Transcriptome and expression profiling analysis link patterns of gene expression to antennal responses in Spodoptera litura

Novel Sex-Specific Genes and Diverse Interspecific Expression in the Antennal Transcriptomes of Ithomiine Butterflies

The olfactory sense is crucial for organisms, facilitating environmental recognition and interindividual communication. Ithomiini butterflies exemplify this importance not only because they rely strongly on olfactory cues for both inter- and intra-sexual behaviors, but also because they show convergent evolution of specialized structures within the antennal lobe, called macroglomerular complexes (MGCs). These structures, widely absent in butterflies, are present in moths where they enable heightened sensitivity to, and integration of, information from various types of pheromones. In this study, we investigate chemosensory evolution across six Ithomiini species and identify possible links between expression profiles and neuroanatomical. To enable this, we sequenced four new high-quality genome assemblies and six sex-specific antennal transcriptomes for three of these species with different MGC morphologies. With extensive genomic analyses, we found that the expression of antennal transcriptomes across species exhibit profound divergence, and identified highly expressed ORs, which we hypothesize may be associated to MGCs, as highly expressed ORs are absent in Methona, an Ithomiini lineage which also lacks MGCs. More broadly, we show how antennal sexual dimorphism is prevalent in both chemosensory genes and non-chemosensory genes, with possible relevance for behavior. As an example, we show how lipid-related genes exhibit consistent sexual dimorphism, potentially linked to lipid transport or host selection. In this study, we investigate the antennal chemosensory adaptations, suggesting a link between genetic diversity, ecological specialization, and sensory perception with the convergent evolution of MCGs. Insights into chemosensory gene evolution, expression patterns, and potential functional implications enhance our knowledge of sensory adaptations and sexual dimorphisms in butterflies, laying the foundation for future investigations into the genetic drivers of insect behavior, adaptation, and speciation.

Comparative transcriptome and proteome reveal the unique genes and proteins of female parasitic wasps, Lysiphlebia japonica Ashmead

BACKGROUND

Lysiphlebia japonica Ashmead (Hymenoptera, Braconidae) is an endophagous parasitoid wasp and its host, Aphis gossypii Glover (Hemiptera, Aphididae) is a major cotton pest. L. japonica affects the growth and fatty acid metabolism of cotton aphids after parasitization and has been widely used as a biocontrol agent. However, there are currently few reports about the molecular characteristics of L. japonica, especially the differences between male and female.

RESULTS

In this study, using transcriptome and proteome analysis of the abdomen of female and male parasitic wasps, respectively, we obtained a total of 27,169 DEGs and 1,194 DEPs, then a total of 909 positively correlated high-expression proteins and genes were obtained by combined omics analysis. Subsequently, 20 differentially expressed abdomen specific proteins were selected for validation by RT-qPCR and Multiple Reaction Monitoring (MRM) protein verification. The result of RT-qPCR demonstrated that all 20 genes were highly expressed in the abdomen of females, and five target proteins with unique peptide fragments and identification profiles were identified by MRM, which were venom protease, tropomyosin, lipase member I, venom serine carboxypeptidase and calreticulin, respectively.

CONCLUSION

Overall, these results provided molecular resources for the differences between males and females in L. japonica and the screened 20 abdomen specific proteins were verified to demonstrate the validity of the data, which offered important molecular data resources for further studies on the related functional genes of parasitic wasps and the mechanism of parasitoids regulating the growth of aphids. © 2023 Society of Chemical Industry.

Transcriptomic Identification and Expression Profile Analysis of Odorant-Degrading Enzymes from the Asian Corn Borer Moth, Ostrinia furnacalis

