Evidence for trade-offs in detoxification and chemosensation gene signatures in Plutella xylostella

Transgenic expression of SeCYP9A186 and PxFMO2 confers resistance to emamectin benzoate in Plutella xylostella

BACKGROUND

The overexpression of metabolic enzymes constitutes a crucial mechanism for insects to detoxify xenobiotics and metabolic pesticides. A flavin-containing monooxygenase gene (PxFMO2) from Plutella xylostella and a P450 gene (SeCYP9A186, F116V mutant allele) from Spodoptera exigua have been reported to be involved in insecticide resistance. In this study, we aim to utilize transgenic technology to validate their in vivo detoxification functions in Plutella xylostella.

RESULTS

We established two transgenic strains of Plutella xylostella with expressing an endogenous PxFMO2 gene from Plutella xylostella and an exogenous SeCYP9A186 gene from S. exigua, respectively. Bioassays demonstrated that the transgenic Plutella xylostella strain (IPP-FMO2) expressing PxFMO2 exhibited a 12-fold resistance to emamectin benzoate and a 6.4-fold resistance to chlorantraniliprole compared to the background strain (IPP-S). In contrast, the transgenic Plutella xylostella strain (IPP-9A186) expressing SeCYP9A186 displayed a 235-fold resistance to emamectin benzoate and a 115-fold resistance to abamectin. Moreover, resistance to emamectin benzoate in the IPP-9A186 strain of Plutella xylostella was inherited as an incompletely dominant trait and was genetically linked to the transgene locus.

CONCLUSIONS

Our results not only elucidated the in vivo contribution of the PxFMO2 and SeCYP9A186 to the insecticide resistance phenotype in Plutella xylostella, but also provided a genetic engineering toolkit to manipulate resistance pathways. These insights and methodologies could further aid in developing sustainable pest management strategies in Plutella xylostella. © 2024 Society of Chemical Industry.

Chemosensory protein 8 confers thiamethoxam resistance in Diaphorina citri

The Asian citrus psyllid (Diaphorina citri) serves as the primary vector of the bacterium Candidatus Liberibacter asiaticus (CLas), which is responsible for causing citrus Huanglongbing. Neonicotinoid pesticides such as thiamethoxam are frequently utilized for D. citri management worldwide, but their use have resulted in considerable development of insecticide resistance within D. citri populations. Recent reports have revealed that chemosensory proteins (CSPs)-mediated resistance represents a novel insecticide resistance mechanism in insects. In this finding, we found that exposure to thiamethoxam at different concentrations resulted in significant upregulation of transcriptional levels of DcitCSP8 by 2.41, 2.24 and 1.74-fold respectively. The expression profiles among different tissues showed that DcitCSP8 highly occurred in the legs and wings, and its expression being significantly induced by thiamethoxam with 2.40-fold increase specifically in the legs. The purified recombinant protein DcitCSP8, derived from Escherichia coli expression, present strong in vitro binding affinity (Ki = 4.74 μM) to thiamethoxam. Furthermore, the nanocarrier star polycation (SPc) was used to enhance silencing efficiency, resulting in a significant reduction of DcitCSP8 by 85.37% and 91.80% at 24 and 48 h post application of SPc + ds DcitCSP8. Knockdown of DcitCSP8 transcription significantly increased the susceptibility of D. citri adults in response to thiamethoxam, while reinjection of DcitCSP8 protein could restore the resistance in DcitCSP8-silenced individuals. Our findings highlight the association between DcitCSP8 and thiamethoxam resistance via high binding affinity that mitigates toxicity in D. citri, shedding light on CSP-mediated insecticide resistance and potentially contributing to the development of novel strategies targeting CSPs for managing resistance in D. citri.

Omics Analysis of Odorant-Binding Proteins and Cuticle-Enriched SfruOBP18 Confers Multi-Insecticide Tolerance in Spodoptera frugiperda

Spodoptera frugiperda is a notorious pest that develops a high resistance to many insecticides. Recently, insect odorant-binding proteins (OBPs) have been proven to participate in insecticide resistance. However, the functional evidence supporting the cross-link between OBPs and insecticide resistance remains unexplored. Here, we identified 50 SfruOBPs from the larval transcriptome and genome. Notably, SfruOBP18 was highly expressed in the larval cuticle and could be induced to upregulate its expression by multi-insecticides. Ligand-binding assays revealed that SfruOBP18 bound strongly with four insecticides; RNAi and insecticide bioassay demonstrated that the knockdown of SfruOBP18 did not affect larval survival and development. However, it can significantly increase the larval susceptibility to multi-insecticides, suggesting an uncommon role of SfruOBP18 in multi-insecticide susceptibility. Our study provides a comprehensive understanding of SfruOBPs and furthermore proves that a larval cuticle-enriched OBP can bind with and confer larval tolerance to multi-insecticides. SfruOBP18 could be a new insecticidal target for controlling Lepidoptera pests.

Identification and Functional Analysis of Odorant Binding Proteins in Apriona germari (Hope)

Apriona germari (Hope) presents a significant threat as a dangerous wood-boring pest, inflicting substantial harm to forest trees. Investigating the olfactory sensory system of A. germari holds substantial theoretical promise for developing eco-friendly control strategies. To date, however, the olfactory perception mechanism in A. germari remains largely unknown. Therefore, we performed transcriptome sequencing of A. germari across four distinct body parts: antennae, foreleg tarsal segments, mouthparts (maxillary and labial palps), and abdomen terminals, pinpointing the odorant binding protein (OBP) genes and analyzing their expression. We found eight AgerOBPs (5, 19, 23, 25, 29, 59, 63, 70) highly expressed in the antennae. In our competitive binding experiments, AgerOBP23 showed strong binding abilities to the pheromone component fuscumol acetate, eight plant volatiles (farnesol, cis-3-hexenal, nerolidol, myristol acetate, cis-3-hexenyl benzoate, (-)-α-cedrene, 3-ethylacetophenone, and decane), and four insecticides (chlorpyrifos, phoxim, indoxacarb, and cypermethrin). However, AgerOBP29 and AgerOBP63 did not show prominent binding activities to these tested chemicals. Through homology modeling and molecular docking, we identified the key amino acid sites involved in the binding process of AgerOBP23 to these ligands, which shed light on the molecular interactions underlying its binding specificity. Our study suggests that AgerOBP23 may serve as a potential target for future investigations of AgerOBP ligand binding. This approach is consistent with the reverse chemical ecology principle, establishing the groundwork for future studies focusing on attractant or repellent development by exploring further the molecular interactions between OBP and various compounds.

Involvement of Chemosensory Protein CrufCSP3 in Perception of the Host Location in a Parasitic Wasp Cotesia ruficrus

Chemosensory proteins (CSPs) constitute a class of olfactory proteins localized in insect sensory organs that serve a crucial function in decoding external chemical stimuli. This study aims to elucidate the involvement of CrufCSP3 in olfactory perception within the context of Cotesia ruficrus, an indigenous endoparasitoid targeting the invasive pest Spodoptera frugiperda. Through fluorescence-competitive binding assays and site-directed mutagenesis, we pinpointed four amino acids as pivotal residues involved in the interaction between CrufCSP3 and five host-related compounds. Subsequent RNA interference experiments targeting CrufCSP3 unveiled a reduced sensitivity to specific host-related compounds and a decline in the parasitism rate of the FAW larvae. These findings unequivocally indicate the essential role of CrufCSP3 in the chemoreception process of C. ruficrus. Consequently, our study not only sheds light on the functional importance of CSPs in parasitic wasp behavior but also contributes to the development of eco-friendly and efficacious wasp behavior modifiers for effectively mitigating pest population surges.

