORs and 1 GR were highly expressed in both the adult antennae and the larval a-m. Although most of the tested IR/iGluR genes were mainly expressed in adult antennae, transcripts of 3 iGluRs were significantly more abundant in the larval a-m than in the adult antennae of both sexes. Comparison of the expression levels of larval a-m expressed chemosensory receptors among the first, fourth, and sixth instars revealed that the expression of some of the genes varied significantly among different larval stages. These results increase our understanding of the chemosensory systems of S. frugiperda larvae and provide a basis for future functional studies aimed at the development of novel strategies to manage this pest.

Antennal transcriptome analysis of olfactory genes and tissue expression profiling of odorant binding proteins in Semanotus bifasciatus (cerambycidae: coleoptera)

Background

Insect olfactory proteins can transmit chemical signals in the environment that serve as the basis for foraging, mate searching, predator avoidance and oviposition selection. Semanotus bifasciatus is an important destructive borer pest, but its olfactory mechanism is not clear. We identified the chemosensory genes of S. bifasciatus in China, then we conducted a phylogenetic analysis of the olfactory genes of S. bifasciatus and other species. And the expression profiles of odorant binding proteins (OBPs) genes in different tissues and different genders of S. bifasciatus were determined by quantitative real-time PCR for the first time.

Results

A total of 32 OBPs, 8 chemosensory proteins (CSPs), 71 odorant receptors (ORs), 34 gustatory receptors (GRs), 18 ionotropic receptors (IRs), and 3 sensory neuron membrane proteins (SNMPs) were identified. In the tissue expression analysis of OBP genes, 7 OBPs were higher expressed in antennae, among them, SbifOBP2, SbifOBP3, SbifOBP6, SbifOBP7 and SbifOBP20 were female-biased expression, while SbifOBP1 was male-biased expression and SbifOBP22 was no-biased expression in antennae. In addition, the expressed levels of SbifOBP4, SbifOBP12, SbifOBP15, SbifOBP27 and SbifOBP29 were very poor in the antennae, and SbifOBP4 and SbifOBP29 was abundant in the head or legs, and both of them were male-biased expression. While SbifOBP15 was highly expressed only at the end of the abdomen with its expression level in females three times than males. Other OBPs were expressed not only in antennae but also in various tissues.

Conclusion

We identified 166 olfactory genes from S. bifasciatus, and classified these genes into groups and predicted their functions by phylogenetic analysis. The majority of OBPs were antenna-biased expressed, which are involved in odor recognition, sex pheromone detection, and/or host plant volatile detection. However, also some OBPs were detected biased expression in the head, legs or end of the abdomen, indicating that they may function in the different physiological processes in S. bifasciatus.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08655-w.

Sex Pheromone Receptors of Lepidopteran Insects

The sex pheromone receptors (SPRs) of Lepidopteran insects play important roles in chemical communication. In the sex pheromone detection processes, sex pheromone molecule (SPM), SPR, co-receptor (Orco), pheromone binding protein (PBP), sensory neuron membrane protein (SNMP), and pheromone degradation enzyme (PDE) play individual and cooperative roles. Commonly known as butterfly and moth, the Lepidopteran insects are widely distributed throughout the world, most of which are pests. Comprehensive knowledge of the SPRs of Lepidopteran insects would help the development of sex lure technology and the sex communication pathway research. In this review, we summarized SPR/Orco information from 10 families of Lepidopteran insects from corresponding studies. According to the research progress in the literature, we speculated the evolution of SPRs/Orcos and phylogenetically analyzed the Lepidopteran SPRs and Orcos with the neighbor-joining tree and further concluded the relationship between the cluster of SPRs and their ligands; we analyzed the predicted structural features of SPRs and gave our prediction results of SPRs and Orcos with Consensus Constrained TOPology Prediction (CCTOP) and SwissModel; we summarized the functional characterization of Lepidopteran SPRs and SPR-ligand interaction and then described the progress in the sex pheromone signaling pathways and metabotropic ion channel. Further studies are needed to work out the cryo-electron microscopy (EM) structure of SPR and the SPR-ligand docking pattern in a biophysical perspective, which will directly facilitate the understanding of sex pheromone signal transduction pathways and provide guidance in the sex lure technology in field pest control.

Sex pheromone receptors of the light brown apple moth, Epiphyas postvittana, support a second major pheromone receptor clade within the Lepidoptera