The Asian corn borer moth Ostrinia furnacalis is an important lepidopteran pest of maize in Asia. Odorant-degrading enzymes (ODEs), including carboxylesterases (CCEs), glutathione S-transferases (GSTs), cytochrome P450s (CYPs), UDP-glycosyltransferases (UGTs), and aldehyde oxidases (AOXs), are responsible for rapid inactivation of odorant signals in the insect antennae. In this study, we performed a transcriptome assembly for the antennae of O. furnacalis to identify putative ODE genes. Transcriptome sequencing revealed 35,056 unigenes, and 21,012 (59.94%) of these were annotated by searching against the reference sequences in the NCBI non-redundant (NR) protein database. For functional classification, these unigenes were subjected to Gene Ontology (GO), Eukaryotic Orthologous Groups (KOG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations. We identified 79 genes encoding putative ODEs: 19 CCEs, 17 GSTs, 24 CYPs, 13 UGTs, and 6 AOXs. BLASTX best hit results indicated that these genes shared quite high amino acid identities with their respective orthologs from other lepidopteran species. Reverse transcription-quantitative PCR showed that OfurCCE2, OfurCCE5, and OfurCCE18 were enriched in male antennae, while OfurCCE7 and OfurCCE10 were enriched in female antennae. OfurCCE14 and OfurCCE15 were expressed at near-equal amounts in the antennae of both sexes. Our findings establish a solid foundation for future studies aimed at understanding the olfactory functions of these genes in O. furnacalis.

Analysis of the Antennal Transcriptome and Identification of Tissue-specific Expression of Olfactory-related Genes in Micromelalopha troglodyta (Lepidoptera: Notodontidae)

Micromelalopha troglodyta (Graeser) has been one of the most serious pests on poplars in China. We used Illumina HiSeq 2000 sequencing to construct an antennal transcriptome and identify olfactory-related genes. In total, 142 transcripts were identified, including 74 odorant receptors (ORs), 32 odorant-binding proteins (OBPs), 13 chemosensory proteins (CSPs), 20 ionotropic receptors (IRs), and 3 sensory neuron membrane proteins (SNMPs). The genetic relationships were obtained by the phylogenetic tree, and the tissue-specific expression of important olfactory-related genes was determined by quantitative real-time PCR (qRT-PCR). The results showed that most of these genes are abundantly expressed in the antennae and head. In most insects, olfaction plays a key role in foraging, host localization, and searching for mates. Our research lays the foundation for future research on the molecular mechanism of the olfactory system in M. troglodyta. In addition, this study provides a theoretical basis for exploring the relationship between M. troglodyta and their host plants, and for the biological control of M. troglodyta using olfactory receptor as targets.

Insights Into Chemosensory Proteins From Non-Model Insects: Advances and Perspectives in the Context of Pest Management

Nowadays, insect chemosensation represents a key aspect of integrated pest management in the Anthropocene epoch. Olfaction-related proteins have been the focus of studies due to their function in vital processes, such ashost finding and reproduction behavior. Hence, most research has been based on the study of model insects, namely Drosophila melanogaster, Bombyx mori or Tribolium castaneum. Over the passage of time and the advance of new molecular techniques, insects considered non-models have been studied, contributing greatly to the knowledge of insect olfactory systems and enhanced pest control methods. In this review, a reference point for non-model insects is proposed and the concept of model and non-model insects is discussed. Likewise, it summarizes and discusses the progress and contribution in the olfaction field of both model and non-model insects considered pests in agriculture.

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.

Transcriptome Analysis and Characterization of Chemosensory Genes in the Forest Pest, Dioryctria abietella (Lepidoptera: Pyralidae)

In Lepidoptera, RNA sequencing has become a useful tool in identifying chemosensory genes from antennal transcriptomes, but little attention is paid to non-antennal tissues. Though the antennae are primarily responsible for olfaction, studies have found that a certain number of chemosensory genes are exclusively or highly expressed in the non-antennal tissues, such as proboscises, legs and abdomens. In this study, we report a global transcriptome of 16 tissues from Dioryctria abietella, including chemosensory and non-chemosensory tissues. Through Illumina sequencing, totally 952,658,466 clean reads were generated, summing to 142.90 gigabases of data. Based on the transcriptome, 235 chemosensory-related genes were identified, comprising 42 odorant binding proteins (OBPs), 23 chemosensory proteins (CSPs), 75 odorant receptors (ORs), 62 gustatory receptors (GRs), 30 ionotropic receptors (IRs), and 3 sensory neuron membrane proteins (SNMPs). Compared to a previous study in this species, 140 novel genes were found. A transcriptome-wide analysis combined with PCR results revealed that except for GRs, the majority of other five chemosensory gene families in Lepidoptera were expressed in the antennae, including 160 chemosensory genes in D. abietella. Using phylogenetic and expression profiling analyses, members of the six chemosensory gene repertoires were characterized, in which 11 DabiORs were candidates for detecting female sex pheromones in D. abietella, and DabiOR23 may be involved in the sensing of plant-derived phenylacetaldehyde. Intriguingly, more than half of the genes were detected in the proboscises, and one fourth of the genes were found to have the expression in the legs. Our study not only greatly extends and improves the description of chemosensory genes in D. abietella, but also identifies potential molecular targets involved in olfaction, gustation and non-chemosensory functions for control of this pest.