Two cuticle-enriched chemosensory proteins confer multi-insecticide resistance in Spodoptera frugiperda

Recent studies revealed that insect chemosensory proteins (CSPs) both play essential roles in insect olfaction and insect resistance. However, functional evidence supporting the crosslink between CSP and insecticide resistance remains unexplored. In the present study, 22 SfruCSP transcripts were identified from the fall armyworm (FAW) and SfruCSP1 and SfruCSP2 are enriched in the larval cuticle and could be induced by multiple insecticides. Both SfruCSP1 and SfruCSP2 are highly expressed in the larval inner endocuticle and outer epicuticle, and these two proteins exhibited high binding affinities with three insecticides (chlorfenapyr, chlorpyrifos and indoxacarb). The knockdown of SfruCSP1 and SfruCSP2 increased the susceptibility of FAW larvae to the above three insecticides, and significantly increased the penetration ratios of these insecticides. Our in vitro and in vivo evidence suggests that SfruCSP1 and SfruCSP2 are insecticide binding proteins and confer FAW larval resistance to chlorfenapyr, chlorpyrifos and indoxacarb by an insecticide sequestration mechanism. The study should aid in the exploration of larval cuticle-enriched CSPs for insect resistance management.

Biochemistry and transcriptomic analyses of Phthorimaea absoluta (Lepidoptera: Gelechiidae) response to insecticides

Phthorimaea absoluta is an invasive solanaceous plant pest with highly devastating effects on tomato plant. Heavy reliance on insecticide use to tackle the pest has been linked to insecticide resistance selection in P. absoluta populations. To underline insights on P. absoluta insecticide resistance mechanisms to diamides and avermectins, we evaluated the transcriptomic profile of parental (field-collected) and F8 (lab-reared) populations. Furthermore, to screen for the presence of organophosphate and pyrethroid resistance, we assessed the gene expression levels of acetylcholinesterase (ace1) and para-type voltage-gated sodium channel (VGSG) genes in the F1 to F8 lab-reared progeny of diamide and avermectin exposed P. absoluta field-collected populations. The VGSG gene showed up-regulation in 12.5% and down-regulation in 87.5% of the screened populations, while ace1 gene showed up-regulation in 37.5% and down-regulation in 62.5% of the screened populations. Gene ontology of the differentially expressed genes from both parental and eighth generations of diamide-sprayed P. absoluta populations revealed three genes involved in the metabolic detoxification of diamides in P. absoluta. Therefore, our study showed that the detoxification enzymes found could be responsible for P. absoluta diamide-based resistance, while behavioural resistance, which is stimulus-dependent, could be attributed to P. absoluta avermectin resistance.

Odorant Binding Protein Expressed in Legs Enhances Malathion Tolerance in Bactrocera dorsalis (Hendel)

Bactrocera dorsalis is a highly invasive species and is one of the most destructive agricultural pests worldwide. Organophosphorus insecticides have been widely and chronically used to control it, leading to the escalating development of resistance. Recently, odorant binding proteins (OBPs) have been found to play a role in reducing insecticide susceptibility. In this study, we used RT-qPCR to measure the expression levels of four highly expressed OBP genes in the legs of B. dorsalis at different developmental stages and observed the effect of malathion exposure on their expression patterns. The results showed that OBP28a-2 had a high expression level in 5 day old adults of B. dorsalis, and its expression increased after exposure to malathion. By CRISPR/Cas9 mutagenesis, we generated OBP28a-2-/- null mutants and found that they were more susceptible to malathion than wild-type adults. Furthermore, in vitro direct affinity assays confirmed that OBP28a-2 has a strong affinity for malathion, suggesting that it plays a role in reducing the susceptibility of B. dorsalis to malathion. Our findings enriched our understanding of the function of OBPs. The results highlighted the potential role of OBPs as buffering proteins that help insects survive exposure to insecticides.

Three chemosensory proteins enriched in antennae and tarsi of Rhaphuma horsfieldi differentially contribute to the binding of insecticides

Antennae and legs (primarily the tarsal segments) of insects are the foremost sensory organs that contact a diverse range of toxic chemicals including insecticides. Binding proteins expressed in the two tissues are potential molecular candidates serving as the binding and sequestering of insecticides, like chemosensory proteins (CSPs). Insect CSPs endowed with multiple roles have been suggested to participate in insecticide resistance, focusing mainly on moths, aphids and mosquitos. Yet, the molecular underpinnings underlying the interactions of cerambycid CSPs and insecticides remain unexplored. Here, we present binding properties of three antenna- and tarsus-enriched RhorCSPs (RhorCSP1, CSP2 and CSP3) in Rhaphuma horsfieldi to eight insecticide classes totaling 15 chemicals. From the transcriptome of this beetle, totally 16 CSP-coding genes were found, with seven full-length sequences. In phylogeny, these RhorCSPs were distributed dispersedly in different clades. Expression profiles revealed the abundant expression of RhorCSP1, CSP2 and CSP3 in antennae and tarsi, thus as representatives for studying the protein-insecticide interactions. Binding assays showed that the three RhorCSPs were tuned differentially to insecticides but exhibited the highest affinities with hexaflumuron, chlorpyrifos and rotenone (dissociation constants <13 μM). In particular, RhorCSP3 could interact strongly with 10 of tested insecticides, of which four residues (Tyr25, Phe42, Val65 and Phe68) contributed significantly to the binding of six, four, three and four ligands, respectively. Of these, the binding of four mutated RhorCSP3s to a botanical insecticide rotenone was significantly weakened compared to the wildtype protein. Furthermore, we also evidenced that RhorCSP3 was a broadly-tuned carrier protein in response to a wide variety of plant odorants outside insecticides. Altogether, our findings shed light on different binding mechanisms and odorant-tuning profiles of three RhorCSPs in R. horsfieldi and identify key residues of the RhorCSP3-insecticide interactions.

An OBP gene highly expressed in non-chemosensory tissues affects the phototaxis and reproduction of Spodoptera frugiperda

Insect odorant binding proteins (OBPs) were initially regarded as carriers of the odorants involved in chemosensation. However, it had been observed that a growing number of OBP genes exhibited broad expression patterns beyond chemosensory tissues. Here, an OBP gene (OBP31) was found to be highly expressed in the larval ventral nerve cord, adult brain and male reproductive organ of Spodoptera frugiperda. An OBP31 knockout strain (OBP31-/- ) was generated by CRISPR/Cas9 mutagenesis. For OBP31-/- , the larvae needed longer time to pupate, but there was no difference in the pupal weight between OBP31-/- and wild type (WT). OBP31-/- larvae showed stronger phototaxis than the WT larvae, indicating the importance of OBP31 in light perception. For mating rhythm of adults, OBP31-/- moths displayed an earlier second mating peak. In the cross-pairing of OBP31-/- and WT moths, the mating duration was longer, and hatchability was lower in OBP31-/- group and OBP31+/- ♂ group than that in the WT group. These results suggested that OBP31 played a vital role in larval light perception and male reproductive process and could provide valuable insights into understanding the biological functions of OBPs that were not specific in chemosensory tissues.

