Identification of odorant-binding proteins of the yellow fever mosquito Aedes aegypti: genome annotation and comparative analyses

Binding Properties of Odorant Binding Protein 37 in Plagiodera versicolora to Host Volatile, o-Cymene

The chemosensory system plays an important role in the host plants location. Plagiodera versicolora (Coleoptera: Chrysomelidae) is a worldwide leaf-eating forest pest that feeds exclusively on salicaceous trees. There is no function study of odorant binding proteins (OBPs) in P. versicolora. In the current study, we found that PverOBP37 has a high expression in male and female antennae, heads, and legs by quantitative real-time PCR. The binding properties of PverOBP37 to 18 host plant volatiles were determined by fluorescence competition binding assays. The results showed that PverOBP37 could bind to the host plant volatile, o-cymene. Furthermore, four candidate key amino acid residues (F8, Y50, F103, and R107) of PverOBP37 to o-cymene were identified by molecular docking. The functional assay to confirm Y50, F103, and R107 mutations were key amino acid residues of PverOBP37 involved in the binding to o-cymene. Knockdown of PverOBP37 and Y-tube behavioral bioassays of mated females led to a significantly reduced attraction to o-cymene. This study not only revealed the molecular mechanism of PverOBP37 but also suggested that PverOBP37 is essential to detect host plant volatiles as cues to search for egg-laying sites in P. versicolora.

Identification of the odorant binding proteins of Western Flower Thrips (Frankliniella occidentalis), characterization and binding analysis of FoccOBP3 with molecular modelling, molecular dynamics simulations and a confirmatory field trial

Olfactory systems are indispensable for insects as they, including Western Flower Thrips (Frankliniella occidentalis), use olfactory cues for ovipositing and feeding. F. occidentalis use odorant binding proteins (OBPs) to transport semiochemicals to odorant receptors to induce a behavioural response from the sensillum lymph of the insect's antennae. This study identifies four OBPs of F. occidentalis and analyses their expression at three stages of growth: larvae, adult males and adult females. Further, it investigates the presence of conserved motifs and their phylogenetic relationship to other insect species. Moreover, FoccOBP3 was in silico characterized to analyse its structure along with molecular docking and molecular dynamics simulations to understand its binding with semiochemicals of F. occidentalis. Molecular docking revealed the interactions of methyl isonicotinate, p-anisaldehyde and (S)-(-)-verbenone with FoccOBP3. Moreover, molecular dynamics simulations showed bonding stability of these ligands with FoccOBP3, and field trials validated that Lurem TR (commercial product) and p-anisaldehyde had greater attraction as compared to (S)-(-)-verbenone, given the compound's binding with FoccOBP3. The current study helps in understanding the tertiary structure and interaction of FoccOBP3 with lures using computational and field data and will help in the identification of novel lures of insects in the future, given the importance of binding with OBPs.Communicated by Ramaswamy H. Sarma.

Comparative transcriptome analysis of the antenna and proboscis reveals feeding state-dependent chemosensory genes in Eupeodes corollae

The physiological state of an insect can affect its olfactory system. However, the molecular mechanism underlying the effect of nutrition-dependent states on odour-guided behaviours in hoverflies remains unclear. In this study, comparative transcriptome analysis of the antenna and proboscis from Eupeodes corollae under different feeding states was conducted. Compared with the previously published antennal transcriptome, a total of 32 novel chemosensory genes were identified, including 4 ionotropic receptors, 17 gustatory receptors, 9 odorant binding proteins and 2 chemosensory proteins. Analysis of differences in gene expression between different feeding states in male and female antennae and proboscises revealed that the expression levels of chemosensory genes were impacted by feeding state. For instance, the expression levels of EcorOBP19 in female antennae, EcorOBP6 in female proboscis, and EcorOR6, EcorOR14, EcorIR5 and EcorIR84a in male antennae were significantly upregulated after feeding. On the other hand, the expression levels of EcorCSP7 in male proboscis and EcorOR40 in male antennae were significantly downregulated. These findings suggest that nutritional state plays a role in the adaptation of hoverflies' olfactory system to food availability. Overall, our study provides important insights into the plasticity and adaptation of chemosensory systems in hoverflies.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Identification and expression profiles of olfactory-related genes based on transcriptome analysis in Plodia interpunctella (Lepidoptera: Pyralidae)

The sophisticated olfactory system of insects is plays a critical role in detecting chemical signals and guiding insect behaviors, such as selecting mates, finding hosts, evading predators, and discovering oviposition sites. Therefore, exploring and clarifying the molecular processes of this system is crucial for developing new insecticides or efficient pest control methods. Plodia interpunctella (Hübner) is a disruptive insect pest damaging the stored grains over the world. However, the olfactory processes of P. interpunctella remain unclear. Herein, we employed a transcriptome analysis to identify olfactory and differentially expressed genes to characterize their expression patterns in different developmental stages and antennal tissue. Subsequently, a total of 172 potential olfactory-related genes included 42 odorant-binding proteins, 12 chemosensory proteins, 51 odorant receptors, 13 gustatory receptors, three sensory neuron membrane proteins, and 51 ionotropic receptors. Furthermore, phylogenetic analysis and BLASTx best-hit analyses showed that these olfactory genes were closely linked with those identified in other lepidopterans. Transcriptome analysis revealed 49 differentially expressed olfactory-related genes, and a semiquantitative reverse transcription polymerase chain reaction showed that 11 olfactory genes were particularly expressed in the legs and wings of female P. interpunctella. Meanwhile, PintOBP29 was notably expressed in female antennae and legs. Genes with high expression levels in the abdomen showed high expression in the legs, but low expression in the antennae. Our findings provide the candidate genetic factors for analysis of the olfactory processes in P. interpunctella.

Identification and functional analysis of odorant-binding proteins of the parasitoid wasp Scleroderma guani reveal a chemosensory synergistic evolution with the host Monochamus alternatus

Scleroderma guani is a generalist ectoparasitoid of wood-boring insects. The chemosensory genes expressed in its antennae play crucial roles in host-seeking. In the present study, we identified 14 OBP genes for the first time from the antennae transcriptomes and genomic data of S. guani. The expression profiles of 14 OBPs were tested by RT-qPCR, and the RT-qPCR results showed that SguaOBP2/5/6/11/12/13 were specifically highly expressed in the female antennae. Then we performed ligand binding assays to test the interactions between six selected SguaOBPs with host specific chemical compounds from M. alternatus and pines. The binding results indicated that SguaOBP12 had a higher binding affinity with longifolene, β-caryophyllene, α-pinene, β-pinene, myrcene, butylated hydroxytoluene, and 3-carene. SguaOBP11 had a high or medium binding affinity with them. Furthermore, both SguaOBP11 and SguaOBP12 had a medium binding affinity with the aggregation pheromone of Monochamus species, 2-undecyloxy-1-ethanol. Finally, by using molecular docking and RNAi, we further explored the molecular interactions and behavioral functions of SguaOBP11 and SguaOBP12 with these vital odor molecules. Our study contributes to the further understanding of chemical communications between S. guani and its host, and further exploration for its role as a more effective biological control agent.