Sex pheromones facilitate species-specific sex communication within the Lepidoptera. They are detected by specialised pheromone receptors (PRs), most of which to date fall into a single monophyletic receptor lineage (frequently referred to as "the PR clade") within the odorant receptor (OR) family. Here we investigated PRs of the invasive horticultural pest, Epiphyas postvittana, commonly known as the light brown apple moth. Ten candidate PRs were selected, based on their male-biased expression in antennae or their relationship to the PR clade, for functional assessment in both HEK293 cells and Xenopus oocytes. Of these, six ORs responded to compounds that include components of the E. postvittana ('Epos') sex pheromone blend or compounds that antagonise sex pheromone attraction. In phylogenies, four of the characterised receptors (EposOR1, 6, 7 and 45) fall within the PR clade and two other male-biased receptors (EposOR30 and 34) group together well outside the PR clade. This new clade of pheromone receptors includes the receptor for (E)-11-tetradecenyl acetate (EposOR30), which is the main component of the sex pheromone blend for this species. Interestingly, receptors of the two clades do not segregate by preference for compounds associated with behavioural response (agonist or antagonist), isomer type (E or Z) or functional group (alcohol or acetate), with examples of each scattered across both clades. Phylogenetic comparison with PRs from other species supports the existence of a second major clade of lepidopteran ORs including, EposOR30 and 34, that has been co-opted into sex pheromone detection in the Lepidoptera. This second clade of sex pheromone receptors has an origin that likely predates the split between the major lepidopteran families.

Comparison of functions of pheromone receptor repertoires in Helicoverpa armigera and Helicoverpa assulta using a Drosophila expression system

Helicoverpa armigera and H. assulta are sympatric closely related species sharing two sex pheromone components, (Z)-11-hexadecenal (Z11-16:Ald) and (Z)-9-hexadecenal (Z9-16:Ald) but in opposite ratios, 97:3 and 3:97 respectively. This feature makes them a feasible model for studying the evolution of pheromone coding mechanisms of lepidopteran insects. Despite a decade-long study to deorphanize the pheromone receptor (PR) repertoires of the two species, the comparison of the function of all PR orthologs between the two species is incomplete. Moreover, the ligands of OR14 and OR15 have so far not been found, likely due to the missing of the active ligand(s) in the compound panel and/or incompatibility of heterologous expression systems used. In the present study, we expressed the PR repertoires of both Helicoverpa species in Drosophila T1 neurons to comparatively study the function of PRs. Among those PRs, OR13, OR6, and OR14 of both species are functionally conserved and narrowly tuned, and the T1 neurons expressing each of them respond to Z11-16:Ald, (Z)-9-hexadecenol (Z9-16:OH), and (Z)-11-hexadecenyl acetate (Z11-16:Ac), respectively. While HarmOR16-expressing neurons respond strongly to (Z)-9-tetradecenal (Z9-14:Ald) and (Z)-11-hexadecenol (Z11-16:OH), the neurons expressing HassOR16 mainly respond to Z9-14:Ald and also weakly respond to (Z)-9-tetradecenol (Z9-14:OH). Moreover, HarmOR14b-expressing neurons are activated by Z9-14:Ald, whereas HassOR14b-expressing neurons are sensitive to Z9-16:Ald, Z9-14:Ald, and (Z)-9-hexadecenol (Z9-16:OH). In addition, HarmOR15-expressing neurons are selectively responsive to Z9-14:Ald. However, the Drosophila T1 neurons expressing either HarmOR11 or HassOR11 are silent to all of the compounds tested. In summary, except for OR11, we have deorphanized all the PRs of these two Helicoverpa species using a Drosophila expression system and a large panel of pheromone compounds, thereby providing a valuable reference for parsing the code of peripheral coding of pheromones.

Chemosensory Proteins (CSPs) in the Cotton Bollworm Helicoverpa armigera

Simple Summary

The insect chemosensory system is crucial in regulating insect behaviors. Chemosensory proteins (CSPs) are a family of small, soluble proteins conventionally known to transport odorant molecules in insect chemosensory system. Besides chemosensation, CSPs have been reported to play important roles in development, nutrient metabolism, and insecticide resistance. Therefore, identification and characterization of previously unknown CSPs will be valuable for further investigation of this protein family. The cotton bollworm, Helicoverpa armigera (Hübner) is among the most serious insect pests in various agricultural and horticultural crops. In this study, 27 CSP genes were identified from H. armigera genome and transcriptome sequences, and their expression patterns were further examined by using transcriptomic data obtained from different tissues and stages. The results demonstrate that H. armigera CSP genes are highly expressed in both chemosensory and non-chemosensory tissues. Moreover, a new recombinant expression method was developed that can significantly increase H. armigera CSP expression levels as soluble proteins in Escherichia coli. This study improves our understanding of insect CSPs and developed a new approach to highly express recombinant CSPs, which can be expanded to examine CSPs in other species for functional characterization.

Abstract

Chemosensory proteins (CSPs) are a family of small, soluble proteins that play a crucial role in transporting odorant and pheromone molecules in the insect chemosensory system. Recent studies reveal that they also function in development, nutrient metabolism and insecticide resistance. In-depth and systematic characterization of previously unknown CSPs will be valuable to investigate more detailed functionalities of this protein family. Here, we identified 27 CSP genes from the genome and transcriptome sequences of cotton bollworm, Helicoverpa armigera (Hübner). The expression patterns of these genes were studied by using transcriptomic data obtained from different tissues and stages. The results demonstrate that H. armigera CSP genes are not only highly expressed in chemosensory tissues, such as antennae, mouthparts, and tarsi, but also in the salivary glands, cuticle epidermis, and hind gut. HarmCSP6 and 22 were selected as candidate CSPs for expression in Escherichia coli and purification. A new method was developed that significantly increased the HarmCSP6 and 22 expression levels as soluble recombinant proteins for purification. This study advances our understanding of insect CSPs and provides a new approach to highly express recombinant CSPs in E. coli.