Characterization of CrufCSP1 and Its Potential Involvement in Host Location by Cotesia ruficrus (Hymenoptera: Braconidae), an Indigenous Parasitoid of Spodoptera frugiperda (Lepidoptera: Noctuidae) in China

Chemosensory proteins (CSPs) are a class of soluble proteins that facilitate the recognition of chemical signals in insects. While CSP genes have been identified in many insect species, studies investigating their function remain limited. Cotesia ruficrus (Hymenoptera: Braconidae) holds promise as an indigenous biological control agent for managing the invasive pest Spodoptera frugiperda (Lepidoptera: Noctuidae) in China. This study aimed to shed light on the gene expression, ligand binding, and molecular docking of CrufCSP1 in C. ruficrus. A RT-qPCR analysis revealed that the expression of CrufCSP1 was higher in the wings, with male adults exhibiting significantly higher relative expression levels than other developmental stages. A fluorescence competitive binding analysis further demonstrated that CrufCSP1 has a high binding ability with several host-related volatiles, with trans-2-hexenal, octanal, and benzaldehyde showing the strongest affinity to CrufCSP1. A molecular docking analysis indicated that specific amino acid residues (Phe24, Asp25, Thr53, and Lys81) of CrufCSP1 can bind to these specific ligands. Together, these findings suggest that CrufCSP1 may play a crucial role in the process of C. ruficrus locating hosts. This knowledge can contribute to the development of more efficient and eco-friendly strategies for protecting crops and managing pests.

Overexpression of the Chemosensory Protein CSP7 Gene Contributed to Lambda-Cyhalothrin Resistance in the Bird Cherry-Oat Aphid Rhopalosiphum padi

Lambda-cyhalothrin is one of the most important pyrethroids used for controlling wheat aphids. Extensive spraying of lambda-cyhalothrin has led to the development of high resistance to this pyrethroid inRhopalosiphum padi. The mechanisms of resistance are complex and not fully understood. In this study, we found that a laboratory-selected strain of R. padi showed extremely high resistance to lambda-cyhalothrin and cross-resistance to bifenthrin and deltamethrin. The expression level of RpCSP7 was significantly elevated in the resistant strain compared to that in the susceptible strain. Knockdown of RpCSP7 increased the susceptibility of R. padi to lambda-cyhalothrin, whereas the susceptibility to bifenthrin and deltamethrin was not significantly changed. The recombinant RpCSP7 displayed a high affinity for lambda-cyhalothrin but no affinities to bifenthrin and deltamethrin. These findings suggest that the overexpression of RpCSP7 contributes to the resistance of R. padi to lambda-cyhalothrin. This study provides valuable insights into CSP-mediated insecticide resistance in insects.

Roles of insect odorant binding proteins in communication and xenobiotic adaptation

Odorant binding proteins (OBPs) are small water-soluble proteins mainly associated with olfaction, facilitating the transport of odorant molecules to their relevant receptors in the sensillum lymph. While traditionally considered essential for olfaction, recent research has revealed that OBPs are engaged in a diverse range of physiological functions in modulating chemical communication and defense. Over the past 10 years, emerging evidence suggests that OBPs play vital roles in purifying the perireceptor space from unwanted xenobiotics including plant volatiles and pesticides, potentially facilitating xenobiotic adaptation, such as host location, adaptation, and pesticide resistance. This multifunctionality can be attributed, in part, to their structural variability and effectiveness in transporting, sequestering, and concealing numerous hydrophobic molecules. Here, we firstly overviewed the classification and structural properties of OBPs in diverse insect orders. Subsequently, we discussed the myriad of functional roles of insect OBPs in communication and their adaptation to xenobiotics. By synthesizing the current knowledge in this field, our review paper contributes to a comprehensive understanding of the significance of insect OBPs in chemical ecology, xenobiotic adaptation, paving the way for future research in this fascinating area of study.

The chemosensory protein 1 contributes to indoxacarb resistance in Plutella xylostella (L.)

BACKGROUND

Insecticide resistance continuously poses a threat to agricultural production. Chemosensory protein-mediated resistance is a new mechanism of insecticide resistance discovered in recent years. In-depth research on resistance mediated by chemosensory proteins (CSPs) provides new insight into aid insecticide resistance management.

RESULTS

Chemosensory protein 1 in Plutella xylostella (PxCSP1) was overexpressed in the two indoxacarb-resistant field populations and PxCSP1 has a high affinity with indoxacarb. PxCSP1 was upregulated when exposed to indoxacarb and the knockdown of this gene elevated sensitivity to indoxacarb, which demonstrate that PxCSP1 is involved in the indoxacarb resistance. Considering that CSPs may confer resistance in insects via binding or sequestering, we explored the binding mechanism of indoxacarb in PxCSP1-mediated resistance. Using molecular dynamics simulations and site-directed mutation, we found that indoxacarb forms a solid complex with PxCSP1 mainly through van der Waals interactions and electrostatic interactions. Between these, the electrostatic interaction provided by the Lys100 side chain in PxCSP1, and especially the hydrogen bonding between the NZ atom and the O of the carbamoyl carbonyl group of indoxacarb, are the key factors for the high affinity of PxCSP1 to indoxacarb.

CONCLUSIONS

The overexpression of PxCPS1 and its high affinity to indoxacarb is partially responsible for indoxacarb resistance in P. xylostella. Modification of indoxacarb's carbamoyl group has the potential to alleviate indoxacarb resistance in P. xylostella. These findings will contribute to solving chemosensory protein-mediated indoxacarb resistance and provide a better understanding of the insecticide resistance mechanism. © 2023 Society of Chemical Industry.

Chemosensory proteins are associated with thiamethoxam tolerance in bird cherry-oat aphid Rhopalosiphum padi

Rhopalosiphum padi (L.) is an important cosmopolitan pest of cereal crops. Thiamethoxam is widely used for control R. padi in some regions. Chemosensory proteins (CSPs) are a class of transporter proteins in arthropods which play a key role in various physiological processes including response to insecticide exposure. However, the role of R. padi CSPs (RpCSPs) in insecticide binding and susceptibility has not been well clarified. In this study, we found that the expression levels of RpCSP1, RpCSP4, RpCSP5, RpCSP7, RpCSP10 were dramatically upregulated after exposure to thiamethoxam. Suppression of RpCSP4 and RpCSP5 transcription by RNA interference significantly enhanced the susceptibility of R. padi to thiamethoxam. Molecular docking and fluorescence competitive binding showed that RpCSP4 and RpCSP5 had high binding affinity with thiamethoxam. The present results prove that RpCSP4 and RpCSP5 are related to insecticide resistance through high binding affinity to reduce the toxicity of insecticide.

Evidence for the Participation of Chemosensory Proteins in Response to Insecticide Challenge in Conopomorpha sinensis

Chemosensory proteins (CSPs) are a type of efficient transporters that can bind various hydrophobic compounds. Previous research has shown that the expression levels of some insect CSPs were significantly increased after insecticide treatment. However, the role of CSPs in response to insecticide challenge is unclear. Conopomorpha sinensis is the most destructive borer pest of litchi (Litchi chinensis) and longan (Euphoria longan) in the Asia-Pacific region. Here, we studied the expression patterns and potential functions of 12 CSP genes (CsCSPs) from C. sinensis in response to λ-cyhalothrin exposure. The spatiotemporal distribution of CsCSPs suggested that they were predominantly expressed in the female abdomen, female legs, and male legs. The expression levels of CsCSPs were affected in a time-dependent manner after λ-cyhalothrin treatment in both sexes of C. sinensis adults. Compared to the control group, the expression levels of CsCSP1, CsCSP2, CsCSP9, and CsCSP12 in females were significantly increased by 2-4 times, while only one CsCSP, three CsCSPs, and two CsCSPs were significantly upregulated in males at three time points post-treatment. The sex-biased variance of CSP expression may be related to sex-specific detoxification enzymatic activities and survival rates of C. sinensis in response to insecticide challenge. Homology modeling and molecular docking analyses showed that the binding energy value of CsCSP1-12 to λ-cyhalothrin was negative and the binding energy between CsCSP9 and λ-cyhalothrin was the lowest (-11.35 kJ/mol). Combined with expression alterations of CsCSP1-12, the results indicate that CsCSP1, CsCSP2, CsCSP9, and CsCSP12 were involved in binding and ferrying of λ-cyhalothrin in C. sinensis.

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

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

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.