Brindley's Glands Volatilome of the Predator Zelus renardii Interacting with Xylella Vectors

Alien species must adapt to new biogeographical regions to acclimatise and survive. We consider a species to have become invasive if it establishes negative interactions after acclimatisation. Xylella fastidiosa Wells, Raju et al., 1986 (XF) represents Italy's and Europe's most recent biological invasion. In Apulia (southern Italy), the XF-encountered Philaenus spumarius L. 1758 (Spittlebugs, Hemiptera: Auchenorrhyncha) can acquire and transmit the bacterium to Olea europaea L., 1753. The management of XF invasion involves various transmission control means, including inundative biological control using Zelus renardii (ZR) Kolenati, 1856 (Hemiptera: Reduviidae). ZR is an alien stenophagous predator of Xylella vectors, recently entered from the Nearctic and acclimated in Europe. Zelus spp. can secrete semiochemicals during interactions with conspecifics and prey, including volatile organic compounds (VOCs) that elicit conspecific defence behavioural responses. Our study describes ZR Brindley's glands, present in males and females of ZR, which can produce semiochemicals, eliciting conspecific behavioural responses. We scrutinised ZR secretion alone or interacting with P. spumarius. The ZR volatilome includes 2-methyl-propanoic acid, 2-methyl-butanoic acid, and 3-methyl-1-butanol, which are consistent for Z. renardii alone. Olfactometric tests show that these three VOCs, individually tested, generate an avoidance (alarm) response in Z. renardii. 3-Methyl-1-butanol elicited the highest significant repellence, followed by 2-methyl-butanoic and 2-methyl-propanoic acids. The concentrations of the VOCs of ZR decrease during the interaction with P. spumarius. We discuss the potential effects of VOC secretions on the interaction of Z. renardii with P. spumarius.

Functional differentiation of two general-odorant binding proteins in Hyphantria cunea (Drury) (Lepidoptera: Erebidae)

BACKGROUND

General odor-binding proteins (GOBPs) play critical roles in insect olfactory recognition of sex pheromones and plant volatiles. Therefore, the identification of GOBPs in Hyphantria cunea (Drury) based on their characterization to pheromone components and plant volatiles is remain unknown.

RESULTS

In this study, two H. cunea (HcunGOBPs) genes were cloned, and their expression profiles and odorant binding characteristics were systematically analyzed. Firstly, the tissue expression study showed that both HcunGOBP1 and HcunGOBP2 were highly expressed in the antennae of both sexes, indicating their potential involvement in the perception of sex pheromones. Secondly, these two HcunGOBPs genes were expressed in Escherichia coli and ligand binding assays were used to assess the binding affinities to its sex pheromone components including two aldehydes and two epoxides, and some plant volatiles. HcunGOBP2 showed high binding affinities to two aldehyde components (Z9, Z12, Z15-18Ald and Z9, Z12-18Ald), and showed low binding affinities to two epoxide components (1, Z3, Z6-9S, 10R-epoxy-21Hy and Z3, Z6-9S, 10R-epoxy-21Hy), whereas HcunGOBP1 showed weak but significant binding to all four sex pheromone components. Furthermore, both HcunGOBPs demonstrated variable binding affinities to the plant volatiles tested. Thirdly, in silico studies of HcunGOBPs utilized homology, structure modeling, and molecular docking revealed critical hydrophobic residues might be involved in the binding of HcunGOBPs to their sex pheromone components and plant volatiles.

CONCLUSION

Our study suggests that these two HcunGOBPs may serve as potential targets for future studies of HcunGOBPs ligand binding, providing insight in the mechanism of olfaction in H. cunea. © 2023 Society of Chemical Industry.

Differentiation of action mechanisms between natural and synthetic repellents through neuronal electroantennogram and proteomic in Aedes aegypti (Diptera: Culicidae)

Natural-based compounds with repellent activity arise nowadays with the possibility to replace commercial synthetic repellents wholly or partially, such as N,N-Diethyl-m-toluamide (DEET). It is due to DEET's demonstrated toxicity and cutaneous irritation for human beings. Besides, research recommends avoiding using it with kids and pregnant women. The search for a repellent product implies early stages of detailed research that resolve the modes of action against the target insect. Therefore the objective of the current study was to analyze neuronal electrophysiological signals and olfactory system protein expression when the Aedes aegypti mosquito with exposition to natural-based repellents. Adult females of Ae. aegypti of Rockefeller strain were exposed to specific concentrations of repellent compounds like geranyl acetate, α-bisabolol, nerolidol, and DEET. The neuronal effect was measured by electroantennography technique, and the effect of exposure to either DEET or a mixture of natural molecules on protein expression was determined with 2D-PAGE followed by MALDI-TOF-mass spectrometry (MS). This approach revealed that DEET affected proteins related to synapses and ATP production, whereas natural-based repellents increased transport, signaling, and detoxification proteins. The proteomic and electrophysiology experiments demonstrated that repellent exposure disrupts ionic channel activity and modifies neuronal synapse and energy production processes.

Transcriptome analysis of megalurothrips usitatus (Bagnall) identifies olfactory genes with ligands binding characteristics of MusiOBP1 and MusiCSP1

The olfactory system is an important component of insect behavior and is vital for survival and reproduction. However, the genomic characterization and molecular basis of the olfactory response of Megalurothrips usitatus remain relatively unknown. RNA sequencing-built developmental transcriptomes of nymphs, pupae, and adult M. usitatus were examined in order to establish the sequence-based background of M. usitatus olfactory responses. A total of 56,669 unigenes were annotated using GO, NR, Pfam, eggNOG, SwissProt, and KEGG. The number of differentially expressed genes between pupae and nymphs, males and nymphs, and females and nymphs were 10,498, 9,235, and 10,964, respectively. One odorant-binding protein (MusiOBP1) and one chemosensory protein (MusiCSP1) were selected from the transcriptome, and their full-length sequences were obtained using RACE PCR. The relative expression of MusiOBP1 was the highest in primordial females, whereas the relative expression of MusiCSP1 was the highest in primordial pupae. The strongest binding ability to the odor-binding protein MusiOBP1 was observed for β-citronellol. 3-Hydroxy-2-methyl-4-pyrone showed the strongest binding affinity to MusiCSP1. Our analysis suggests that MusiOBP1 and MusiCSP1 may play significant roles in mediating M. usitatus host recognition. This research will improve our knowledge of odorant-binding proteins and chemosensory proteins, which will in turn improve our understanding of insect olfactory systems.

Elevated expression of odorant receptors and odorant-binding proteins genes detected in antennae of Culex quinquefasciatus field females

Culex quinquefasciatus is responsible for the transmission of filarial worms and several arboviruses. Olfaction plays a crucial role in disease transmission as it influences behaviors that are essential for the survival and reproduction of the mosquito, such as the host-seeking behavior, courtship, and oviposition. Understanding the molecular events that coordinate how mosquitoes find their host may lead to alternative methods to reduce diseases transmission. Our aim was to investigate the differential expression profile of odorant receptor (ORs) and odorant-binding proteins (OBPs) genes in Cx. quinquefasciatus field females compared with CqSLab laboratory mosquito colony. Seventeen genes of interest were evaluated for their qualitative and specific expression by RT-PCR on RNAs extracted from female antennae, female legs, complete male bodies, incomplete female bodies (no head and no legs), and L4 larvae. The general expression mapping of olfactory genes revealed that all analyzed genes were expressed in antennae. Some genes showed different qualitative expression profiles, such as CquiOR2, CquiOR64, CquiOR93, CquiOBP11, and CquiOBP16, which were expressed exclusively in female antennae. On the other hand, CquiOR37, CquiOBP2, and CquiOBP43 are expressed in all sample types, and CquiOBP10 was expressed in female antennae and legs and in the complete male bodies. The expression of CquiOBP5 was detected in the female’s antennae and body, but it was absent in the legs. The quantitative differential expression analysis of six of the 17 genes by RT-qPCR was performed from RNA samples from antenna pools collected in three physiological states, post-emergence, post-mating, and post-blood feeding of the field females and CqSLab. A total of 3,600 antennae were analyzed, in pools containing 100 pairs. Most genes screened showed a higher expression level in field mosquitoes when compared with the laboratory strain CqSLab. The expression of CquiOBP5 and CquiOBP10 genes was significantly different between the post-mating and post blood-meal samples of laboratory females (p < 0.05). Our results suggest specialization of the function of the genes studied and divergence in the expression pattern of field mosquitoes compared with laboratory mosquitoes, and therefore, caution should be exercised in the interpretation of data from laboratory mosquito studies.