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.

Combined transcriptomic, proteomic and genomic analysis identifies reproductive-related proteins and potential modulators of female behaviors in Spodoptera litura

The common cutworm, Spodoptera litura, is a polyandrous moth with high reproductive ability. Sexual reproduction is a unique strategy for survival and reproduction of population in this species. However, to date available information about its reproductive genes is rare. Here, we combined transcriptomics, genomics and proteomics approaches to characterize reproductive-related proteins in S. litura. Illumina sequencing in parallel with the reference genome led to the yields of 12,161 reproductive genes, representing 47.83% of genes annotated in the genome. Further, 524 genes of 19 specific gene families annotated in the genome were detected in reproductive tissues of both sexes, some of which exhibited sex-biased and/or tissue-enriched expression. Of these, manual efforts together with the transcriptome analyses re-annotated 54 odorant binding proteins (OBPs) and 23 chemosensory proteins (CSPs) with an increase of 18 OBPs and one CSP compared to those previously annotated in the genome. Interestingly, at least 35 OBPs and 22 CSPs were transcribed in at least one reproductive tissue, suggestive of their involvement in reproduction. Further proteomic analysis revealed 2381 common proteins between virgin and mated female reproductive systems, 79 of which were differentially expressed. More importantly, 74 proteins exclusive to mated females were identified as transferred relatives, coupled with their specific or high expression in male reproductive systems. Of the transferred proteins, several conserved protein classes across insects were observed including OBPs, serpins, trypsins and juvenile hormone-binding proteins. Our current study has extensively surveyed reproductive genes in S. litura with an emphasis on the roles of OBPs and CSPs in reproduction, and identifies potentially transferred proteins serving as modulators of female post-mating behaviors.

Identification and analysis of olfactory genes in Dioryctria abietella based on the antennal transcriptome

The coneworm Dioryctria abietella (Lepidoptera: Pyralidae) is an economy devastating pest that infests many valuable conifer species in the Holarctic regions, such as Pinus koraiensis Siebold and Zucc. The chemosensory system plays a crucial role in the mating, foraging, and ovipositing of this pest, and therefore it is desirable to identify chemosensory molecules for pest control. However, little is known at molecular level about the olfactory mechanisms in D. abietella. In the present study, we first established antennal transcriptomes of D. abietella and identified 132 putative chemosensory genes, including 15 odorant-binding proteins, 18 chemosensory proteins, 65 odorant receptors, 5 sensory neuron membrane proteins, 24 ionotropic receptors, and 5 gustatory receptors. In addition, phylogenetic trees were constructed for chemosensory genes to investigate the orthologs between D. abietella and other species of insects. Furthermore, we also compared the patterns of motifs between OBPs and CSPs using MEME. Additionally, we observed that most of DabiOBPs and DabiCSPs had the antenna-biased expression by quantitative real-time PCR (RT-qPCR), and there was a higher expression of DabiPBP1 and DabiPBP2 in male antennae than in female antennae. The binding sites of DabiPBPs (DabiPBP1, DabiPBP2) and DabiPRs (DabiOR19, DabiOR31) to the sex pheromone were predicted well by three-dimensional docking structure modelling and molecular docking. Our finding supplied a foundation for further research on the binding process of OBPs or CSPs and sensing process of ORs, SNMPs, IRs or GRs in D. abietella.

Sexual-biased gene expression of olfactory-related genes in the antennae of Conogethes pinicolalis (Lepidoptera: Crambidae)

BACKGROUND

Conogethes pinicolalis (Lepidoptera: Crambidae), is similar to Conogethes punctiferalis (yellow peach moth) and its host plant is gymnosperms, especially for masson pine. So far, less literature was reported on this pest. In the present study, we sequenced and characterized the antennal transcriptomes of male and female C. pinicolalis for the first time.