Olfactory Sensilla and Olfactory Genes in the Parasitoid Wasp Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae)

Simple Summary

Parasitic wasps are the major natural enemies of many organisms, and therefore they are broadly used in the biological control of numerous agricultural and horticultural pests. For example, Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) is a tiny natural egg parasitoid of various agricultural pest insects, including Plutella xylostella, Helicoverpa armigera, Spodoptera frugiperda and Ectomyelois ceratoniae. However, how T. pretiosum seek and localise host insect eggs is still not clear. The olfactory system is critical in guiding insect behaviours, including mating, feeding and oviposition, which play pivotal roles in the interactions between parasitoid wasps and their hosts. This project aimed to investigate T. pretiosum major olfactory tissue (antennae) and the olfactory genes, including odorant binding proteins (OBPs) and odorant receptors (ORs). T. pretiosum adult antennae were examined under scanning electron microscopy, and four types of olfactory sensilla were observed. Using T. pretiosum genome, 22 OBPs and 105 ORs were identified, which were further compared with olfactory genes of other Hymenoptera insect species. The expression patterns of OBPs between T. pretiosum male and female adults were examined to identify female- or male-specific OBPs. This study enriches our knowledge of T. pretiosum olfactory system and will help better use it in the integrated pest management (IPM) for many insect pest species.

Abstract

Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) is a tiny natural egg parasitoid of several agricultural pest insects, which has been widely used in the biological control for Plutella xylostella, Helicoverpa armigera, Spodoptera frugiperda and Ectomyelois ceratoniae. However, limited studies have been conducted on T. pretiosum olfactory system, which is critical in regulating insect behaviours. In this study, T. pretiosum adult antennae were investigated under ascanning electron microscopy (SEM). Four types of olfactory sensilla were observed, including chaetica sensilla (CS), trichoid sensilla (TS), faleate sensilla (FS) and placoid sensilla (PS). Using T. pretiosum genome, 22 putative odorant binding proteins (OBPs) and 105 odorant receptors (ORs) were identified, which were further compared with olfactory genes of Apis mellifera, Nasonia vitripennis and Diachasma alloeum. The expression patterns of OBPs between T. pretiosum male and female adults were examined by quantitative real time PCR (qRT-PCR) approaches. Three female-specific OBPs (TpreOBP19, TpreOBP15 and TpreOBP3) were identified, which may play crucial roles in T. pretiosum host-seeking and oviposition behaviours. This study enriches our knowledge of T. pretiosum olfactory genes and improves our understanding of its olfactory system.

Chemosensory genes in the head of Spodoptera litura larvae

The tobacco cutworm Spodoptera litura (Lepidoptera: Noctuidae) is a polyphagous pest with a highly selective and sensitive chemosensory system involved in complex physiological behaviors such as searching for food sources, feeding, courtship, and oviposition. However, effective management strategies for controlling the insect pest populations under threshold levels are lacking. Therefore, there is an urgent need to formulate eco-friendly pest control strategies based on the disruption of the insect chemosensory system. In this study, we identified 158 putative chemosensory genes based on transcriptomic and genomic data for S. litura, including 45 odorant-binding proteins (OBPs, nine were new), 23 chemosensory proteins (CSPs), 60 odorant receptors (ORs, three were new), and 30 gustatory receptors (GRs, three were new), a number higher than those reported by previous transcriptome studies. Subsequently, we constructed phylogenetic trees based on these genes in moths and analyzed the dynamic expression of various genes in head capsules across larval instars using quantitative real-time polymerase chain reaction. Nine genes-SlitOBP8, SlitOBP9, SlitOBP25, SlitCSP1, SlitCSP7, SlitCSP18, SlitOR34, SlitGR240, and SlitGR242-were highly expressed in the heads of 3- to 5-day-old S. litura larvae. The genes differentially expressed in olfactory organs during larval development might play crucial roles in the chemosensory system of S. litura larvae. Our findings substantially expand the gene inventory for S. litura and present potential target genes for further studies on larval feeding in S. litura.

The Draft Genome of Yellow Stem Borer, an Agriculturally Important Pest, Provides Molecular Insights into Its Biology, Development and Specificity Towards Rice for Infestation

Simple Summary

Yellow stem borer (YSB), is the most destructive and widely occurring pest that attacks rice throughout the growing season. Rice (Oryza sativa L.) is a major staple cereal worldwide, providing essential caloric requirements for more than half of the world’s population. Annual losses to rice borers are approximately 5–10%, but losses in individual fields may reach up to 50–60%. The use of traditional pest management strategies in controlling YSB is somewhat challenging due to its unique internal feeding habit. Genome sequence information of economically important crop pests is important for designing or developing pest-resistant rice varieties. In an approach to achieve this, we present our first-ever study on the draft genome sequence of YSB. The information provided from our current study might be useful in developing genome-based approaches for the management of pest species.