The Antennal Sensilla and Expression Patterns of Olfactory Genes in the Lower Termite Reticulitermes aculabialis (Isoptera: Rhinotermitidae)

The insect olfactory system plays pivotal roles in insect survival and reproduction through odor detection. Morphological and physiological adaptations are caste-specific and evolved independently in workers, soldiers, and reproductives in termites. However, it is unclear whether the olfactory system is involved in the division of labor in termite colonies. In the present study, the antennal sensilla of alates, workers, soldiers, nymphs, and larvae of the termite Reticulitermes aculabialis Tsai et Hwang ( Isoptera: Rhinotermitidae) were investigated. Transcriptomes were used to detect olfactory genes, and differential expression levels of olfactory genes were confirmed in various castes by qRT-PCR analysis. Nine types of sensilla were identified on the antennae of R. aculabialis, and soldiers possessed all 9 types. In 89,475 assembled unigenes, we found 16 olfactory genes, including 6 chemosensory protein (CSP) and 10 odorant-binding protein (OBP) genes. These OBP genes included 8 general odorant-binding protein genes (GOBPs) and 2 pheromone-binding protein-related protein (PBP) genes. Five CSP genes were more highly expressed in alates than in workers, soldiers, larvae, and nymphs, and the expression levels of CSP6 were significantly higher in nymphs. Seven GOBP and two PBP genes exhibited significantly higher expression levels in alates, and there were no significant differences in the expression levels of GOBP2 among workers, soldiers, alates, and larvae. These results suggest that alates, as primary reproductives, have unique expression patterns of olfactory genes, which play key roles in nuptial flight, mate seeking, and new colony foundation.

Identification of Chemosensory Genes, Including Candidate Pheromone Receptors, in Phauda flammans (Walker) (Lepidoptera: Phaudidae) Through Transcriptomic Analyses

Olfactory and gustatory systems play an irreplaceable role in all cycles of growth of insects, such as host location, mating, and oviposition. Many chemosensory genes in many nocturnal moths have been identified via omics technology, but knowledge of these genes in diurnal moths is lacking. In our recent studies, we reported two sex pheromone compounds and three host plant volatiles that play a vital role in attracting the diurnal moth, Phauda flammans. The antennal full-length transcriptome sequence of P. flammans was obtained using the Pacbio sequencing to further explore the process of sex pheromone and host plant volatile recognition in P. flammans. Transcriptome analysis identified 166 candidate olfactory and gustatory genes, including 58 odorant-binding proteins (OBPs), 19 chemosensory proteins (CSPs), 59 olfactory receptors (ORs), 16 ionotropic receptors (IRs), 14 gustatory receptors (GRs), and 2 sensory neuron membrane proteins (SNMPs). Subsequently, a phylogenetic tree was established using P. flammans and other lepidopteran species to investigate orthologs. Among the 17 candidate pheromone receptor (PR) genes, the expression levels of PflaOR21, PflaOR25, PflaOR35, PflaOR40, PflaOR41, PflaOR42, PflaOR44, PflaOR49, PflaOR51, PflaOR61, and PflaOR63 in the antennae were significantly higher than those in other non-antennae tissues. Among these PR genes, PflaOR21, PflaOR27, PflaOR29, PflaOR35, PflaOR37, PflaOR40, PflaOR42, PflaOR44, PflaOR60, and PflaOR62 showed male-biased expression, whereas PflaOR49, PflaOR61, and PflaOR63 revealed female-biased expression. The functions of related OR genes were also discussed. This research filled the gap of the chemosensory genes of P. flammans and provided basic data for future functional molecular mechanisms studies on P. flammans olfaction.

Odorant Binding Proteins (OBPs) and Odorant Receptors (ORs) of Anopheles stephensi: Identification and comparative insights

Anopheles stephensi is an important vector of malaria in the South Asia, the Middle East, and Eastern Africa. The olfactory system of An. stephensi plays an important role in host-seeking, oviposition, and feeding. Odorant binding proteins (OBPs) are globular proteins that play a pivotal role in insect olfaction by transporting semiochemicals through the sensillum lymph to odorant receptors (ORs). Custom motifs designed from annotated OBPs of Aedes aegypti, Drosophila melanogaster, and Anopheles gambiae were used for the identification of putative OBPs from protein sequences of the An. stephensi Indian strain. Further, BLASTp was also performed to identify missing OBPs and ORs. Subsequently, the presence of domains common to OBPs was confirmed. Identified OBPs were further classified into three sub-classes. Phylogenetic and syntenic analyses were carried out to find homology, and thus the evolutionary relationship between An. stephensi OBPs and ORs with those of An. gambiae, Ae. aegypti and D. melanogaster. Gene structure and physicochemical properties of the OBPs and ORs were also predicted. A total of 44 OBPs and 45 ORs were predicted from the protein sequences of An. stephensi. OBPs were further classified into the classic (27), atypical (10) and plus-C (7) OBP subclasses. The phylogeny revealed close relationship of An. stephensi OBPs and ORs with An. gambiae homologs whereas only five OBPs and two ORs of An. stephensi were related to Ae. aegypti OBPs and ORs, respectively. However, D. melanogaster OBPs and ORs were distantly rooted. Synteny analyses showed the presence of collinear block between the OBPs and ORs of An. stephensi and An. gambiae as well as Ae. aegypti’s. No homology was found with D. melanogaster OBPs and ORs. As an important component of the olfactory system, correctly identifying a species’ OBPs and ORs provide a valuable resource for downstream translational research that will ultimately aim to better control the malaria vector An. stephensi.

Seminal fluid proteins induce transcriptome changes in the Aedes aegypti female lower reproductive tract

BACKGROUND

Mating induces behavioral and physiological changes in the arbovirus vector Aedes aegypti, including stimulation of egg development and oviposition, increased survival, and reluctance to re-mate with subsequent males. Transferred seminal fluid proteins and peptides derived from the male accessory glands induce these changes, though the mechanism by which they do this is not known.

RESULTS

To determine transcriptome changes induced by seminal proteins, we injected extract from male accessory glands and seminal vesicles (MAG extract) into females and examined female lower reproductive tract (LRT) transcriptomes 24 h later, relative to non-injected controls. MAG extract induced 87 transcript-level changes, 31 of which were also seen in a previous study of the LRT 24 h after a natural mating, including 15 genes with transcript-level changes similarly observed in the spermathecae of mated females. The differentially-regulated genes are involved in diverse molecular processes, including immunity, proteolysis, neuronal function, transcription control, or contain predicted small-molecule binding and transport domains.

CONCLUSIONS

Our results reveal that seminal fluid proteins, specifically, can induce gene expression responses after mating and identify gene targets to further investigate for roles in post-mating responses and potential use in vector control.

Pleiotropic Odorant-Binding Proteins Promote Aedes aegypti Reproduction and Flavivirus Transmission

Insect odorant-binding proteins (OBPs) are small soluble proteins that have been assigned roles in olfaction, but their other potential functions have not been extensively explored. Using CRISPR/Cas9-mediated disruption of Aedes aegyptiObp10 and Obp22, we demonstrate the pleiotropic contribution of these proteins to multiple processes that are essential for vectorial capacity. Mutant mosquitoes have impaired host-seeking and oviposition behavior, reproduction, and arbovirus transmission. Here, we show that Obp22 is linked to the male-determining sex locus (M) on chromosome 1 and is involved in male reproduction, likely by mediating the development of spermatozoa. Although OBP10 and OBP22 are not involved in flavivirus replication, abolition of these proteins significantly reduces transmission of dengue and Zika viruses through a mechanism affecting secretion of viral particles into the saliva. These results extend our current understanding of the role of insect OBPs in insect reproduction and transmission of human pathogens, making them essential determinants of vectorial capacity.