RESULTS

Totally, 26 odorant-binding protein (OBP) genes, 19 chemosensory protein (CSP) genes, 55 odorant receptor (OR) genes and 20 ionotropic receptor (IR) genes were identified from the C. pinicolalis antennae transcriptome and amino sequences were annotated against homologs of C. punctiferalis. The neighbor-joining tree indicated that the amino acid sequence of olfactory related genes is highly homologous with C. punctiferalis. Furthermore, the reference genes were selected, and we recommended the phosphate dehydrogenase gene (GAPDH) or ribosomal protein 49 gene (RP49) to verify the target gene expression during larval development stages and RP49 or ribosomal protein L13 gene (RPL13) for adult tissues.

CONCLUSIONS

Our study provides a starting point on the molecular level characterization between C. pinicolalis and C. punctiferalis, which might be supportive for pest management studies in future.

Identification and phylogenetics of Spodoptera frugiperda chemosensory proteins based on antennal transcriptome data

Understanding the interaction between the insect olfactory system and the environment is crucial for fully explaining the molecular mechanisms underlying insect behavior, and providing new strategies for integrated pest management. Although there is good evidence that olfactory proteins play a vital role in mediating insect behaviors, the olfactory mechanism of insects remains poorly understood. We identified a total of 71 chemosensory genes; 25 odorant-binding proteins (OBPs), 27 odorant receptors (ORs), 8 ionotropic receptors (IRs), 8 chemosensory proteins (CSPs) and 3 sensory neuron membrane proteins (SNMPs), in the antennae of male and female fall armyworms, Spodoptera frugiperda, an invasive global pest that causes significant economic damage worldwide. We used differential gene expression (DGE) and fragments per kilobase per million fragments (FPKM) values to compare the transcript levels of candidate chemosensory genes, and qRT-PCR to compare the expression levels of the OR gene, in male and female antennae. The expression of candidate OR genes in male and female antennae was consistent with the DGE data, and the expression of the SfruCL4419.Contig1-All and SfruUnigene1070-All genes was sex-biased. These results not only provide new information on the olfactory mechanism of S. frugiperda, and insects in general, but also suggest new gene targets for pest control.

Antennal transcriptome analysis and expression profiles of putative chemosensory soluble proteins in Histia rhodope Cramer (Lepidoptera: Zygaenidae)

Histia rhodope Cramer (Lepidoptera: Zygaenidae) is one of the most destructive defoliators of landscape tree Bischofia polycarpa (Levl.) Airy Shaw in China stretching to other Southeast Asia regions. Olfactory genes, encoding proteins such as odorant carrier proteins believed to initiate olfactory signal transduction in insects, have been acknowledged to be novel targets for pest control. In this study, we established antennal transcriptome of H. rhodope and ultimately identified 19 odorant binding proteins (OBPs), 23 chemosensory proteins (CSPs) and 4 Niemann-Pick type C2 proteins (NPC2s). The 19 OBPs, 6 CSPs and 4 NPC2s were assessed to validate the differential expressions between sexes, and between olfactory and non-olfactory tissues. 8 OBPs and 2 CSPs exhibited male-biased antennae expression, while 6 OBPs, 2 CSPs and HrhoNPC2a exhibited female-biased antennae expression. Moreover, 17 OBPs, 4 CSPs and 2 NPC2s were predominantly expressed in the antennae compared with non-olfactory tissues. HrhoOBP1 and HrhoOBP8 were predominantly expressed in the antennae and heads, HrhoCSP8 and HrhoCSP14 were highly expressed in abdomens and legs, HrhoNPC2c was highly expressed in abdomens, while HrhoNPC2d was expressed in all tissues. Phylogenetic analysis revealed that most H. rhodope proteins were closely related to proteins from other moths. Moreover, compared with other nocturnal moths, acting as a diurnal moth, we found that H. rhodope may have lost a PBP gene. Our results provide important molecular information for further studies on olfactory mechanisms of H. rhodope.