Abstract

Yellow stem borer (YSB), Scirpophaga incertulas (Walker) (Lepidoptera: Crambidae), a major monophagous insect pest of rice, causes significant yield losses. The rice–YSB interaction is very dynamic, making it difficult for management. The development of resistant lines has been unsuccessful as there are no effective resistant sources in the germplasm. Genome information is necessary for a better understanding of interaction with rice in terms of its recognition, response, and infestation mechanism. The draft genome of YSB is predicted to have 46,057 genes with an estimated size of 308 Mb, being correlated with the flow cytometry analysis. The existence of complex metabolic mechanisms and genes related to specific behavior was identified, being conditioned by a higher level of regulation. We deciphered the possible visual, olfactory, and gustatory mechanisms responsible for its evolution as a monophagous pest. Comparative genomic analysis revealed that YSB is unique in the way it has evolved. The obvious presence of high-immunity-related genes, well-developed RNAi machinery, and diverse effectors provides a means for developing genomic tools for its management. The identified 21,696 SSR markers can be utilized for diversity analysis of populations across the rice-growing regions. We present the first draft genome of YSB. The information emanated paves a way for biologists to design novel pest management strategies as well as for the industry to design new classes of safer and specific insecticide molecules.

Characterization of sensory neuron membrane proteins (SNMPs) in cotton bollworm Helicoverpa armigera (Lepidoptera: Noctuidae)

Sensory neuron membrane proteins (SNMPs) play a critical role in insect chemosensory system. Previously, three SNMPs were identified, characterized and functionally investigated in a lepidopteran model insect, Bombyx mori. However, whether these results are consistent across other lepidopteran species are unknown. Here genome and transcriptome data analysis, expression profiling, quantitative real-time PCR (qRT-PCR) and the yeast hybridization system were utilized to examine snmp genes of Helicoverpa armigera, one of the most destructive lepidopteran pests in cropping areas. In silico expression and qRT-PCR analyses showed that, just as the B. mori snmp genes, H. armigera snmp1 (Harmsnmp1) is specifically expressed in adult antennae. Harmsnmp2 is broadly expressed in multiple tissues including adult antennae, tarsi, larval antennae and mouthparts. Harmsnmp3 is specifically expressed in larval midguts. Further RNAseq analysis suggested that the expression levels of Harmsnmp2 and Harmsnmp3 differed significantly depending on the plant species on which the larvae fed, indicating they may be involved in plant-feeding behaviours. Yeast hybridization results revealed a protein-protein interaction between HarmSNMP1 and the sex pheromone receptor, HarmOR13. This study demonstrated that SNMPs may share same functions and mechanisms in different lepidopteran species, which improved our understanding of insect snmp genes and their functions in lepidopterans.

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.

A moth odorant receptor highly expressed in the ovipositor is involved in detecting host-plant volatiles

Antennae are often considered to be the nostrils of insects. Here, we sequenced the transcriptome of the pheromone gland-ovipositor complex of Helicoverpa assulta and discovered that an odorant receptor (OR) gene, HassOR31, had much higher expression in the ovipositor than in antennae or other tissues. To determine whether the ovipositor was involved in odorant detection, we co-expressed HassOR31 and its co-receptor, HassORco, in a Xenopus oocyte model system, and demonstrated that the OR was responsive to 12 plant odorants, especially Z-3-hexenyl butyrate. These odorants elicited electrophysiological responses of some sensilla in the ovipositor, and HassOR31 and HassORco were co-expressed within ovipositor sensilla. Two oviposition preference experiments showed that female moths lacking antennae still preferentially selected oviposition sites containing plant volatiles. We suggest that the expression of HassOR31 in the ovipositor of H. assulta helps females to determine precise egg-laying sites in host plants.

When most insects reproduce they lay eggs that hatch into juveniles known as larvae. To provide good sources of food for the larvae, the adult insects have to carefully select where to lay the eggs. Host plants produce specific sets of chemicals known as odorants that the adult insects are able to smell using proteins called odorant receptors.

It is generally thought that odorant receptors in the antennae on the head are responsible for guiding adult insects to good egg-laying sites. However, recent studies have reported that odorant receptors are also present in the egg-laying organs of several different species of moth. It remains unclear what role these odorant receptors may play in egg-laying.

The oriental tobacco budworm (Helicoverpa assulta) is considered a serious pest in agriculture. The adult moths lay their eggs on a narrow range of plants in the nightshade family including tobacco and hot pepper. Li et al. have now investigated the odorant receptors of H. assulta and found that one gene for an odorant receptor called HassOR31 was expressed much more in the egg-laying organs of the moths than in the antennae. Further experiments showed that this receptor was tuned to respond to 12 odorants that also stimulated responses in the egg-laying organ of H. assulta. Together these findings suggest that this odorant receptor in the egg-laying organ helps the moths find suitable host plants to lay their eggs on.