Identification of Candidate Olfactory Genes in Scolytus schevyrewi Based on Transcriptomic Analysis

The bark beetle, Scolytus schevyrewi (S. schevyrewi), is an economically important pest in China that causes serious damage to the fruit industry, particularly, in Xinjiang Province. Chemical signals play an important role in the behavior of most insects, accordingly, ecofriendly traps can be used to monitor and control the target pests in agriculture. In order to lay a foundation for future research on chemical communication mechanisms at the molecular level, we generate antennal transcriptome databases for male and female S. schevyrewi using RNA sequencing (RNA-seq) analysis. By assembling and analyzing the adult male and female antennal transcriptomes, we identified 47 odorant receptors (ORs), 22 ionotropic receptors (IRs), 22 odorant-binding proteins (OBPs), and 11 chemosensory proteins (CSPs). Furthermore, expression levels of all the candidate OBPs and CSPs were validated in different tissues of male and female adults by semiquantitative reverse transcription PCR (RT-PCR). ScosOBP2 and ScosOBP18 were highly expressed in female antennae. ScosCSP2, ScosCSP3, and ScosCSP5 were specifically expressed in the antennae of both males and females. These results provide new potential molecular targets to inform and improve future management strategies of S. schevyrewi.

A Detailed Spatial Expression Analysis of Wing Phenotypes Reveals Novel Patterns of Odorant Binding Proteins in the Soybean Aphid, Aphis glycines

The wide range of insect niches has led to a rapid expansion of chemosensory gene families as well as their relatively independent evolution and a high variation. Previous studies have revealed some functions for odorant-binding proteins (OBPs) in processes beyond olfaction, such as gustation and reproduction. In this study, a comparative transcriptomic analysis strategy was applied for the soybean aphid, Aphis glycines, focusing on various functional tissues and organs of winged aphids, including the antenna, head, leg, wing, thorax, cauda, and cornicle. Detailed spatial OBP expression patterns in winged and wingless parthenogenetic aphids were detected by RT-qPCR. Twelve OBPs were identified, and three new OBPs in A. glycines are first reported. All OBPs showed comparatively higher expression in sensory organs and tissues, such as the antenna, head, or leg. Additionally, we found some novel expression patterns for aphid OBPs (Beckendorf et al., 2008). Five OBPs exhibited high-expression levels in the cauda and four in the cornicle (Biasio et al., 2015). Three genes (OBP2/3/15) were highly expressed in the wing (Calvello et al., 2003). Two (OBP3/15) were significantly more highly expressed in the wingless thorax than in the winged thorax with the wings removed, and these transcripts were significantly enriched in the removed wings. More details regarding OBP spatial expression were revealed under our strategy. These findings supported the existence of carrier transport functions other than for foreign chemicals and therefore broader ligand ranges of aphid OBPs. It is important for understanding how insect OBPs function in chemical perception as well as their other potential physiological functions.

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.

Identification and Functional Analysis of SlitOBP11 From Spodoptera litura

Odorant binding proteins (OBPs) play a key role in the olfactory recognition of insects, whose functions have been extensively studied in adult insects but rarely in larvae. In this study, one OBP (SlitOBP11) with high expression in larval antenna but low expression in adult antenna of Spodoptera litura was screened by RNA-seq and verified by quantitative real-time PCR. Furthermore, the function of SlitOBP11 was explored by analysis of the expression patterns and prokaryotic expression of proteins as well as assays of competitive binding. Competitive binding assay demonstrated that SlitOBP11 had high binding affinity to all four female sex pheromone components, but exhibited almost no binding affinity to plant volatiles except for a low affinity to Phenylacetaldehyde and Phenethyl acetate. Homology modeling and molecular docking implied that the shape of these four sex pheromones were linear, which were appropriate for the binding channel of SlitOBP11 and the amino acid residue Asn99 of SlitOBP11 might play an important role in binding. Taken together, our results indicate that SlitOBP11 may be involved in the perception of female sex pheromones by S. litura larvae, and OBPs in the larvae of S. litura play an important role in the olfactory perception process.

A coffee berry borer (Hypothenemus hampei) genome assembly reveals a reduced chemosensory receptor gene repertoire and male-specific genome sequences

Coffee berry borer-CBB (Hypothenemus hampei) is a globally important economic pest of coffee (Coffea spp.). Despite current insect control methods for managing CBB, development of future control strategies requires a better understanding of its biology and interaction with its host plant. Towards this objective, we performed de novo CBB genome and transcriptome sequencing, improved CBB genome assembly and predicted 18,765 protein-encoding genes. Using genome and transcriptome data, we annotated the genes associated with chemosensation and found a reduced gene repertoire composed by 67 odorant receptors (ORs), 62 gustatory receptors (GRs), 33 ionotropic receptors (IRs) and 29 odorant-binding proteins (OBPs). In silico transcript abundance analysis of these chemosensory genes revealed expression enrichment in CBB adults compared with larva. Detection of differentially expressed chemosensory genes between males and females is likely associated with differences in host-finding behavior between sexes. Additionally, we discovered male-specific genome content and identified candidate male-specific expressed genes on these scaffolds, suggesting that a Y-like chromosome may be involved in the CBB's functional haplodiploid mechanism of sex determination.

Characterization of the binding ability of the odorant binding protein BminOBP9 of Bactrocera minax to citrus volatiles

BACKGROUND

Bactrocera minax, one of the most important citrus pests, oviposits exclusively on citrus fruit. In the insect olfactory system, odorant-binding proteins (OBPs) facilitate the initial recognition role of host odor molecules. The aim of this study was to characterize the functional OBPs of B. minax and identify specific volatile organic compounds in the Citrus genus as OBP targets.

RESULTS

BminOBP9 (BminGOBP99a), a closely related homolog of BdorGOBP99a, which reduces the egg-laying behavior of Bactrocera dorsalis through silencing technology, was cloned, expressed, and purified. The binding ability of BminOBP9 to 11 citrus volatiles was then examined using fluorescence competition binding assays (FCBA). The results demonstrated that BminOBP9 could bind to all tested citrus volatiles, as could BdorGOBP99a, ZcucGOBP99a, and ZtauGOBP99a. Interestingly, the binding ability of BminOBP9 was the strongest among the four, suggesting that BminOBP9 may have a function in the specific recognition of citrus volatiles. Furthermore, we aligned the above four proteins and found nine distinctive amino acid sites in BminOBP9. To identify the unique binding sites of BminOBP9, we produced the nine mutants using site-directed mutagenesis. Further FCBA showed that the binding ability of the nine mutants to citrus volatiles significantly reduced, and six of them (substitutes S24P, L36F, E53K, N68D, D112A, and S118R) had the weakest binding ability.

CONCLUSION

The results demonstrated that BminOBP9 was the specific protein involved in the perception of citrus host volatiles by B. minax. Moreover, BminOBP9 could prove efficient in screening the candidate odors for pest management. © 2020 Society of Chemical Industry.

Olfactory systems across mosquito species

There are 3559 species of mosquitoes in the world (Harbach 2018) but, so far, only a handful of them have been a focus of olfactory neuroscience and neurobiology research. Here we discuss mosquito olfactory anatomy and function and connect these to mosquito ecology. We highlight the least well-known and thus most interesting aspects of mosquito olfactory systems and discuss promising future directions. We hope this review will encourage the insect neuroscience community to work more broadly across mosquito species instead of focusing narrowly on the main disease vectors.