Transcriptomic modulation in response to an intoxication with deltamethrin in a population of Triatoma infestans with low resistance to pyrethroids

Background

Triatoma infestans is the main vector of Chagas disease in the Southern Cone. The resistance to pyrethroid insecticides developed by populations of this species impairs the effectiveness of vector control campaigns in wide regions of Argentina. The study of the global transcriptomic response to pyrethroid insecticides is important to deepen the knowledge about detoxification in triatomines.

Methodology and findings

We used RNA-Seq to explore the early transcriptomic response after intoxication with deltamethrin in a population of T. infestans which presents low resistance to pyrethroids. We were able to assemble a complete transcriptome of this vector and found evidence of differentially expressed genes belonging to diverse families such as chemosensory and odorant-binding proteins, ABC transporters and heat-shock proteins. Moreover, genes related to transcription and translation, energetic metabolism and cuticle rearrangements were also modulated. Finally, we characterized the repertoire of previously uncharacterized detoxification-related gene families in T. infestans and Rhodnius prolixus.

Conclusions and significance

Our work contributes to the understanding of the detoxification response in vectors of Chagas disease. Given the absence of an annotated genome from T. infestans, the analysis presented here constitutes a resource for molecular and physiological studies in this species. The results increase the knowledge on detoxification processes in vectors of Chagas disease, and provide relevant information to explore undescribed potential insecticide resistance mechanisms in populations of these insects.

Chemosensory proteins confer adaptation to the ryanoid anthranilic diamide insecticide cyantraniliprole in Aphis gossypii glover

Chemosensory proteins (CSPs) are a class of small transporter proteins expressed only in arthropods with various functions beyond chemoreception. Previous studies have been reported that CSPs are involved in the insecticide resistance. In this study, we found that AgoCSP1, AgoCSP4, and AgoCSP5 were constitutively overexpressed in an insecticide-resistant strain of Aphis gossypii and showed higher expression in broad body tissue (including fat bodies) than in the midgut but without tissue specificity. However, the function of these three upregulated AgoCSPs remains unknown. Here, we investigated the function of AgoCSPs in resistance to the diamide insecticide cyantraniliprole. Suppression of AgoCSP1, AgoCSP4 and AgoCSP5 transcription by RNAi significantly increased the sensitivity of resistant aphids to cyantraniliprole. Molecular docking and competitive binding assays indicated that these AgoCSPs bind moderate with cyantraniliprole. Transgenic Drosophila melanogaster expressing these AgoCSPs in the broad body or midgut showed higher tolerance to cyantraniliprole than control flies with the same genetic background; AgoCSP4 was more effective in broad body tissue, and AgoCSP1 and AgoCSP5 were more effective in the midgut, indicating that broad body and midgut tissues may be involved in the insecticide resistance mediated by the AgoCSPs examined. The present results strongly indicate that AgoCSPs participate in xenobiotic detoxification by sequestering and masking toxic insecticide molecules, providing insights into new factors involved in resistance development in A. gossypii.

Odorant binding protein 3 is associated with nitenpyram and sulfoxaflor resistance in Nilaparvata lugens

Odorant binding protein (OBP) can interact with small-molecule compounds insecticides and thereby modulate variation in insecticide susceptibility in insects. However, the regulatory mechanism of OBP-mediated insecticide resistance in Nilaparvata lugens, a destructive rice pest in Asia, remains unclear. Here, we explored the role of NlOBP3 in the resistance of N. lugens to nitenpyram and sulfoxaflor. The results showed that NlOBP3 was overexpressed in association with nitenpyram and sulfoxaflor resistance, and NlOBP3 silencing significantly increased the mortality of N. lugens to nitenpyram and sulfoxaflor, suggesting that NlOBP3 may be associated with nitenpyram and sulfoxaflor resistance in N. lugens. OBP localization revealed that NlOBP3 was highly expressed in all nymph stages and was enriched in the antennae, legs, body wall, and fat body. RT-qPCR analyses showed that the mRNA levels of NlOBP3 were significantly affected by nitenpyram and sulfoxaflor. Additionally, molecular docking predicted that there were multiple binding sites that may played key roles in the binding of NlOBP3 with nitenpyram and sulfoxaflor. The current study identifies a previously undescribed mechanism of insecticide resistance in N. lugens, showing that NlOBP3 is likely to be involved in the evolution of nitenpyram and sulfoxaflor resistance in N. lugens.

Odorant Binding Protein C17 Contributes to the Response to Artemisia vulgaris Oil in Tribolium castaneum

The red flour beetle, Tribolium castaneum (T. castaneum), generates great financial losses to the grain storage and food processing industries. Previous studies have shown that essential oil (EO) from Artemisia vulgaris (A. vulgaris) has strong contact toxicity to larvae of the beetle, and odorant-binding proteins (OBPs) contribute to the defense of larvae against A. vulgaris. However, the functions of OBPs in insects defending against plant oil is still not clear. Here, expression of one OBP gene, TcOBPC17, was significantly induced 12–72 h after EO exposure. Furthermore, compared to the control group, RNA interference (RNAi) against TcOBPC17 resulted in a higher mortality rate after EO treatment, which suggests that TcOBPC17 involves in the defense against EO and induces a declining sensitivity to EO. In addition, the tissue expression profile analysis revealed that the expression of TcOBPC17 was more abundant in the metabolic detoxification organs of the head, fat body, epidermis, and hemolymph than in other larval tissue. The expression profile of developmental stages showed that TcOBPC17 had a higher level in early and late adult stages than in other developmental stages. Taken together, these results suggest that TcOBPC17 could participate in the sequestration process of exogenous toxicants in T. castaneum larvae.

Distinct molecular impact patterns of abamectin on Apis mellifera ligustica and Apis cerana cerana

The effects of insecticides on bee health are a topic of intensive research. Although abamectin is toxic to bees, the molecular impact of abamectin needs to be clarified. Here, we found that Apis cerana cerana exhibited a higher mortality rate when exposed to abamectin than Apis mellifera ligustica. In addition, A. cerana cerana had markedly higher numbers of differentially expressed genes (DEGs), differentially expressed proteins (DEPs) and differentially expressed metabolites (DEMs) than A. mellifera ligustica during exposure to abamectin. These results indicate that abamectin exposure exerts stronger effects on A. cerana cerana than on A. mellifera ligustica. In addition, six DEGs, two DEPs and two DEMs overlapped between the two bee species under abamectin exposure; however, some genes or proteins from the zinc finger protein, superoxide dismutase and peroxiredoxin families and the energy metabolism pathway were only unregulated in A. cerana cerana, which indicates a significant difference in the impact of abamectin on the two bee species. Despite these differences, several of the same gene families, such as heat shock proteins, cytochrome P450, odorant-binding proteins and cuticle proteins, and pathways, including the carbohydrate metabolism, immune system, lipid metabolism, amino acid metabolism, sensory system, locomotion and development pathways, were influenced by abamectin exposure in both A. cerana cerana and A. mellifera ligustica. Together, our results indicate that abamectin causes adverse effects on bees and thus poses a risk to bee populations and that abamectin exposure affects A. cerana cerana more strongly than A. mellifera ligustica. These findings improve our understanding of the behavioural and physiological effects of abamectin on bees.