The work of Li et al. may help us understand how H. assulta evolved to lay its eggs on specific members of the nightshade family and lead to new methods of controlling this pest. An insect’s sense of smell guides many other behaviors including finding food, mates and avoiding enemies. Therefore, these findings may inspire researchers to investigate whether odorant receptors in the antennae or other organs guide these behaviors.

An Orphan Pheromone Receptor Affects the Mating Behavior of Helicoverpa armigera

The Lepidoptera is the second largest insect order, which has the most extensive knowledge of sex pheromones and mechanisms of pheromone communication since the identification of the first insect pheromone in Bombyx mori. In the past 15 years, pheromone receptors have been identified and functionally characterized in many moth species. HarmOR14 is a typical pheromone receptor of Helicoverpa armigera which showed no response to the tested pheromones in Xenopus oocyte expression system, but its orthologous gene in Heliothis virescens, HvirOR14 could be activated by pheromones in the same expression system. To assess the possible functions of OR14 in vivo, in this study, we knocked out this gene using CRISPR/Cas9 system and compared the mating behaviors and EAG response to pheromones between the wild type and mutant strains. Our results showed that OR14 mutants did not affect the mating rate or the EAG response to pheromones but could prolong the mating duration and change the mating time in undefined manners, which extends our understanding to this kind of pheromone receptors.

Functional characterization of pheromone receptors in the moth Athetis dissimilis (Lepidoptera: Noctuidae)

Sex pheromones are crucial for communication between females and males in moths, and pheromone receptors (PRs) play a key role in peripheral coding of sex pheromones. During the last decade, many PR candidates have been identified based on transcriptome sequencing and bioinformatic analysis, but their detailed functions remain mostly unknown. Here, focusing on four PR candidates of Athetis dissimilis (AdisOR1, AdisOR6, AdisOR11 and AdisOR14) identified in a previous study, we first cloned the full-length cDNAs and determined the tissue expression profiles by quantitative real-time PCR (qPCR). The results revealed that expression of three of these genes were male antennae-specific, while AdisOR11 was similar in expression between male and female antennae. Furthermore, the expression level of AdisOR1 was much higher than those of the other three genes. Then, functional analysis was conducted using Xenopus oocyte system. AdisOR1 responded strongly to the sex pheromone component Z9-14:OH and the potential pheromone component Z9,E12-14:OH, suggesting its important role in the sex pheromone perception; AdisOR14 showed specificity for Z9,E12-14:OH; while AdisOR6 and AdisOR11 did not respond to any of the pheromone components and analogs tested. Taken together, this study contributes to elucidate the molecular mechanism of sex pheromone reception and provides potential targets for development of OR based pest control techniques in A. dissimilis.

Identification of candidate chemosensory genes in Mythimna separata by transcriptomic analysis

Background

The oriental armyworm, Mythimna separata, is an economically important and common Lepidopteran pest of cereal crops. Chemoreception plays a key role in insect life, such as foraging, oviposition site selection, and mating partners. To better understand the chemosensory mechanisms in M. separata, transcriptomic analysis of antennae, labial palps, and proboscises were conducted using next-generation sequencing technology to identify members of the major chemosensory related genes.

Results

In this study, 62 putative odorant receptors (OR), 20 ionotropic receptors (IR), 16 gustatory receptors (GR), 38 odorant binding proteins (OBP), 26 chemosensory proteins (CSP), and 2 sensory neuron membrane proteins (SNMP) were identified in M. separata by bioinformatics analysis. Phylogenetic analysis of these candidate proteins was performed. Differentially expressed genes (DEGs) analysis was used to determine the expressions of all candidate chemosensory genes and then the expression profiles of the three families of receptor genes were confirmed by real-time quantitative RT-PCR (qPCR).

Conclusions

The important genes for chemoreception have now been identified in M. separata. This study will provide valuable information for further functional studies of chemoreception mechanisms in this important agricultural pest.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4898-0) contains supplementary material, which is available to authorized users.

Identification of Chemosensory Genes Based on the Transcriptomic Analysis of Six Different Chemosensory Organs in Spodoptera exigua

Insects have a complex chemosensory system that accurately perceives external chemicals and plays a pivotal role in many insect life activities. Thus, the study of the chemosensory mechanism has become an important research topic in entomology. Spodoptera exigua Hübner (Lepidoptera: Noctuidae) is a major agricultural polyphagous pest that causes significant agricultural economic losses worldwide. However, except for a few genes that have been discovered, its olfactory and gustatory mechanisms remain uncertain. In the present study, we acquired 144,479 unigenes of S. exigua by assembling 65.81 giga base reads from 6 chemosensory organs (female and male antennae, female and male proboscises, and female and male labial palps), and identified many differentially expressed genes in the gustatory and olfactory organs. Analysis of the transcriptome data obtained 159 putative chemosensory genes, including 24 odorant binding proteins (OBPs; 3 were new), 19 chemosensory proteins (4 were new), 64 odorant receptors (57 were new), 22 ionotropic receptors (16 were new), and 30 new gustatory receptors. Phylogenetic analyses of all genes and SexiGRs expression patterns using quantitative real-time polymerase chain reactions were investigated. Our results found that several of these genes had differential expression features in the olfactory organs compared to the gustatory organs that might play crucial roles in the chemosensory system of S. exigua, and could be utilized as targets for future functional studies to assist in the interpretation of the molecular mechanism of the system. They could also be used for developing novel behavioral disturbance agents to control the population of the moths in the future.