Antennal transcriptome analysis and expression profiles of odorant binding proteins in Clostera restitura

Clostera restitura Walker (Lepidoptera: Notodontidae) is one of the most destructive defoliators of poplars in China. We constructed an antennal transcriptome using Illumina Hiseq 2500™ sequencing and characterized the expression profiles of odorant binding proteins for better understanding of the olfactory receptive system and the role of putative olfactory proteins in C. restitura. A total of 165 transcripts were identified, including 43 transcripts encoding putative odorant-binding proteins (OBPs), 13 chemosensory proteins (CSPs), 78 odorant receptors (ORs), 15 ionotropic receptors (IRs), 13 gustatory receptors (GRs), and 3 sensory neuron membrane proteins (SNMPs). Furthermore, we systematically analyzed expression patterns of eight OBPs from different tissues of both C. restitura sexes by using reverse transcription PCR and quantitative real time PCR (RT-qPCR). The expression level of CresGOBP2 in female antennae was approximately two times higher than in males, and two pheromone binding proteins PBPs (CresPBP1 and -PBP3) and three OBPs (CresOBP9, -10, and -16) were more highly enriched in male antennae than in female antennae. CresOBP10 showed a remarkably high expression in legs compared to other studied insects. Our results suggested that these proteins might play a key role in foraging, seeking mates, and host recognition in C. restitura. Our findings provided a foundation for future studies on the molecular mechanisms controlling the olfactory system in C. restitura and potential novel targets for pest control strategies.

Candidate olfactory genes identified in Heortia vitessoides (Lepidoptera: Crambidae) by antennal transcriptome analysis

Heortia vitessoides Moore is the most severe defoliating pest of Aquilaria sinensis (Lour.) Gilg (Thymelaeaceae) forests. Olfaction in insects is essential for host identification, mating, and oviposition, in which olfactory proteins, including odorant-binding proteins (OBPs), chemosensory proteins (CSPs), olfactory receptors (ORs), ionotropic receptors (IRs), and sensory neuron membrane proteins (SNMPs), are responsible for chemical signaling. Here, we determined the transcriptomes of male and female adult antennae of H. vitessoides. We assembled 52,383 unigenes and annotated their putative gene functions based on the gene ontology (GO), eukaryotic ortholog groups (KOG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. Overall, 61 olfactory-related transcripts, including nine OBPs, 10 CSPs, 28 ORs, 12 IRs, and two SNMPs, were identified. Expression patterns of OBPs and CSPs in the female antennae, male antennae, and legs were performed using reverse transcription quantitative PCR (RT-qPCR). The results revealed that HvitOBP1, HvitOBP6, and HvitGOBP1 were enriched in the female antennae, while HvitOBP2, HvitOBP3, HvitOBP5, HvitGOBP2, and HvitPBP1 were enriched in the male antennae. HvitOBP4 was expressed at nearly the same level in the antennae of both males and females. Four CSPs (HvitCSP3, HvitCSP5, HvitCSP7, and HvitCSP10) and two CSPs (HvitCSP1 and HvitCSP4) were expressed at higher levels in the female and male antennae, respectively. HvitCSP6 was expressed at higher levels both in the female antennae and legs. Three CSP genes (HvitCSP2, HvitCSP8, and HvitCSP9) were expressed at higher levels in the legs. These results provide a basis for further studies on the molecular olfactory mechanisms of H. vitessoides.

Effects of insecticides chlorpyrifos, emamectin benzoate and fipronil on Spodoptera litura might be mediated by OBPs and CSPs

Spodoptera litura is a widespread polyphagous insect pest that can develop resistance and cross-resistance to insecticides, making it difficult to control. Insecticide exposure has previously been linked with induction of specific olfactory-related proteins, including some chemosensory proteins (CSPs) and odorant-binding proteins (OPBs), which may disrupt detection of environmental factors and reduce fitness. However, functional evidence supporting insecticide and OBPs/CSPs mediation remains unknown. Here we fed male S. litura moths with sucrose water containing one of three insecticides, chlorpyrifos, emamectin benzoate or fipronil, and used real-time quantitative polymerase chain reaction and RNAi to investigate OBPs and CSPs expression and their correlations with survival. Chlorpyrifos and emamectin benzoate increased expression of 78% of OBPs, plus 63 and 56% of CSP genes, respectively, indicating a major impact on these gene families. RNAi knockdown of SlituCSP18, followed by feeding with chlorpyrifos or fipronil, decreased survival rates of male moths significantly compared with controls. Survival rate also decreased significantly with the downregulation of SlituOBP9 followed by feeding with chlorpyrifos. Thus, although these three insecticides had different effects on OBP and CSP gene expression, we hypothesize that SlituOBPs and SlituCSPs might mediate their effects by increasing their expression levels to improve survival. Moreover, the differential response of S. litura male moths to the three insecticides indicated the potential specificity of chlorpyrifos affect SlituCSP18 and SlituOBP9 expression.