Chemosensory Proteins Are Associated with Thiamethoxam and Spirotetramat Tolerance in Aphis gossypii Glover

Chemosensory proteins (CSPs) are a class of transporters in arthropods. Deeper research on CSPs showed that CSPs may be involved in some physiological processes beyond chemoreception, such as insect resistance to pesticides. We identified two upregulated CSPs in two resistant strains of Aphis gossypii Glover. To understand their role in the resistance of aphids to pesticides, we performed the functional verification of CSP1 and CSP4 in vivo and in vitro. Results showed that the sensitivity of the thiamethoxam-resistant strain to thiamethoxam increased significantly with the silencing of CSP1 and CSP4 by RNAi (RNA interference), and the sensitivity of the spirotetramat-resistant strain to spirotetramat increased significantly with the silencing of CSP4. Transgenic Drosophila melanogaster expressing CSPs exhibited stronger resistance to thiamethoxam, spirotetramat, and alpha-cypermethrin than the control did. In the bioassay of transgenic Drosophila, CSPs showed different tolerance mechanisms for different pesticides, and the overexpressed CSPs may play a role in processes other than resistance to pesticides. In brief, the present results prove that CSPs are related to the resistance of cotton aphids to insecticides.

BarH1 regulates odorant-binding proteins expression and olfactory perception of Monochamus alternatus Hope

Insect odorant-binding proteins (OBPs) are a class of small soluble proteins that can be found in various tissues wherein binding and transport of small molecules are required. Thus, OBPs are not only involved in typical olfactory function by specific activities with odorants but also participate in other physiological processes in non-chemosensory tissues. To better understand the complex biological functions of OBPs, it is necessary to study the transcriptional regulation of their expression patterns. In this paper, an apparent gradient expression pattern of Obp19, that was highly and specifically expressed in antennae and played an essential role in the detection of camphene, was defined in the antennae of the Japanese pine sawyer. Further, the transcription factor BarH1, that also presented gradient expression pattern in antennae, was found to regulate expression of Obp19 directly through binding to its upstream DNA sequence. The condition of BarH1 gene silence, the gene expression levels of Obp19 significantly decreased. At the same time, additional olfactory genes also were regulated and thus influence camphene reception. These findings provide us an opportunity to incorporate Obps in the gene regulatory networks of insects, which contribute to a better understanding of the multiplicity and diversity of OBPs and the olfactory mediated behaviors.

Transcriptomic analysis of resistance and short-term induction response to pyrethroids, in Anopheles coluzzii legs

BACKGROUND

Insecticide-treated bed nets and indoor residual spraying comprise the major control measures against Anopheles gambiae sl, the dominant vector in sub-Saharan Africa. The primary site of contact with insecticide is through the mosquitoes' legs, which represents the first barrier insecticides have to bypass to reach their neuronal targets. Proteomic changes and leg cuticle modifications have been associated with insecticide resistance that may reduce the rate of penetration of insecticides. Here, we performed a multiple transcriptomic analyses focusing on An. coluzzii legs.

RESULTS

Firstly, leg-specific enrichment analysis identified 359 genes including the pyrethroid-binder SAP2 and 2 other chemosensory proteins, along with 4 ABCG transporters previously shown to be leg enriched. Enrichment of gene families included those involved in detecting chemical stimuli, including gustatory and ionotropic receptors and genes implicated in hydrocarbon-synthesis. Subsequently, we compared transcript expression in the legs of a highly resistant strain (VK7-HR) to both a strain with very similar genetic background which has reverted to susceptibility after several generations without insecticide pressure (VK7-LR) and a lab susceptible population (NG). Two hundred thirty-two differentially expressed genes (73 up-regulated and 159 down-regulated) were identified in the resistant strain when compared to the two susceptible counterparts, indicating an over-expression of phase I detoxification enzymes and cuticular proteins, with decrease in hormone-related metabolic processes in legs from the insecticide resistant population. Finally, we analysed the short-term effect of pyrethroid exposure on An. coluzzii legs, comparing legs of 1 h-deltamethrin-exposed An. coluzzii (VK7-IN) to those of unexposed mosquitoes (VK7-HR) and identified 348 up-regulated genes including those encoding for GPCRs, ABC transporters, odorant-binding proteins and members of the divergent salivary gland protein family.

CONCLUSIONS

The data on An. coluzzii leg-specific transcriptome provides valuable insights into the first line of defense in pyrethroid resistant and short-term deltamethrin-exposed mosquitoes. Our results suggest that xenobiotic detoxification is likely occurring in legs, while the enrichment of sensory proteins, ABCG transporters and cuticular genes is also evident. Constitutive resistance is primarily associated with elevated levels of detoxification and cuticular genes, while short-term insecticide-induced tolerance is linked with overexpression of transporters, GPCRs and GPCR-related genes, sensory/binding and salivary gland proteins.

Odorant binding protein C12 is involved in the defense against eugenol in Tribolium castaneum

Tribolium castaneum (T. castaneum) is a worldwide pest of stored grain that mainly harms flour, and not only causes serious loss of flour quality but also leads to deterioration of flour quality. Chemical detection plays a key role in insect behavior, and the role of odorant-binding proteins (OBPs) in insect chemical detection has been widely studied. However, the mechanism of OBPs in insect defense against exogenous toxic substances is still unclear. In this study, biochemical analysis showed that eugenol, the active component of A. vulgaris essential oil, significantly induced the expression of the OBP gene OBPC12 from T. castaneum (TcOBPC12). The mortality of late larvae treated with eugenol was higher than that of the control group after RNA interference (RNAi) against TcOBPC12, which indicates that the OBP gene is involved in the eugenol defense mechanism and leads to a decrease in sensitivity to eugenol. Tissue expression profiling showed that the expression of TcOBPC12 in the epidermis, hemolymph, and intestine was higher than in other larval tissues, and TcOBPC12 was expressed mainly in the epidermis, head, and fat body of adults. The developmental expression profile showed that the expression of TcOBPC12 in late eggs, early and late larval stages, and late adult stages was higher than in other developmental stages. These data suggest that TcOBPC12 may be involved in the absorption of exogenous toxic substances by the larvae from T. castaneum. Our results provide a theoretical basis for the metabolism and degradation mechanism of exogenous toxic substances and help explore more potential target genes of insect pests.

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.

Three chemosensory proteins from the sweet potato weevil, Cylas formicarius, are involved in the perception of host plant volatiles

BACKGROUND

Chemosensory proteins (CSPs) play important roles in chemical communication, but their precise physiological functions are still unclear. Cylas formicarius is the most serious pest attacking sweet potato around the world. At present, there is no effective way to control this pest.

RESULTS

Our results showed that CforCSP1, 5 and 6 genes were highly expressed in the antennae of both sexes of C. formicarius. In addition, results from a fluorescence competitive binding assay showed that the CforCSP1, 5 and 6 proteins had high binding affinities for 17 plant volatiles including eight host plant volatiles. This indicated that the three proteins may be involved in the detection of host plant volatiles. Furthermore, results from four-arm olfactometer bioassays showed that there was a significant tendency for C. formicarius to be attracted to eucalyptol, β-carotene, benzaldehyde, vanillin and phenethyl alcohol, while it was repelled by β-ionone. Finally, the levels of expression of the three CforCSPs in C. formicarius were successfully inhibited by RNA interference (RNAi). Behavioral experiments showed that CforCSP1, 5 and 6-deficient C. formicarius were partly anosmic to β-cyclocitral, benzaldehyde, octyl aldehyde, and β-ionone and exhibited a reduced ability to locate the host plant volatiles β-carotene and vanillin.

CONCLUSION

Our data suggest that CforCSP1, 5 and 6 likely are involved in the chemical communication between C. formicarius and host plant volatiles, which may play pivotal roles in oviposition and feeding site preferences. More importantly, these results could provide information for the development of monitoring and push-pull strategies for the control of C. formicarius. © 2021 Society of Chemical Industry.