Identification and Expression Patterns of Anoplophora chinensis (Forster) Chemosensory Receptor Genes from the Antennal Transcriptome

The citrus long-horned beetle (CLB), Anoplophora chinensis (Forster) is a destructive native pest in China. Chemosensory receptors including odorant receptors (ORs), gustatory receptors (GRs), and ionotropic receptors (IRs) function to interface the insect with its chemical environment. In the current study, we assembled the antennal transcriptome of A. chinensis by next-generation sequencing. We assembled 44,938 unigenes from 64,787,784 clean reads and annotated their putative gene functions based on gene ontology (GO) and Clusters of Orthologous Groups of proteins (COG). Overall, 74 putative receptor genes from chemosensory receptor gene families, including 53 ORs, 17 GRs, and 4 IRs were identified. Expression patterns of these receptors on the antennae, maxillary and labial palps, and remaining body segments of both male and female A. chinensis were performed using quantitative real time-PCR (RT-qPCR). The results revealed that 23 ORs, 6 GRs, and 1 IR showed male-biased expression profiles, suggesting that they may play a significant role in sensing female-produced sex pheromones; whereas 8 ORs, 5 GRs, and 1 IR showed female-biased expression profiles, indicating that these receptors may be involved in some female-specific behaviors such as oviposition site seeking. These results lay a solid foundation for deeply understanding CLB olfactory processing mechanisms. Moreover, by comparing our results with those from chemosensory receptor studies in other cerambycid species, several highly probable pheromone receptor candidates were highlighted, which may facilitate the identification of additional pheromone and/or host attractants in CLB.

Molecular characterization of sugar taste receptors in the cotton bollworm Helicoverpa armigera

Insects utilize sugars as their essential energy and nutrient sources; therefore, the sense of sugar detection plays a critical role in insect behaviours. Previously, using genomic and transcriptomic approaches, we identified eight putative sugar gustatory receptor (GR) genes from the cotton bollworm Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). Here, we further validated these annotated sugar receptor genes (HarmGr4–HarmGr8 and HarmGr10–HarmGr12) and found HarmGr10 may be a pseudogene carrying a stop codon in the open reading frame. Sequence alignment revealed H. armigera sugar GR sequences are conserved at C-terminus and phylogenetic analysis showed that insect sugar GRs have evolved in a family-specific manner. Interestingly, all eight H. armigera sugar GRs are localized in a tandem array on the same scaffold of the genome. In silico gene expression and reverse transcription (RT)-PCR analysis showed that HarmGr10 is specifically expressed in male adult testes while HarmGr11 is specifically expressed in female adult ovaries, suggesting H. armigera sugar GRs may be involved in reproduction-related functions. This study improves our knowledge on insect sugar receptors and gustatory systems.

Two single-point mutations shift the ligand selectivity of a pheromone receptor between two closely related moth species

Male moths possess highly sensitive and selective olfactory systems that detect sex pheromones produced by their females. Pheromone receptors (PRs) play a key role in this process. The PR HassOr14b is found to be tuned to (Z)-9-hexadecenal, the major sex-pheromone component, in Helicoverpa assulta. HassOr14b is co-localized with HassOr6 or HassOr16 in two olfactory sensory neurons within the same sensilla. As HarmOr14b, the ortholog of HassOr14b in the closely related species Helicoverpa armigera, is tuned to another chemical (Z)-9-tetradecenal, we study the amino acid residues that determine their ligand selectivity. Two amino acids located in the transmembrane domains F232I and T355I together determine the functional difference between the two orthologs. We conclude that species-specific changes in the tuning specificity of the PRs in the two Helicoverpa moth species could be achieved with just a few amino acid substitutions, which provides new insights into the evolution of closely related moth species.

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.

Candidate odorant binding proteins and chemosensory proteins in the larval chemosensory tissues of two closely related noctuidae moths, Helicoverpa armigera and H. assulta

In order to acquire enough nutrients and energy for further development, larvae need to invest a large portion of their sensory equipments to identify food sources. Yet, the molecular basis of odor-driven behavior in larvae has been poorly investigated. Information on olfactory genes, particularly odorant binding proteins (OBPs) and chemosensory proteins (CSPs) which are involved in the initial steps of olfaction is very scarce. In this study, we have identified 26 OBP and 21 CSP genes from the transcriptomes of Helicoverpa armigera larval antennae and mouthparts. A comparison with the 34 OBP and 18 CSP genes of the adult antenna, revealed four novel OBPs and seven novel CSPs. Similarly, 27 OBPs (six novel OBPs) and 20 CSPs (6 novel CSPs) were identified in the transcriptomes of Helicoverpa assulta larval antennae and mouthparts. Tissue-specific profiles of these soluble proteins in H. armigera showed that 6 OBP and 4 CSP genes are larval tissue-specific, 15 OBPs and 13 CSPs are expressed in both larvae and adult, while the rest are adult- specific. Our data provide useful information for functional studies of genes involved in larval foraging.