Volatile allosteric antagonists of mosquito odorant receptors inhibit human-host attraction

Odorant-dependent behaviors in insects are triggered by the binding of odorant ligands to the variable subunits of heteromeric olfactory receptors. Previous studies have shown, however, that specific odor binding to ORco, the common subunit of odorant receptor heteromers, may allosterically alter olfactory receptor function and profoundly affect subsequent behavioral responses. Using an insect cell-based screening platform, we identified and characterized several antagonists of the odorant receptor coreceptor of the African malaria vector Anopheles gambiae (AgamORco) in a small collection of natural volatile organic compounds. Because some of the identified antagonists were previously shown to strongly repel Anopheles and Culex mosquitoes, we examined the bioactivities of the identified antagonists against Aedes, the third major genus of the Culicidae family. The tested antagonists inhibited the function of Ae. aegypti ORco ex vivo and repelled adult Asian tiger mosquitoes (Ae. albopictus). Binary mixtures of specific antagonists elicited higher repellency than single antagonists, and binding competition assays suggested that this enhanced repellence is due to antagonist interaction with distinct ORco sites. Our results also suggest that the enhanced mosquito repellency by antagonist mixtures is due to additive rather than synergistic effects of the specific antagonist combinations on ORco function. Taken together, these findings provide novel insights concerning the molecular aspects of odorant receptor function. Moreover, our results demonstrate that a simple screening assay may be used for the identification of allosteric modifiers of olfactory-driven behaviors capable of providing enhanced personal protection against multiple mosquito-borne infectious diseases.

Odorant-binding proteins and chemosensory proteins potentially involved in host plant recognition in the Asian citrus psyllid, Diaphorina citri

BACKGROUND

Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) are two families of small water-soluble proteins involved in odor detection and subsequent signal transmission. Determination of their binding mechanisms and specificity towards different odorants is important for developing OBPs/CSPs as targets in pest control management.

RESULTS

We re-annotated genes encoding putative OBPs and CSPs in the Asian citrus psyllid (Diaphorina citri) draft genome using various bioinformatic tools. Genes encoding nine OBPs (seven Classic and two Plus-C) and 12 CSPs were identified, consistent with our previous transcriptomic results. Tissue-specific and developmental expression analyses suggested that genes encoding six OBPs and four CSPs were predominantly expressed in antennae, and displayed various expression patterns in different development stages, suggesting potential involvement in olfactory perception. Competitive fluorescence binding assays with 13 candidate ligands, including known host plant volatiles, sex pheromone components and repellents, showed that DcitOBP3 could bind to various odorants, whereas DcitOBP6, 8 and 9 bound specifically to host plant terpenoids. DcitCSP1 and 12 could also bind to certain terpenoids with high binding specificity.

CONCLUSION

OBP- and CSP-encoding genes were systematically identified by annotating the draft D. citri genome and those potentially involved in odorant detection and signal transmission were identified by analyzing their tissue-expression profiles and odorant-binding affinities, particularly to the peripheral molecular perception of host plant terpenoids. The identified genes may provide potential targets for efficient pest control. © 2020 Society of Chemical Industry.

Brown marmorated stink bug, Halyomorpha halys (Stål), genome: putative underpinnings of polyphagy, insecticide resistance potential and biology of a top worldwide pest

BACKGROUND

Halyomorpha halys (Stål), the brown marmorated stink bug, is a highly invasive insect species due in part to its exceptionally high levels of polyphagy. This species is also a nuisance due to overwintering in human-made structures. It has caused significant agricultural losses in recent years along the Atlantic seaboard of North America and in continental Europe. Genomic resources will assist with determining the molecular basis for this species' feeding and habitat traits, defining potential targets for pest management strategies.

RESULTS

Analysis of the 1.15-Gb draft genome assembly has identified a wide variety of genetic elements underpinning the biological characteristics of this formidable pest species, encompassing the roles of sensory functions, digestion, immunity, detoxification and development, all of which likely support H. halys' capacity for invasiveness. Many of the genes identified herein have potential for biomolecular pesticide applications.

CONCLUSIONS

Availability of the H. halys genome sequence will be useful for the development of environmentally friendly biomolecular pesticides to be applied in concert with more traditional, synthetic chemical-based controls.

Design of a Repellent Against Aedes aegypti (Diptera: Culicidae) Using in silico Simulations With AaegOBP1 Protein

Skin irritation has been reported to be the main adverse effect of excessive use of N,N-diethyl-m-toluamide (DEET) and ethyl 3-acetyl(butyl)amino (IR3535) commercial repellents. Therefore, there is an interest in alternatives of natural origin such as essential oils (EOs) and major compounds, which have repellent effects but have no contraindications. The main purpose of the present study was to identify the repellent effect of selected terpenes on Aedes aegypti Linnaeus, 1762 (Diptera: Culicidae) by in silico analysis based on their affinity with the odorant protein AaegOBP1. The protein-metabolite interactions in 20 terpenes were analyzed using the SwissDock tool. Terpenes presenting the highest affinity compared with commercial repellents were selected to evaluate repellent activity at concentrations 0.1, 10, and 25% against Ae. aegypti. Different periods (0-2, 2-15, 15-60 min) were evaluated with DEET as a positive control. The toxicity of terpenes was verified through Osiris and Molinspiration Cheminformatics Software, and cytotoxicity assays in Vero and HepaRG cells were performed using the MTT method. Two formulations were prepared with polyethylene glycol to evaluate skin long-lasting in vivo assay. The results showed four terpenes: geranyl acetate, nerolidol, α-bisabolol, and nerol, with affinity to AaegOBP1 comparable with DEET and IR3535. Geranyl acetate, nerolidol, and their mixtures showed no cytotoxicity and protection percentages close to 100% during the test at concentrations 10 and 25%. Long-lasting assays with geranyl acetate and nerolidol formulate showed 3 h as maximum protection time with 100% protection percentage. These metabolites and their mixtures are candidates to repellent formulations with times and protection percentages similar to DEET.

Molecular characteristics of odorant-binding protein 1 in Anopheles maculipennis

Background

Anopheles maculipennis complex, the historic vector of malaria, causes serious medical problems worldwide and exhibits different behaviours. Studying the odorant-binding proteins (OBPs), which influence the chemosensory system and behavioural responses, is essential to understanding the population structure and developing effective control measures against this vector. The present study was designed to identify and analyse the obp1 gene in An. maculipennis.

Methods

Adults of An. maculipennis sensu stricto were collected in Zanjan Province, northwest of Iran, and gDNAs of female mosquitoes were extracted. Fragments of An. maculipennis obp1 (Amacobp1) gene were amplified using degenerate and specific primers, and some of amplicons were selected for sequencing.

Results

Analysis of amplified products identified that the sequence of Amacobp1 gene was 1341 bp long. This gene contains three exons (5′, internal, and 3′of 160, 256, and 18 bp, respectively) and encodes 144 amino acids. The sizes of introns I and II in deduced gene are 268 and 358 nucleotides, respectively. The amino acid sequence in the C-terminal of AmacOBP1 is similar to that of major malaria vector Anopheles species. However, its N-terminal has a specific signal peptide with 19 amino acids. This peptide is conserved in different studied populations, and its sequence of amino acids shows the most variation among anopheline species.

Conclusions

Degenerate primers in this study are suggested for studying obp1 gene in Anopheles species. Amacobp1 gene is proposed as a molecular marker for the detection of intraspecific ecotypes and diagnosis of different species within Maculipennis Group. Moreover, the N-terminal of AmacOBP1 peptide is recommended as a molecular marker to identify the Amacobp1 expression patterns in different chemosensory organs for assessing the molecular mechanisms and developing novel behavioural disturbance agents to control An. maculipennis.