Transcription factor CncC potentially regulates the expression of multiple detoxification genes that mediate indoxacarb resistance in Spodoptera litura

The xenobiotic transcription factor cap 'n' collar isoform C (CncC) is considered the central regulator of antioxidant and detoxification genes. Previous research indicated that CncC regulates three-phase enzymes responsible for insecticide resistance. In this study, the SlituCncC gene from Spodoptera litura was obtained and characterized. Quantitative polymerase chain reaction (qPCR) analysis showed that SlituCncC was expressed in all developmental stages and tissues, but was highly expressed in 3rd- and 4th-instar larvae, and in the Malpighian tubule, fat body, and midgut. In addition, SlituCncC was up-regulated and more highly induced with indoxacarb treatment in the indoxacarb-resistant strains compared with the susceptible strain. RNA interference-mediated gene silencing of SlituCncC significantly increased mortality of S. litura when exposed to indoxacarb. Furthermore, comparative transcriptome analysis showed that 842 genes were down-regulated and 127 genes were up-regulated in SlituCncC knockdown S. litura. Further analysis indicated that 18 three-phase enzymes were identified in the down-regulated genes, of which seven were associated with indoxacarb resistance in S. litura. qPCR analysis confirmed that expression of six of these seven genes was consistent with RNA sequencing data. All six detoxification genes were induced by indoxacarb, and the expression patterns were similar to that of SlituCncC. Finally, the CncC-Maf binding site was predicted in all six gene promoters. This study indicates that the transcription factor SlituCncC may regulate multiple detoxification genes that mediate indoxacarb resistance in S. litura.

Transcriptomic analysis and molecular docking reveal genes involved in the response of Aedes aegypti larvae to an essential oil extracted from Eucalyptus

BACKGROUND

Aedes aegypti (L.) is an urban mosquito, vector of several arboviruses that cause severe diseases in hundreds of million people each year. The resistance to synthetic insecticides developed by Ae. aegypti populations worldwide has contributed to failures in vector control campaigns, increasing the impact of arbovirus diseases. In this context, plant-derived essential oils with larvicidal activity could be an attractive alternative for vector control. However, the mode of action and the detoxificant response of mosquitoes to plant derived compounds have not been established, impairing the optimization of their use.

METHODS AND FINDINGS

Here we compare gene expression in Ae. aegypti larvae after 14 hrs of exposure to Eucalyptus camaldulensis essential oil with a control group exposed to vehicle (acetone) for the same lapse, by using RNA-Seq. We found differentially expressed genes encoding for cuticle proteins, fatty-acid synthesis, membrane transporters and detoxificant related gene families (i.e. heat shock proteins, cytochromes P450, glutathione transferases, UDP-glycosyltransferases and ABC transporters). Finally, our RNA-Seq and molecular docking results provide evidence pointing to a central involvement of chemosensory proteins in the detoxificant response in mosquitoes.

CONCLUSIONS AND SIGNIFICANCE

Our work contributes to the understanding of the physiological response of Ae. aegypti larvae to an intoxication with a natural toxic distilled from Eucalyptus leafs. The results suggest an involvement of most of the gene families associated to detoxification of xenobiotics in insects. Noteworthy, this work provides important information regarding the implication of chemosensory proteins in the detoxification of a natural larvicide. Understanding the mode of detoxification of Eucalyptus distilled compounds could contribute to their implementation as a tool in mosquito control.

The Crystal Structure of the Spodoptera litura Chemosensory Protein CSP8

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

Evidence for the Involvement of the Chemosensory Protein AgosCSP5 in Resistance to Insecticides in the Cotton Aphid, Aphis gossypii

Simple Summary

Insect chemosensory proteins (CSPs) are potential targets for insect pest control strategies and are proposed to function in insect chemoreception, because they play a role in crop host location by binding and transporting odorant molecules. They are also thought to have other functions, for example, in tissue regeneration and in insecticide resistance, because they also express in nonolfactory tissues and are capable of binding insecticides. However, there are few reports that provide direct evidence for this proposal. In this study, we discovered gene gain-and-loss among aphid populations, possibly associated with different insecticide resistance, and then identified and cloned a CSP gene responsive to insecticide treatments. The introduction of such gene in Drosophila fruit flies made the transgenic flies less sensitive to the treatment of different insecticides. Our study advances the research of insect CSP functions and offers valuable new information to target CSPs for pest management.

Abstract

It has been speculated that insect chemosensory proteins (CSPs) may have additional roles beyond olfaction. In this study, the phylogenetic and genomic analyses of the CSPs of the cotton aphid, Aphis gossypii, revealed the presence of gene gain-and-loss among different aphid field populations. Differential expressions of eight CSP genes were demonstrated after treatments with insecticides of different modes of action. The expression of AgosCSP5 was significantly upregulated by the insecticide treatments in a dose-dependent manner. The Drosophila flies overexpressing AgosCSP5 were significantly less susceptible to the insecticides, omethoate, imidacloprid and cypermethrin but not to deltamethrin and tau-fluvalinate, compared with control flies. The transgenic Drosophila flies exhibited an LC₅₀ resistance ratio of 2.6 to omethoate, compared with control flies. Likewise, the mortality of the transgenic flies to imidacloprid and cypermethrin was significantly lower than that of the control flies (p < 0.01). Homology modelling, molecular docking and dynamic simulation supported the interactions and revealed a higher stability of AgosCSP5/insecticide complexes than AgosCSP5/semiochemical complexes. Our study demonstrates for first time the in vivo evidence for the involvement of CSP genes in insecticide resistance of crop insect pests and provides new insights of the newly discovered CSP-mediated insect resistance mechanism to insecticides.

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

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

Altered Expression of Chemosensory and Odorant Binding Proteins in Response to Fungal Infection in the Red Imported Fire Ant, Solenopsis invicta

Social insects have evolved acute mechanisms for sensing and mitigating the spread of microbial pathogens within their communities that include complex behaviors such as grooming and sanitation. Chemical sensing involves detection and transport of olfactory and other chemicals that are mediated by at least two distinct classes of small molecular weight soluble proteins known as chemosensory- and odorant binding proteins (CSPs and OBPs, respectively) that exist as protein families in all insects. However, to date, a systematic examination of the expression of these genes involved in olfactory and other pathways to microbial infection has yet to be reported. The red imported fire ant, Solenopsis invicta, is one of the most successful invasive organisms on our planet. Here, we examined the temporal gene expression profiles of a suite of S. invicta CSPs (SiCSPs1-22) and OBPs (SiOBPs1-16) in response to infection by the broad host range fungal insect pathogen, Beauveria bassiana. Our data show that within 24 h post-infection, i.e., before the fungus has penetrated the host cuticle, the expression of SiCSPs and SiOBPs is altered (mainly increased compared to uninfected controls), followed by suppression of SiCSP and select SiOBP expression 48 h post-infection and mixed responses at 72 h post-infection. A smaller group of SiBOPs, however, appeared to respond to fungal infection, with expression of SiOBP15 consistently higher during fungal infection over the time course examined. These data indicate dynamic gene expression responses of CSPs and OBPs to fungal infection that provide clues to mechanisms that might mediate detection of microbial pathogens, triggering grooming, and nest sanitation.

Identification and motif analyses of candidate nonreceptor olfactory genes of Dendroctonus adjunctus Blandford (Coleoptera: Curculionidae) from the head transcriptome

The round-headed pine beetle Dendroctonus adjunctus, whose dispersion and colonization behaviors are linked to a communication system mediated by semiochemicals, is one of the five most critical primary pests in forest ecosystems in Mexico. This study provides the first head transcriptome analysis of D. adjunctus and the identification of the nonreceptor olfactory genes involved in the perception of odors. De novo assembly yielded 44,420 unigenes, and GO annotations were similar to those of antennal transcriptomes of other beetle species, which reflect metabolic processes related to smell and signal transduction. A total of 36 new transcripts of nonreceptor olfactory genes were identified, of which 27 encode OBPs, 7 encode CSPs, and 2 encode SNMP candidates, which were subsequently compared to homologous proteins from other bark beetles and Coleoptera species by searching for sequence motifs and performing phylogenetic analyses. Our study provides information on genes encoding nonreceptor proteins in D. adjunctus and broadens the knowledge of olfactory genes in Coleoptera and bark beetle species, and will help to understand colonization and aggregation behaviors for the development of tools that complement management strategies.