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.

Molecular Characterization and Sex Distribution of Chemosensory Receptor Gene Family Based on Transcriptome Analysis of Scaeva pyrastri

Chemosensory receptors play key roles in insect behavior. Thus, genes encoding these receptors have great potential for use in integrated pest management. The hover fly Scaeva pyrastri (L.) is an important pollinating insect and a natural enemy of aphids, mainly distributed in the Palearctic and Nearctic regions. However, a systematic identification of their chemosensory receptor genes in the antennae has not been reported. In the present study, we assembled the antennal transcriptome of S. pyrastri by using Illumina sequencing technology. Analysis of the transcriptome data identified 60 candidate chemosensory genes, including 38 for odorant receptors (ORs), 16 for ionotropic receptors (IRs), and 6 for gustatory receptors (GRs). The numbers are similar to those of other Diptera species, suggesting that we were able to successfully identify S. pyrastri chemosensory genes. We analyzed the expression patterns of all genes by using reverse transcriptase PCR (RT-PCR), and found that some genes exhibited sex-biased or sex-specific expression. These candidate chemosensory genes and their tissue expression profiles provide information for further studies aimed at fully understanding the molecular basis behind chemoreception-related behaviors in S. pyrastri.

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.

Olfactory perception and behavioral effects of sex pheromone gland components in Helicoverpa armigera and Helicoverpa assulta

Two sympatric species Helicoverpa armigera and Helicoverpa assulta use (Z)-11-hexadecenal and (Z)-9-hexadecenal as sex pheromone components in reverse ratio. They also share several other pheromone gland components (PGCs). We present a comparative study on the olfactory coding mechanism and behavioral effects of these additional PGCs in pheromone communication of the two species using single sensillum recording, in situ hybridization, calcium imaging, and wind tunnel. We classify antennal sensilla types A, B and C into A, B1, B2, C1, C2 and C3 based on the response profiles, and identify the glomeruli responsible for antagonist detection in both species. The abundance of these sensilla types when compared with the number of OSNs expressing each of six pheromone receptors suggests that HarmOR13 and HassOR13 are expressed in OSNs housed within A type sensilla, HarmOR14b within B and C type sensilla, while HassOR6 and HassOR16 within some of C type sensilla. We find that for H. armigera, (Z)-11-hexadecenol and (Z)-11-hexadecenyl acetate act as behavioral antagonists. For H. assulta, instead, (Z)-11-hexadecenyl acetate acts as an agonist, while (Z)-9-hexadecenol, (Z)-11-hexadecenol and (Z)-9-hexadecenyl acetate are antagonists. The results provide an overall picture of intra- and interspecific olfactory and behavioral responses to all PGCs in two sister species.

Identification of Putative Chemosensory Receptor Genes from the Athetis dissimilis Antennal Transcriptome

Olfaction plays a crucial role in insect population survival and reproduction. Identification of the genes associated with the olfactory system, without the doubt will promote studying the insect chemical communication system. In this study, RNA-seq technology was used to sequence the antennae transcriptome of Athetis dissimilis, an emerging crop pest in China with limited genomic information, with the purpose of identifying the gene set involved in olfactory recognition. Analysis of the transcriptome of female and male antennae generated 13.74 Gb clean reads in total from which 98,001 unigenes were assembled, and 25,930 unigenes were annotated. Total of 60 olfactory receptors (ORs), 18 gustatory receptors (GRs), and 12 ionotropic receptors (IRs) were identified by Blast and sequence similarity analyzes. One obligated olfactory receptor co-receptor (Orco) and four conserved sex pheromone receptors (PRs) were annotated in 60 ORs. Among the putative GRs, five genes (AdisGR1, 6, 7, 8 and 94) clustered in the sugar receptor family, and two genes (AdisGR3 and 93) involved in CO₂ detection were identified. Finally, AdisIR8a.1 and AdisIR8a.2 co-receptors were identified in the group of candidate IRs. Furthermore, expression levels of these chemosensory receptor genes in female and male antennae were analyzed by mapping the Illumina reads.

Candidate chemosensory genes identified in Colaphellus bowringi by antennal transcriptome analysis

BACKGROUND

Since chemosensory genes play key roles in insect behaviour, they can potentially be used as new targets for pest control. The cabbage beetle, Colaphellus bowringi, is a serious insect pest of cruciferous vegetables in China and other Asian countries. However, a systematic identification of the chemosensory genes expressed in the antennae has not been reported.