Odorant Receptors and Odorant-Binding Proteins as Insect Pest Control Targets: A Comparative Analysis

Recently, two alternative targets in insect periphery nerve system have been explored for environmentally-friendly approaches in insect pest management, namely odorant-binding proteins (OBPs) and odorant receptors (ORs). Located in insect antennae, OBPs are thought to be involved in the transport of odorants to ORs for the specific signal transduction of behaviorally active odorants. There is rich information on OBP binding affinity and molecular docking to bioactive compounds as well as ample 3D crystal structures due to feasible production of recombinant proteins. Although these provide excellent opportunities for them to be considered as pest control targets and a tool to design pest control agents, the debates on their binding specificity represent an obstacle. On the other hand, ORs have recently been functionally characterized with increasing evidence for their specificity, sensitivity and functional roles in pest behaviors. However, a major barrier to use ORs for semiochemical discovery is the lack of 3D crystal structures. Thus, OBPs and ORs have not been analyzed comparatively together so far for their feasibility as pest control targets. Here, we summarize the state of OBPs and ORs research in terms of its application in insect pest management. We discuss the suitability of both proteins as pest control targets and their selection toward the discovery of new potent semiochemicals. We argue that both proteins represent promising targets for pest control and can be used to identify new super-ligands likely present in nature and with reduced risk of resistance development than insect pesticides currently used in agriculture. We discuss that with the massive identification of OBPs through RNA-seq and improved binding affinity measurements, these proteins could be reconsidered as suitable targets for semiochemical discovery.

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

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

Key genes of the sex pheromone biosynthesis pathway in female moths are required for pheromone quality and possibly mediate olfactory plasticity in conspecific male moths in Spodoptera litura

Ninety sex pheromone biosynthesis genes in Spodoptera litura were identified in transcriptome data and were investigated and classified into the following five groups: fatty acid synthase, beta oxidase, fatty acyl-coenzyme A (CoA) reductase, desaturase and acetyl-CoA acetyltransferase. Fourteen female-specific genes were identified through semi-quantitative PCR, and 15 additional genes had expression levels that were 3- to 10-fold higher in females than in males. The majority of the genes had higher expression levels in the sex pheromone glands. Injection of double-stranded RNA (dsRNA) against nine selected genes showed that down-regulation of Desaturase 3 (Des3), Des5 or fatty acyl coenzyme A reductase 17 (FAR17) significantly changed the ratio of the four sex pheromone components (Z,E) -9,11-tetradecadienyl acetate (Z9E11-14:Ac), (Z,E)-9,12-Tetradecadienyl acetate(Z9E12-14:Ac), (Z)-9-tetradecenyl acetate (Z9-14:Ac), (E)-11-Tetradecenyl acetate(E11-14:Ac). These key genes were differentially expressed in female moths collected from different geographical regions. Furthermore, field bioassays demonstrated geographical variation in the olfactory profile of male moths in response to the different sex pheromone mixtures, which therefore indicates that a significant variation in the sex pheromone components exists in the natural population. Our results suggest that a change in the expression of these key genes, Des3, Des5 and FAR17, in the sex pheromone biosynthesis pathway could change the ratio of the sex pheromone components. We surmise that the differential expression levels of the key genes of the sex pheromone biosynthesis pathway may lead to differential ratios of the sex pheromones in the field. Our field trapping experiment suggested that the change of the ratio of the sex pheromone components may have been adapted by the olfactory system and possibly mediate olfactory plasticity in conspecific male moths.

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.