Organophosphorus insecticide interacts with the pheromone-binding proteins of Athetis lepigone: Implication for olfactory dysfunction

Athetis lepigone is one of the most severe polyphagous pests, and it has developed resistance to different chemical insecticides. Insects primarily rely on the olfactory system to recognize various environmental chemicals, including xenobiotics such as insecticides. Here, we expressed two A. lepigone pheromone-binding proteins (AlepPBP2 and AlepPBP3), and observed they had higher binding affinities to phoxim than other insecticides, with Ki was 3.30 ± 0.38 μM and 3.27 ± 0.10 μM, respectively. Molecular dynamics simulation, binding mode analysis, and computational alanine scanning showed that six residues (Phe15, Phe39, Ile55, Leu65, Ile97, and Phe122) of AlepPBP2 and three residues (Phe12, Ile52, and Ile134) of AlepPBP3 maybe as potential residues that can change protein ability to bind an organophosphorus insecticide phoxim. Then, we used site-directed mutagenesis assay to mutate these residues into alanine, respectively. Subsequently, the binding assays displayed that Phe15, Phe39, and Ile97 of AlepPBP2, Phe12 and Ile134 of AlepPBP3 caused a significant decrease of AlepPBPs binding ability to phoxim, suggesting they should play crucial roles in the AlepPBPs/phoxim interactions. Our findings could further advance in using PBPs as unique targets to design and develop precise and environmentally-friendly pest control agents with high insecticidal potential using a computer-aided drug design (CADD) approach.

Odorant binding protein 2 reduces imidacloprid susceptibility of Diaphorina citri

The Asian citrus psyllid, Diaphorina citri, is the principal vector of Huanglongbing pathogen Candidatus Liberibacter asiaticus (CLas), which causes severe economic losses to citrus industry worldwide. Use of broad-spectrum insecticides to control D. citri has resulted in considerable resistance development. Specific chemosensory proteins such as odorant binding proteins (OBPs) are potentially involved in reduced insecticide susceptibility. However, functional data on the contribution of OBPs to reduced susceptibility of D. citri are unavailable. We found that DcitOBP2 was stably expressed in different developmental stages and highly expressed in the legs, head and cuticle of D. citri. Expression of DcitOBP2 was significantly induced by 12 to 48 h of imidacloprid exposure and ranged from a 1.34- to 2.44-fold increase. RNAi of DcitOBP2 increased the susceptibility of D. citri adults to imidacloprid. The purified recombinant protein of DcitOBP2 expressed in Escherichia coli showed strong in vitro binding activity (K_d = 62.39 nM) to imidacloprid using microscale thermophoresis technology (MST). DcitOBP2 also had strong binding ability to thiamethoxam and dinotefuran but it had no response to abamectin, fenpropathrin and chlorpyrifos. The results showed that DcitOBP2 can interact with several neonicotinoid insecticides. This suggests that DcitOBP2 is involved in the decreased susceptibility of D. citri to imidacloprid. Our data reveal a new function of insect OBPs as a buffering protein that helps insects survive insecticide exposure. Our investigation may also aid in the development of new methods for resistance management of D. citri.

Odorant-Binding Proteins Contribute to the Defense of the Red Flour Beetle, Tribolium castaneum, Against Essential Oil of Artemisia vulgaris

The function of odorant-binding proteins (OBPs) in insect chemodetection has been extensively studied. However, the role of OBPs in the defense of insects against exogenous toxic substances remains elusive. The red flour beetle, Tribolium castaneum, a major pest of stored grains, causes serious economic losses for the agricultural grain and food processing industries. Here, biochemical analysis showed that essential oil (EO) from Artemisia vulgaris, a traditional Chinese medicine, has a strong contact killing effect against larvae of the red flour beetle. Furthermore, one OBP gene, TcOBPC11, was significantly induced after exposure to EO. RNA interference (RNAi) against TcOBPC11 led to higher mortality compared with the controls after EO treatment, suggesting that this OBP gene is associated with defense of the beetle against EO and leads to a decrease in sensitivity to the EO. Tissue expression profiling showed that expression of TcOBPC11 was higher in the fat body, Malpighian tubule, and hemolymph than in other larval tissues, and was mainly expressed in epidermis, fat body, and antennae from the early adult. The developmental expression profile revealed that expression of TcOBPC11 was higher in late larval stages and adult stages than in other developmental stages. These data indicate that TcOBPC11 may be involved in sequestration of exogenous toxicants in the larvae of T. castaneum. Our results provide a theoretical basis for the degradation mechanism of exogenous toxicants and identify potential novel targets for controlling the beetle.

Insecticidal Activity of Artemisia vulgaris Essential Oil and Transcriptome Analysis of Tribolium castaneum in Response to Oil Exposure

Red flour beetle (Tribolium castaneum) is one of the most destructive pests of stored cereals worldwide. The essential oil (EO) of Artemisia vulgaris (mugwort) is known to be a strong toxicant that inhibits the growth, development, and reproduction of T. castaneum. However, the molecular mechanisms underlying the toxic effects of A. vulgaris EO on T. castaneum remain unclear. Here, two detoxifying enzymes, carboxylesterase (CarEs) and cytochrome oxidase P450 (CYPs), were dramatically increased in red flour beetle larvae when they were exposed to A. vulgaris EO. Further, 758 genes were differentially expressed between EO treated and control samples. Based on Gene Ontology (GO) analysis, numerous differentially expressed genes (DEGs) were enriched for terms related to the regulation of biological processes, response to stimulus, and antigen processing and presentation. Our results indicated that A. vulgaris EO disturbed the antioxidant activity in larvae and partially inhibited serine protease (SP), cathepsin (CAT), and lipase signaling pathways, thus disrupting larval development and reproduction as well as down-regulating the stress response. Moreover, these DEGs showed that A. vulgaris indirectly affected the development and reproduction of beetles by inducing the expression of genes encoding copper-zinc-superoxide dismutase (CuZnSOD), heme peroxidase (HPX), antioxidant enzymes, and transcription factors. Moreover, the majority of DEGs were mapped to the drug metabolism pathway in the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. Notably, the following genes were detected: 6 odorant binding proteins (OBPs), 5 chemosensory proteins (CSPs), 14 CYPs, 3 esterases (ESTs), 5 glutathione S-transferases (GSTs), 6 UDP-glucuronosyltransferases (UGTs), and 2 multidrug resistance proteins (MRPs), of which 8 CYPs, 2 ESTs, 2 GSTs, and 3 UGTs were up-regulated dramatically after exposure to A. vulgaris EO. The residual DEGs were significantly down-regulated in EO exposed larvae, implying that partial compensation of metabolism detoxification existed in treated beetles. Furthermore, A. vulgaris EO induced overexpression of OBP/CYP, and RNAi against these genes significantly increased mortality of larvae exposed to EO, providing further evidence for the involvement of OBP/CYP in EO metabolic detoxification in T. castaneum. Our results provide an overview of the transcriptomic changes in T. castaneum in response to A. vulgaris EO.