RESULTS

We assembled the antennal transcriptome of C. bowringi by using Illumina sequencing technology and identified 104 candidate chemosensory genes by analyzing transcriptomic data, which included transcripts encoding 26 odorant-binding proteins (OBPs), 12 chemosensory proteins (CSPs), four sensory neuron membrane proteins (SNMPs), 43 odorant receptors (ORs), nine ionotropic receptors (IRs), and ten gustatory receptors (GRs). The data obtained are similar to those found in other coleopteran species, suggesting that our approach successfully identified the chemosensory genes of C. bowringi. The expression patterns of 43 OR genes, some of which were predominately found in the antenna or associated with sex-biased expression, were analyzed using quantitative real time RT-PCR (qPCR).

CONCLUSIONS

Our study revealed that a large number of chemosensory genes are expressed in C. bowringi. These candidate chemosensory genes and their expression profiles in various tissues provide further information on understanding their function in C. bowringi as well as other insects, and identifying potential targets to disrupt the odorant system in C. bowringi so that new methods for pest management can be developed.

A reference gene set for chemosensory receptor genes of Manduca sexta

The order of Lepidoptera has historically been crucial for chemosensory research, with many important advances coming from the analysis of species like Bombyx mori or the tobacco hornworm, Manduca sexta. Specifically M. sexta has long been a major model species in the field, especially regarding the importance of olfaction in an ecological context, mainly the interaction with its host plants. In recent years transcriptomic data has led to the discovery of members of all major chemosensory receptor families in the species, but the data was fragmentary and incomplete. Here we present the analysis of the newly available high-quality genome data for the species, supplemented by additional transcriptome data to generate a high quality reference gene set for the three major chemosensory receptor gene families, the gustatory (GR), olfactory (OR) and antennal ionotropic receptors (IR). Coupled with gene expression analysis our approach allows association of specific receptor types and behaviors, like pheromone and host detection. The dataset will provide valuable support for future analysis of these essential chemosensory modalities in this species and in Lepidoptera in general.

Identification and Characterization of Candidate Chemosensory Gene Families from Spodoptera exigua Developmental Transcriptomes

Insect chemosensory genes have been considered as potential molecular targets to develop alternative strategies for pest control. However, in Spodoptera exigua, a seriously polyphagous agricultural pest, only a small part of such genes have been identified and characterized to date. Here, using a bioinformatics screen a total of 79 chemosensory genes were identified from a public transcriptomic data of different developmental stages (eggs, 1st to 5th instar larvae, pupae, female and male adults), including 34 odorant binding proteins (OBPs), 20 chemosensory proteins (CSPs), 22 chemosensory receptors (10 odorant receptors (ORs), six gustatory receptors (GRs) and six ionotropic receptors (IRs)) and three sensory neuron membrane proteins (SNMPs). Notably, a new group of lepidopteran SNMPs (SNMP3 group) was found for the first time in S. exigua, and confirmed in four other moth species. Further, reverse transcription PCR (RT-PCR) and quantitative real time PCR (qPCR) were employed respectively to validate the sequences and determine the expression patterns of 69 identified chemosensory genes regarding to sexes, tissues and stages. Results showed that 67 of these genes could be detected and reconstructed in at least one tissue tested. Further, 60 chemosensory genes were expressed in adult antennae and 52 in larval heads with the antennae, whereas over half of the genes were also detected in non-olfactory tissues like egg and thorax. Particularly, S. exigua OBP2 showed a predominantly larval head-biased expression, and functional studies further indicated its potentially olfactory roles in guiding food searching of larvae. This work suggests functional diversities of S. exigua chemosensory genes and could greatly facilitate the understanding of olfactory system in S. exigua and other lepidopteran species.

Targeted disruption of a single sex pheromone receptor gene completely abolishes in vivo pheromone response in the silkmoth

Male moths use species-specific sex pheromones to identify and orientate toward conspecific females. Odorant receptors (ORs) for sex pheromone substances have been identified as sex pheromone receptors in various moth species. However, direct in vivo evidence linking the functional role of these ORs with behavioural responses is lacking. In the silkmoth, Bombyx mori, female moths emit two sex pheromone components, bombykol and bombykal, but only bombykol elicits sexual behaviour in male moths. A sex pheromone receptor BmOR1 is specifically tuned to bombykol and is expressed in specialized olfactory receptor neurons (ORNs) in the pheromone sensitive long sensilla trichodea of male silkmoth antennae. Here, we show that disruption of the BmOR1 gene, mediated by transcription activator-like effector nucleases (TALENs), completely removes ORN sensitivity to bombykol and corresponding pheromone-source searching behaviour in male moths. Furthermore, transgenic rescue of BmOR1 restored normal behavioural responses to bombykol. Our results demonstrate that BmOR1 is required for the physiological and behavioural response to bombykol, demonstrating that it is the receptor that mediates sex pheromone responses in male silkmoths. This study provides the first direct evidence that a member of the sex pheromone receptor family in moth species mediates conspecific sex pheromone information for sexual behaviour.