Molecular and functional characterization of a candidate sex pheromone receptor OR1 in Spodoptera litura

Olfaction is primarily mediated by highly specified olfactory receptors (ORs). Here, we cloned and identified an olfactory receptor, named SlituOR1 (Genbank no. JN835269), from Spodoptera litura and found evidence that it is a candidate pheromone receptor. It exhibited male-biased expression in the antennae, where it was localized at the base of sensilla trichoidea, the antennal sensilla mainly responsive to pheromones in moths. Conserved orthologues of this receptor, found among known pheromone receptors within the Lepidoptera, and SlituOR1 were placed among a clade of candidate pheromone receptors in a phylogeny tree of insect OR gene sequences. SlituOR1 showed differential expression in S. litura populations attracted to traps baited with different ratios of the two sex pheromone components (9Z,11E)-tetradecadienyl acetate (Z9E11-14:OAc) and (9Z,12E)-tetradecadienyl acetate (Z9E12-14:OAc). Knocking down of SlituOR1 by RNA interference reduced the electroantennogram (EAG) response to Z9E11-14:OAc, and this result is consistent with the field trapping experiment. We infer that variation in transcription levels of olfactory receptors may modulate sex pheromone perception in male moths and could provide some of the flexibility required to maintain the functionality of communication with females when a population is adapting to a new niche and reproductive isolation becomes an advantage.

Characterization of Spodoptera litura (Lepidoptera: Noctuidae) Takeout Genes and Their Differential Responses to Insecticides and Sex Pheromone

Spodoptera litura (S. litura) is one of the most serious agricultural insect pests worldwide. Takeout (TO) is involved in a variety of physiological and biochemical pathways and performs various biological functions. We characterized 18 S. litura TO genes and investigated their differential responses to insecticides and sex pheromones. All predicted TO proteins have two Cysteines that are unique to the N-terminal of the TO family proteins and contain four highly conserved Prolines, two Glycines, and one Tyrosine. The expression levels of seven TO genes in the male antennae were higher than those in the female antennae, although the expression levels of 10 TO genes in the female were higher than those in the male. We investigated the effects of the sex pheromone and three insecticides, that is, chlorpyrifos (Ch), emamectin benzoate (EB), and fipronil (Fi), on the expression levels of the TO genes in the antennae. The results showed that the insecticides and sex pheromone affect the expression levels of the TO genes. One day after the treatment, the expression levels of SlTO15 and SlTO4 were significantly induced by the Ch/EB treatment. Two days after the S. litura moths were treated with Fi, the expression of SlTO4 was significantly induced (28.35-fold). The expression of SlTO10 changed significantly after the Ch and EB treatment, although the expression of SlTO12 and SlTO15 was inhibited by the three insecticides after two days of treatment. Our results lay a foundation for studying the role of TO genes in the interaction between insecticides and sex pheromone.

Antennal transcriptome analysis of the piercing moth Oraesia emarginata (Lepidoptera: Noctuidae)

The piercing fruit moth Oraesia emarginata is an economically significant pest; however, our understanding of its olfactory mechanisms in infestation is limited. The present study conducted antennal transcriptome analysis of olfactory genes using real-time quantitative reverse transcription PCR analysis (RT-qPCR). We identified a total of 104 candidate chemosensory genes from several gene families, including 35 olfactory receptors (ORs), 41 odorant-binding proteins, 20 chemosensory proteins, 6 ionotropic receptors, and 2 sensory neuron membrane proteins. Seven candidate pheromone receptors (PRs) and 3 candidate pheromone-binding proteins (PBPs) for sex pheromone recognition were found. OemaOR29 and OemaPBP1 had the highest fragments per kb per million fragments (FPKM) values in all ORs and OBPs, respectively. Eighteen olfactory genes were upregulated in females, including 5 candidate PRs, and 20 olfactory genes were upregulated in males, including 2 candidate PRs (OemaOR29 and 4) and 2 PBPs (OemaPBP1 and 3). These genes may have roles in mediating sex-specific behaviors. Most candidate olfactory genes of sex pheromone recognition (except OemaOR29 and OemaPBP3) in O. emarginata were not clustered with those of studied noctuid species (type I pheromone). In addition, OemaOR29 was belonged to cluster PRIII, which comprise proteins that recognize type II pheromones instead of type I pheromones. The structure and function of olfactory genes that encode sex pheromones in O. emarginata might thus differ from those of other studied noctuids. The findings of the present study may help explain the molecular mechanism underlying olfaction and the evolution of olfactory genes encoding sex pheromones in O. emarginata.