Sex- and tissue-specific transcriptome analyses and expression profiling of olfactory-related genes in Ceracris nigricornis Walker (Orthoptera: Acrididae)

BACKGROUND

The sophisticated insect olfactory system plays an important role in recognizing external odors and enabling insects to adapt to environment. Foraging, host seeking, mating, ovipositing and other forms of chemical communication are based on olfaction, which requires the participation of multiple olfactory genes. The exclusive evolutionary trend of the olfactory system in Orthoptera insects is an excellent model for studying olfactory evolution, but limited olfaction research is available for these species. The olfactory-related genes of Ceracris nigricornis Walker (Orthoptera: Acrididae), a severe pest of bamboos, have not yet been reported.

RESULTS

We sequenced and analyzed the transcriptomes from different tissues of C. nigricornis and obtained 223.76 Gb clean data that were assembled into 43,603 unigenes with an N50 length of 2235 bp. Among the transcripts, 66.79% of unigenes were annotated. Based on annotation and tBLASTn results, 112 candidate olfactory-related genes were identified for the first time, including 20 odorant-binding proteins (OBPs), 10 chemosensory-binding proteins (CSPs), 71 odorant receptors (ORs), eight ionotropic receptors (IRs) and three sensory neuron membrane proteins (SNMPs). The fragments per kilobase per million mapped fragments (FPKM) values showed that most olfactory-related differentially expressed genes (DEGs) were enriched in the antennae, and these results were confirmed by detecting the expression of olfactory-related genes with quantitative real-time PCR (qRT-PCR). Among these antennae-enriched genes, some were sex-biased, indicating their different roles in the olfactory system of C. nigricornis.

CONCLUSIONS

This study provides the first comprehensive list and expression profiles of olfactory-related genes in C. nigricornis and a foundation for functional studies of these olfactory-related genes at the molecular level.

Evolution and functional analysis of odorant-binding proteins in three rice planthoppers: Nilaparvata lugens, Sogatella furcifera, and Laodelphax striatellus

BACKGROUND

The white-backed planthopper (WBPH) Sogatella furcifera, the brown planthopper (BPH) Nilaparvata lugens, and the small brown planthopper (SBPH) Laodelphax striatellus are three notorious rice pests that cause annual losses in rice yield through sap-sucking and virus transmission. Odorant-binding proteins (OBPs) are crucial olfactory genes involved in host-seeking behavior.

RESULTS

We discovered the presence of 12, 12, and 16 OBPs in WBPH, BPH, and SBPH, respectively, including two novel OBPs in BPH and seven novel OBPs in SBPH. Phylogenetic analysis indicated that most of these OBPs have homologous genes, and one group (SfurOBP11, NlugOBP8, and LstrOBP2) show a slower evolution rate and are more conserved. Further, in vitro binding studies demonstrated that the three OBPs have similar binding affinities for some rice plant volatiles. Finally, RNA interference (RNAi) successfully inhibited the mRNA expression of the three OBPs, and in vivo behavioral tests showed that the OBP-deficient rice planthoppers were partly anosmic and lost some of their ability to locate rice plants.

CONCLUSION

These results demonstrate the crucial role of the rice planthopper OBP genes in seeking rice plants. This information complements the current genetic resources for the development of RNAi-based transgenic rice and other pest management technologies. © 2018 Society of Chemical Industry.

Antennal transcriptome analysis of the maize weevil Sitophilus zeamais: Identification and tissue expression profiling of candidate odorant-binding protein genes

Our bioassays reviewed that antennae played crucial roles in the responses of maize weevil (Sitophilus zeamais) to food and sex volatiles. In order to identify the maize weevil odorant-binding protein (OBP) genes, we analyzed its antennal transcriptome. In total, 21,587,928 high-quality clean reads were obtained from RNA-seq, 52,206 unigenes were assembled, and 25,744 unigenes showed significant similarity ( E value < 10 ^-5 ) to known proteins in the NCBI nonredundant protein database. From those unigenes, we identified 41 candidate OBP proteins, which could be categorized into dimeric OBPs subfamily, minus-C OBPs subfamily, and classical OBPs subfamily. Phylogenic analysis indicated that most maize weevil OBPs were closely related to their orthologues in other beetles of the Superfamily Curculionoidea. We further investigated the expression profiles of those candidate OBP genes by quantitative real-time polymerase chain reaction. Twenty-six of forty-one maize weevil OBP genes were highly expressed in the antennae or other parts of the head. The rest were expressed in the legs, wings, or other tested tissues. The antennal transcriptomic data and candidate OBP genes described here provide a basis for the functional studies of the maize weevil chemical perception, which are potential novel targets for pest control strategies.

Integrative transcriptomic and genomic analysis of odorant binding proteins and chemosensory proteins in aphids

Odorant binding proteins (OBPs) and chemosensory proteins (CSPs) play essential roles in insect chemosensory recognition. Here, we identified nine OBPs and nine CSPs from the Myzus persicae transcriptome and genome. Genomic structure analysis showed that the number and length of the introns are much higher, and this appears to be a unique feature of aphid OBP genes. Three M. persicae OBP genes (OBP3/7/8) as well as CSP1/4/6, CSP2/9 and CSP5/8 are tandem arrayed in the genome. Phylogenetic analyses of five different aphid species suggest that aphid OBPs and CSPs are conserved in single copy across all aphids (with occasional losses), indicating that each OBP and CSP class evolved from a single gene in the common ancestor of aphids without subsequent duplication. Motif pattern analysis revealed that aphid OBP and CSP motifs are highly conserved, and this could suggest the conserved functions of aphid OBPs and CSPs. Three OBPs (MperOBP6/7/10) are expressed antennae specifically, and five OBPs (MperOBP2/4/5/8/9) are expressed antennae enriched, consistent with their putative olfactory roles. M. persicae CSPs showed much broader expression profiles in nonsensory organs than OBPs. None of the nine MperCSPs were found to be antennae specific, but five of them (MperCSP1/2/4/5/6) showed higher expression levels in the legs than in other tissues. MperCSP10 mainly expressed in the antennae and legs. The broad and diverse expression patterns of M. persicae CSPs suggest their multifunctions in olfactory perception, development and other processes.

Functional Characterization of Odorant Binding Protein 27 (RproOBP27) From Rhodnius prolixus Antennae

Olfactory proteins mediate a wide range of essential behaviors for insect survival. Odorant binding proteins (OBPs) are small soluble olfactory proteins involved in the transport of odor molecules (=odorants) through the sensillum lymph to odorant receptors, which are housed on the dendritic membrane of olfactory sensory neurons also known as olfactory receptor neurons. Thus, a better understanding of the role(s) of OBPs from Rhodnius prolixus, one of the main vectors of Chagas disease, may ultimately lead to new strategies for vector management. Here we aimed at functionally characterize OBPs from R. prolixus. Genes of interest were selected using conventional bioinformatics approaches and subsequent quantification by qPCR. We screened and estimated expression in different tissues of 17 OBPs from R. prolixus adults. These analyses showed that 11 OBPs were expressed in all tissues, whereas six OBP genes were specific to antennae. Two OBP genes, RproOBP6 and RproOBP13, were expressed in both male and female antennae thus suggesting that they might be involved in the recognition of semiochemicals mediating behaviors common to both sexes, such host finding (for a blood meal). Transcripts for RproOBP17 and RproOBP21 were enriched in female antennae and possibly involved in the detection of oviposition attractants or other semiochemicals mediating female-specific behaviors. By contrast, RproOBP26 and RproOBP27 might be involved in the reception of sex pheromones given that their transcripts were highly expressed in male antennae. To test this hypothesis, we silenced RproOBP27 using RNAi and examined the sexual behavior of the phenotype. Indeed, adult males treated with dsOBP27 spent significantly less time close to females as compared to controls. Additionally, docking analysis suggested that RproOBP27 binds to putative sex pheromones. We therefore concluded that RproOBP27 might be a pheromone-binding protein.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Expressional and functional comparisons of two general odorant binding proteins in Agrotis ipsilon