A sensory appendage protein protects malaria vectors from pyrethroids

Pyrethroid-impregnated bed nets have driven considerable reductions in malaria-associated morbidity and mortality in Africa since the beginning of the century1. The intense selection pressure exerted by bed nets has precipitated widespread and escalating resistance to pyrethroids in African Anopheles populations, threatening to reverse the gains that been made by malaria control2. Here we show that expression of a sensory appendage protein (SAP2), which is enriched in the legs, confers pyrethroid resistance to Anopheles gambiae. Expression of SAP2 is increased in insecticide-resistant populations and is further induced after the mosquito comes into contact with pyrethroids. SAP2 silencing fully restores mortality of the mosquitoes, whereas SAP2 overexpression results in increased resistance, probably owing to high-affinity binding of SAP2 to pyrethroid insecticides. Mining of genome sequence data reveals a selective sweep near the SAP2 locus in the mosquito populations of three West African countries (Cameroon, Guinea and Burkina Faso) with the observed increase in haplotype-associated single-nucleotide polymorphisms mirroring the increasing resistance of mosquitoes to pyrethroids reported in Burkina Faso. Our study identifies a previously undescribed mechanism of insecticide resistance that is likely to be highly relevant to malaria control efforts.

Transcriptome-based identification and characterization of genes responding to imidacloprid in Myzus persicae

Myzus persicae is a serious and widespread agricultural pest, against which, imidacloprid remains an effective control measure. However, recent reports indicate that this aphid has evolved and developed resistance to imidacloprid. This study aimed to elucidate the underlying mechanisms and genetic basis of this resistance by conducting comparative transcriptomics studies on both imidacloprid-resistant (IR) and imidacloprid-susceptible (IS) M. persicae. The comparative analysis identified 252 differentially expressed genes (DEGs) among the IR and IS M. persicae transcriptomes. These candidate genes included 160 and 92 genes that were down- and up-regulated, respectively, in the imidacloprid-resistant strain. Using functional classification in the GO and KEGG databases, 187 DEGs were assigned to 303 functional subcategories and 100 DEGs were classified into 45 pathway groups. Moreover, several genes were associated with known insecticide targets, cuticle, metabolic processes, and oxidative phosphorylation. Quantitative real-time PCR of 10 DEGs confirmed the trends observed in the RNA sequencing expression profiles. These findings provide a valuable basis for further investigation into the complicated mechanisms of imidacloprid resistance in M. persicae.

Latrophilin participates in insecticide susceptibility through positively regulating CSP10 and partially compensated by OBPC01 in Tribolium castaneum

Latrophilin (LPH) is an adhesion G protein-coupled receptor (aGPCR) that participates in multiple essential physiological processes. Our previous studies have shown that lph is not only indispensable for the development and reproduction of red flour beetles (Tribolium castaneum), but also for their resistance against dichlorvos or carbofuran insecticides. However, the regulatory mechanism of lph-mediated insecticide susceptibility remains unclear. Here, we revealed that knockdown of lph in beetles resulted in opposing changes in two chemoreception genes, chemosensory protein 10 (CSP10) and odorant-binding protein C01 (OBPC01), in which the expression of TcCSP10 was downregulated, whereas the expression of TcOBPC01 was upregulated. TcCSP10 and TcOBPC01 were expressed at the highest levels in early pupal and late larval stages, respectively. High levels of expression of both these genes were observed in the heads (without antennae) of adults. TcCSP10 and TcOBPC01 were significantly induced by dichlorvos or carbofuran between 12 and 72 h (hrs) after exposure, suggesting that they are likely associated with increasing the binding affinity of insecticides, leading to a decrease in sensitivity to the insecticides. Moreover, once these two genes were knocked down, the susceptibility of the beetles to dichlorvos or carbofuran was enhanced. Additionally, RNA interference (RNAi) targeting of lph followed by exposure to dichlorvos or carbofuran also caused the opposing expression levels of TcCSP10 and TcOBPC01 compared to the expression levels of wild-type larvae treated with insecticides alone. All these results indicate that lph is involved in insecticide susceptibility through positively regulating TcCSP10; and the susceptibility could also further partially compensated for through the negative regulation of TcOBPC01 when lph was knockdown in the red flour beetle. Our studies shed new light on the molecular regulatory mechanisms of lph related to insecticide susceptibility.

Mosquitoes cloak their legs to resist insecticides

Malaria incidence has halved since the year 2000, with 80% of the reduction attributable to the use of insecticides. However, insecticide resistance is now widespread, is rapidly increasing in spectrum and intensity across Africa, and may be contributing to the increase of malaria incidence in 2018. The role of detoxification enzymes and target site mutations has been documented in the major malaria vector Anopheles gambiae; however, the emergence of striking resistant phenotypes suggests the occurrence of additional mechanisms. By comparing legs, the most relevant insect tissue for insecticide uptake, we show that resistant mosquitoes largely remodel their leg cuticles via enhanced deposition of cuticular proteins and chitin, corroborating a leg-thickening phenotype. Moreover, we show that resistant female mosquitoes seal their leg cuticles with higher total and different relative amounts of cuticular hydrocarbons, compared with susceptible ones. The structural and functional alterations in Anopheles female mosquito legs are associated with a reduced uptake of insecticides, substantially contributing to the resistance phenotype.

Binding affinity characterization of an antennae-enriched chemosensory protein from the white-backed planthopper, Sogatella furcifera (Horváth), with host plant volatiles

The white-backed planthopper (WBPH) Sogatella furcifera is a notorious rice pest in Asia. Olfaction is crucial for the WBPH to seek and locate rice plants. However, its mechanism is still not fully understood. Chemosensory proteins (CSPs) are some of the important olfactory-related proteins. In this study, we first used a bacterial system to successfully express the recombinant, antennae-enriched protein SfurCSP5. Further, competitive fluorescence binding assays with 86 candidate ligands, including some known rice plant volatiles, showed that SfurCSP5 has high affinities for 2-tridecanone, 2-pentadecanone, and β-ionone, which are known to be present in volatile mixtures that can attract rice planthoppers, and produced Ki values of 4.89, 4.09, and 1.39 μmol/L, respectively. Additionally, homology modeling of the protein structure of SfurCSP5 showed that it possesses five α-helixes (α-1, α-2, α-3, α-4, and α-5), which is a non-typical feature of the insect CSPs. Finally, ligand docking results revealed that Leu-44, Ile-64, Phe-90, Trp-98, and Phe-101 are five hydrophobic residues that interact with all of the ligands, indicating their key involvement in the binding of SfurCSP5. Our study lays the foundation for an understanding of the olfaction mechanism of rice planthoppers.

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.

Characterization and functional analysis of a carboxylesterase gene associated with chlorpyrifos resistance in Nilaparvata lugens (Stål)

The widespread and extensive application of insecticides have promoted the development of resistance in the brown planthopper Nilaparvata lugens (Stål), one of the most important rice pests in Asia. To better understand the underlying molecular mechanisms of metabolic resistance to insecticides, a chlorpyrifos-resistant (CR) strain of N. lugens was selected and its possible resistance mechanism was investigated. Synergistic tests using carboxylesterases (CarEs) inhibitor triphenyl phosphate (TPP) decreased the resistance of N. lugens to chlorpyrifos, and CarE activities could be induced by low concentrations of chlorpyrifos. Subsequently, a gene putatively encoding CarE, namely NlCarE, predominant in the midgut and ovary was isolated and characterized. The expression levels of NlCarE were detected and compared between the CR and a susceptible (SS) strain of N. lugens. Consistent with the increased CarE activity, this gene was overexpressed in the CR strain compared to the SS strain. The transcript levels of NlCarE were up-regulated by chlorpyrifos exposure, showing dose- and time-dependent expression patterns. Furthermore, RNA interference (RNAi)-mediated knockdown of NlCarE followed by insecticide application significantly increased the susceptibility of N. lugens to chlorpyrifos. These results demonstrate that NlCarE plays an important role in chlorpyrifos detoxification and its overexpression may be involved in chlorpyrifos resistance in N. lugens.