Age influences the olfactory profiles of the migratory oriental armyworm mythimna separate at the molecular level

Background

The oriental armyworm Mythimna separata (Walk) is a serious migratory pest; however, studies on its olfactory response and its underlying molecular mechanism are limited. To gain insights to the olfactory mechanism of migration, olfactory genes were identified using antennal transcriptome analysis. The olfactory response and the expression of olfactory genes for 1-day and 5-day-old moths were respectively investigated by EAG and RT-qPCR analyses.

Results

Putative 126 olfactory genes were identified in M. separata, which included 43 ORs, 13 GRs, 16 IRs, 37 OBPs, 14 CSPs, and 3 SNMPs. RPKM values of IR75d and 10 ORs were larger than co-receptors IR25a and ORco, and the RPKM value of PR2 was larger than that of other ORs. Expression of GR1 (sweet receptor) was higher than that of other GRs. Several sex pheromones activated evident EAG responses where the responses of 5-day-old male moths to the sex pheromones were significantly greater than those of female and 1-day old male moths. In accordance with the EAG response, 11 pheromone genes, including 6 PRs and 5 PBPs were identified in M. separate, and the expression levels of 7 pheromone genes in 5-day-old moths were significantly higher than those of females and 1-day-old moths. PR2 and PBP2 might be used in identifying Z11-16: Ald, which is the main sex pheromone component of M. separata. EAG responses to 16 plant volatiles and the expression levels of 43 olfactory genes in 1-day-old moths were significantly greater than that observed in the 5-day-old moths. Heptanal, Z6-nonenal, and benzaldehyde might be very important floral volatiles for host searching and recognized by several olfactory genes with high expression. Some plant volatiles might be important to male moths because the EAG response to 16 plant volatiles and the expression of 43 olfactory genes were significantly larger in males than in females.

Conclusions

The findings of the present study show the effect of adult age on olfactory responses and expression profile of olfactory genes in the migratory pest M. separate.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3427-2) contains supplementary material, which is available to authorized users.

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

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

Identification and tissue expression profile of genes from three chemoreceptor families in an urban pest, Periplaneta americana

Periplaneta americana is a notorious urban pest prevalent in human habitats; very little is known about its chemosensory mechanism. Employing the advanced next-generation sequencing technique, in the present study, we conducted transcriptome sequencing and analysis of the antennae of the adult males and females as well as their mouthparts using an Illumina platform. This resulted in the discovery of a huge number of the members of all major known chemosensory receptor families in P. americana, including 96 odorant receptors (ORs), 53 ionotropic receptors (IRs), and 33 gustatory receptors (GRs). Tissue expression profiles showed most of them mainly expressed in antennae and phylogenetic analysis demonstrated the expansion in the clade distinguishing them from other functionally well-known Lepidoptera species. A high percentage of chemosensory receptor genes (ORs in particular) showing female antenna bias in mRNA expression was observed. Our results provide a basis for further investigations on how P. americana coordinates its chemosensory receptor genes in chemical communication with environments, and for development of novel pest management approaches.

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

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

Identification and comparative expression analysis of odorant binding protein genes in the tobacco cutworm Spodoptera litura

Insect odorant binding proteins (OBPs) are thought to involve in insects' olfaction perception. In the present study, we identified 38 OBP genes from the antennal transcriptomes of Spodoptera litura. Tissue expression profiles analysis revealed that 17 of the 38 SlitOBP transcripts were uniquely or primarily expressed in the antennae of both sexes, suggesting their putative role in chemoreception. The RPKM value analysis revealed that seven OBPs (SlitPBP1-3, SlitGOBP1-2, SlitOBP3 and SlitOBP5) are highly abundant in male and female antennae. Most S. litura antennal unigenes had high homology with Lepidoptera insects, especially genes of the genus Spodoptera. Phylogenetic analysis of the Lepidoptera OBPs demonstrated that the OBP genes from the genus Spodoptera (S. litura, Spodoptera littoralis and Spodoptera exigua) had a relatively close evolutionary relationship. Some regular patterns and key conserved motifs of OBPs in genus Spodoptera are identified by MEME, and their putative roles in detecting odorants are discussed here. The motif-patterns between Lepidoptera OBPs and CSPs are also compared. The SlitOBPs identified here provide a starting point to facilitate functional studies of insect OBPs at the molecular level both in vivo and in vitro.