Insect general odorant binding proteins (GOBPs) have been long thought to bind and transport host plant volatiles to the olfactory receptors on the dendrite membrane of the olfactory neurons. Recent studies indicate that they can also bind female sex pheromones. In present study, two GOBP genes, AipsGOBP1 and AipsGOBP2 were cloned from the adult antennae of Agrotis ipsilon. Tissue expression profiles indicated that both of them are antennae-specific and more abundant in the female antennae than in the male antennae. Temporal expression profiles showed that both AipsGOBP1 and AipsGOBP2 began to express in antennae 3 days prior to adult emergence from pupae, and reached their highest expression level 3 and 4 days after adult emergence, respectively. Mating increased their expression in the female antennae but reduced their expression in the male antennae. In situ hybridization and immunolocalization demonstrated that both AipsGOBP1 and AipsGOBP2 are expressed and co-localized in sensilla basiconica and sensilla trichodea of both sexes. AipsGOBP2 exhibited a high binding affinity in vitro with the two major sex pheromone components Z7-12:Ac and Z9-14:Ac and the four plant volatiles cis-3-hexen-1-ol, oleic acid, dibutyl phthalate and β-caryophyllene with Ki values less than 5 μM. AipsGOBP1, on the other hand, showed medium binding affinities with the five A. ipsilon sex pheromones and six plant volatiles. AipsGOBP2 also showed a broader ligand-binding spectrum and a greater ligand-binding affinity than AipsGOBP1 with the tested aldehyde and alcohol sex pheromones of Lepidoptera species. Taken together, our results indicate that AipsGOBP2 may play greater roles than AipsGOBP1 does in binding sex pheromones and host plant volatiles.

A head transcriptome provides insights into odorant binding proteins of the bamboo grasshopper

The bamboo grasshopper Ceracris kiangsu is a famous bamboo pest in China. The identification of genes involved in olfactory behavior of C. kiangsu is necessary for better understanding the molecular basis and expression profiles of behavior ecology. However, necessary genomic and transcriptomic data are lacking in the species, limiting control efficiency. The primary objective of this study was to find and describe odorant binding proteins in the head of the bamboo grasshopper. We performed the paired-end sequencing on an Illumina Hiseq2000 following the vendor's recommended protocol. Functional annotation was performed by comparison with public databases. OBP genes were first identified using BLASTN and BLASTX results from our C. kiangsu datebase, which was established from the date of transcriptome sequencing. The gene-specific primers were used to conduct RT-PCR to detect the tissue distribution of OBPs using a SYBR Premix ExTaq kit following the manufacturer's instructions with a real-time thermal cycler. We obtained more than 133 million clean reads derived from the C. Kiangsu heads using the next-generation sequencing, which were assembled into 260,822 unique sequences (average 814 bp). We have detected eight putative odorant binding protein genes (OBPs) of C. kiangsu for the first time, and analyzed the expression profiles of the OBPs in different tissues (head, antenna, mouthpart, body and leg). Our results reveal that the eight OBPs display a clear divergence, strongly indicating that they possessed diverse functions, and thus provides comprehensive sequence analysis for elucidating the molecular basis of OBPs in C. kiangsu. In addition, we find that the relative expression levels of OBP1, OBP2 and OBP8 are significantly higher in the antennae as compared to the other OBP genes, suggesting that these three OBP genes play crucial roles in the locust's odorant discrimination. In general, this is the first study to characterize the complete head transcriptome of C. kiangsu using high-throughput sequencing. The study opens a window for functional characterization of the OBPs of C. kiangsu, with potential for new or refined applications of semiochemicals for control of this notorious pest.

Dynamic Changes in Chemosensory Gene Expression during the Dendrolimus punctatus Mating Process

The insect chemosensory system is pivotal for interactions with their environments, and moths have especially sensitive olfaction. Exploration of the connection between the plasticity of olfactory-guided and molecular level pathways in insects is important for understanding the olfactory recognition mechanisms of insects. The pine caterpillar moth, Dendrolimus punctatus Walker, is a dominant conifer defoliator in China, and mating is the priority for adults of this species, during which sex pheromone recognition and oviposition site location are the main activities; these activities are all closely related to chemosensory genes. Thus, we aimed to identify chemosensory related genes and monitor the spectrum of their dynamic expression during the entire mating process in D. punctatus. In this study, we generated transcriptome data from male and female adult D. punctatus specimens at four mating stages: eclosion, calling, copulation, and post-coitum. These data were analyzed using bioinformatics tools to identify the major olfactory-related gene families and determine their expression patterns during mating. Levels of odorant binding proteins (OBPs), chemosensory proteins (CSPs), and odorant receptors (ORs) were closely correlated with mating behavior. Comparison with ORs from other Dendrolimus and Lepidoptera species led to the discovery of a group of ORs specific to Dendrolimus. Furthermore, we identified several genes encoding OBPs and ORs that were upregulated after mating in females; these genes may mediate the location of host plants for oviposition via plant-emitted volatiles. This work will facilitate functional research into D. punctatus chemosensory genes, provide information about the relationship between chemosensory genes and important physiological activities, and promote research into the mechanisms underlying insect olfactory recognition.

Identification and Characterization of Odorant Binding Proteins in the Forelegs of Adelphocoris lineolatus (Goeze)

The chemosensory system is essential for insects to detect exogenous compounds, and odorant binding proteins (OBPs) play crucial roles in odorant binding and transduction. In the alfalfa plant bug Adelphocoris lineolatus, an important pest of multiple crops, our understanding of the physiological roles of antenna-biased OBPs has increased dramatically, whereas OBPs related to gustation have remained mostly unexplored. In this study, we employed RNA sequencing and RACE PCR methods to identify putative OBPs from the adult forelegs of both sexes. Eight candidate OBPs were identified, and three OBPs (AlinOBP15, 16, and 17) were novel. Full-length sequence alignment and phylogenetic analyses suggested that these three candidate OBPs had characteristics typical of the insect OBP family. AlinOBP16 and 17 displayed six highly conserved cysteines, placing them in the classic OBP subfamily, whereas AlinOBP15 resembled AlinOBP14 and clustered with the Plus-C clade. Quantitative real-time PCR (qRT-PCR) revealed distinct and significant tissue- and sex-biased expression patterns. AlinOBP15 was highly expressed in female heads, and AlinOBP16 and 17 were strongly expressed in female antennae. In particular, AlinOBP11, the most abundant OBP gene in our foreleg transcriptome dataset, was predominately expressed in adult legs. Furthermore, four types of sensilla hairs were observed on the forelegs of adult A. lineolatus, including sensilla trichodea, setae, and two types of sensilla chaetica (Sch1 and Sch2). Anti-AlinOBP11 antiserum strongly labeled the outer sensillum lymph of Sch2, implying that it has important gustatory functions in A. lineolatus. Our current findings provide evidence that OBPs can be functionally expressed in the tarsal gustatory sensilla of hemipteran mirid species, broadening our understanding of OBP chemosensory function in insects and facilitating the discovery of new functional targets for the regulation of insect host-searching behaviors.

Identification and Expression Profiling of Odorant-Binding Proteins and Chemosensory Proteins of Daktulosphaira vitifoliae (Hemiptera: Phylloxeridae)

In insects, odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) are primary peripheral olfactory proteins playing critical roles in odorant detection. In this study, we present the first identification of OBPs and CSPs from the transcriptome of grape phylloxera Daktulosphaira vitifoliae Fitch, an important pest that damages both roots and leaves of grapes. The OBPs contained six conserved cysteine residues and the CSPs contained four conserved cysteine residues in this insect. Phylogenetic analysis showed that most of the olfactory proteins were closely related to OBPs and CSPs from other aphids. However, DviOBP7 and DviCSP9 were different because they were classified into different independent branches, respectively. Real-time polymerase chain reaction (RT-PCR) was used to examine the tissue expression of these transcripts. DviOBP1, DviOBP6, and DviOBP7 were uniquely or primarily expressed in antennae and not in the body. DviOBP2 was more abundantly expressed in the body than in the antennae. The expression levels of OBPs and CSPs of phylloxera varied depending upon where they were expressed in different body tissues.

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.

RNA Interference for Mosquito and Mosquito-Borne Disease Control

RNA interference (RNAi) is a powerful tool to silence endogenous mosquito and mosquito-borne pathogen genes in vivo. As the number of studies utilizing RNAi in basic research grows, so too does the arsenal of physiological targets that can be developed into products that interrupt mosquito life cycles and behaviors and, thereby, relieve the burden of mosquitoes on human health and well-being. As this technology becomes more viable for use in beneficial and pest insect management in agricultural settings, it is exciting to consider its role in public health entomology. Existing and burgeoning strategies for insecticide delivery could be adapted to function as RNAi trigger delivery systems and thereby expedite transformation of RNAi from the lab to the field for mosquito control. Taken together, development of RNAi-based vector and pathogen management techniques & strategies are within reach. That said, tools for successful RNAi design, studies exploring RNAi in the context of vector control, and studies demonstrating field efficacy of RNAi trigger delivery have yet to be honed and/or developed for mosquito control.

A look inside odorant-binding proteins in insect chemoreception

Detection of chemical signals from the environment through olfaction is an indispensable mechanism for maintaining an insect's life, evoking critical behavioral responses. Among several proteins involved in the olfactory perception process, the odorant binding protein (OBP) has been shown to be essential for a normally functioning olfactory system. This paper discusses the role of OBPs in insect chemoreception. Here, structural aspects, mechanisms of action and binding affinity of such proteins are reviewed, as well as their promising application as molecular targets for the development of new strategies for insect population management and other technological purposes.

Molecular evolution of Odorant-binding proteins gene family in two closely related Anastrepha fruit flies

BACKGROUND

Odorant-binding proteins (OBPs) are of great importance for survival and reproduction since they participate in initial steps of the olfactory signal transduction cascade, solubilizing and transporting chemical signals to the olfactory receptors. A comparative analysis of OBPs between closely related species may help explain how these genes evolve and are maintained under natural selection and how differences in these proteins can affect olfactory responses. We studied OBP genes in the closely related species Anastrepha fraterculus and A. obliqua, which have different host preferences, using data from RNA-seq cDNA libraries of head and reproductive tissues from male and female adults, aiming to understand the speciation process occurred between them.

RESULTS

We identified 23 different OBP sequences from Anastrepha fraterculus and 24 from A. obliqua, which correspond to 20 Drosophila melanogaster OBP genes. Phylogenetic analysis separated Anastrepha OBPs sequences in four branches that represent four subfamilies: classic, minus-C, plus-C and dimer. Both species showed five plus-C members, which is the biggest number found in tephritids until now. We found evidence of positive selection in four genes and at least one duplication event that preceded the speciation of these two species. Inferences on tertiary structures of putative proteins from these genes revealed that at least one positively selected change involves the binding cavity (the odorant binding region) in the plus-C OBP50a.

CONCLUSIONS

A. fraterculus and A. obliqua have a bigger OBP repertoire than the other tephritids studied, though the total number of Anastrepha OBPs may be larger, since we studied only a limited number of tissues. The contrast of these closely related species reveals that there are several amino acid changes between the homologous genes, which might be related to their host preferences. The plus-C OBP that has one amino acid under positive selection located in the binding cavity may be under a selection pressure to recognize and bind a new odorant. The other positively selected sites found may be involved in important structural and functional changes, especially ones in which site-specific changes would radically change amino acid properties.

Identification and Expression Analysis of Candidate Odorant-Binding Protein and Chemosensory Protein Genes by Antennal Transcriptome of Sitobion avenae

Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) of aphids are thought to be responsible for the initial molecular interactions during olfaction that mediate detection of chemical signals. Analysis of the diversity of proteins involved comprises critical basic research work that will facilitate the development of sustainable pest control strategies. To help us better understand differences in the olfactory system between winged and wingless grain aphids, we constructed an antennal transcriptome from winged and wingless Sitobion avenae (Fabricius), one of the most serious pests of cereal fields worldwide. Among the 133,331 unigenes in the antennal assembly, 13 OBP and 5 CSP putative transcripts were identified with 6 OBP and 3 CSP sequences representing new S. avenae annotations. We used qPCR to examine the expression profile of these genes sets across S. avenae development and in various tissues. We found 7 SaveOBPs and 1 SaveCSP were specifically or significantly elevated in antennae compared with other tissues, and that some transcripts (SaveOBP8, SaveCSP2 and SaveCSP5) were abundantly expressed in the legs of winged or wingless aphids. The expression levels of the SaveOBPs and SaveCSPs varied depending on the developmental stage. Possible physiological functions of these genes are discussed. Further molecular and functional studies of these olfactory related genes will explore their potential as novel targets for controlling S. avenae.

Antennal and Abdominal Transcriptomes Reveal Chemosensory Genes in the Asian Citrus Psyllid, Diaphorina citri

The Asian citrus psyllid, Diaphorina citri is the principal vector of the highly destructive citrus disease called Huanglongbing (HLB) or citrus greening, which is a major threat to citrus cultivation worldwide. More effective pest control strategies against this pest entail the identification of potential chemosensory proteins that could be used in the development of attractants or repellents. However, the molecular basis of olfaction in the Asian citrus psyllid is not completely understood. Therefore, we performed this study to analyze the antennal and abdominal transcriptome of the Asian citrus psyllid. We identified a large number of transcripts belonging to nine chemoreception-related gene families and compared their expression in male and female adult antennae and terminal abdomen. In total, 9 odorant binding proteins (OBPs), 12 chemosensory proteins (CSPs), 46 odorant receptors (ORs), 20 gustatory receptors (GRs), 35 ionotropic receptors (IRs), 4 sensory neuron membrane proteins (SNMPs) and 4 different gene families encoding odorant-degrading enzymes (ODEs): 80 cytochrome P450s (CYPs), 12 esterase (ESTs), and 5 aldehyde dehydrogenases (ADE) were annotated in the D. citri antennal and abdominal transcriptomes. Our results revealed that a large proportion of chemosensory genes exhibited no distinct differences in their expression patterns in the antennae and terminal abdominal tissues. Notably, RNA sequencing (RNA-seq) data and quantitative real time-PCR (qPCR) analyses showed that 4 DictOBPs, 4 DictCSPs, 4 DictIRs, 1 DictSNMP, and 2 DictCYPs were upregulated in the antennae relative to that in terminal abdominal tissues. Furthermore, 2 DictOBPs (DictOBP8 and DictOBP9), 2 DictCSPs (DictOBP8 and DictOBP12), 4 DictIRs (DictIR3, DictIR6, DictIR10, and DictIR35), and 1 DictCYP (DictCYP57) were expressed at higher levels in the male antennae than in the female antennae. Our study provides the first insights into the molecular basis of chemoreception in this insect pest. Further studies on the identified differentially expressed genes would facilitate the understanding of insect olfaction and their role in the interactions between olfactory system and biological processes.