Identification, Expression Patterns, and Functional Characterization of Chemosensory Proteins in Dendroctonus armandi (Coleoptera: Curculionidae: Scolytinae)

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.

Double-stranded RNA (dsRNA) technology to control forest insect pests and fungal pathogens: challenges and opportunities

Climate change alters the seasonal synchronization between plants and respective pests plus pathogens. The geographical infiltration helps to shift their hosts, resulting in novel outbreaks that damage forests and ecology. Traditional management schemes are unable to control such outbreaks, therefore unconventional and competitive governance is needed to manage forest pests and pathogens. RNA interference (RNAi) mediated double-stranded RNA (dsRNA) treatment method can be implemented to protect forest trees. Exogenous dsRNA triggers the RNAi-mediated gene silencing of a vital gene, and suspends protein production, resulting in the death of targeted pathogens and pests. The dsRNA treatment method is successful for many crop insects and fungi, however, studies of dsRNA against forest pests and pathogens are depleting. Pesticides and fungicides based on dsRNA could be used to combat pathogens that caused outbreaks in different parts of the world. Although the dsRNA has proved its potential, the crucial dilemma and risks including species-specific gene selection, and dsRNA delivery methods cannot be overlooked. Here, we summarized the major fungi pathogens and insect pests that have caused outbreaks, their genomic information, and studies on dsRNA fungi-and pesticides. Current challenges and opportunities in dsRNA target decision, delivery using nanoparticles, direct applications, and a new method using mycorrhiza for forest tree protection are discussed. The importance of affordable next-generation sequencing to minimize the impact on non-target species is discussed. We suggest that collaborative research among forest genomics and pathology institutes could develop necessary dsRNA strategies to protect forest tree species.

Odorant-Binding and Chemosensory Proteins in Anthonomus eugenii (Coleoptera: Curculionidae) and Their Tissue Expression

The pepper weevil Anthonomus eugenii is one of the most damaging pests to the pepper crop. To offer alternative management strategies to insecticides, several studies have identified the semiochemicals that are involved in the pepper weevil's aggregation and mating behavior; however, there is no information on its perireceptor molecular mechanism, to date. In this study, bioinformatics tools were used to functionally annotate and characterize the A. eugenii head transcriptome and their probable coding proteins. We identified twenty-two transcripts belonging to families related to chemosensory processes, seventeen corresponding to odorant-binding proteins (OBP), and six to chemosensory proteins (CSP). All results matched with closely related Coleoptera: Curculionidae homologous proteins. Likewise, twelve OBP and three CSP transcripts were experimentally characterized by RT-PCR in different female and male tissues. The results by sex and tissue display the different expression patterns of the AeugOBPs and AeugCSPs; some are present in both sexes and all tissues, while others show expressions with higher specificity, which suggests diverse physiological functions in addition to chemo-detection. This study provides information to support the understanding of odor perception in the pepper weevil.

Roles of Krüppel Homolog 1 and Broad-Complex in the Development of Dendroctonus armandi (Coleoptera: Scolytinae)

In insects, metamorphosis is controlled by juvenile hormone (JH) and 20-hydroxyecdysone (20E). Krüppel homolog 1 (Kr-h1), a key JH-early inducible gene, is responsible for the suppression of metamorphosis and the regulation of the Broad-Complex (Br-C) gene, which is induced by 20E and functions as a “pupal specifier”. In this study, we identified and characterized the expression patterns and tissue distribution of DaKr-h1 and DaBr-C at various developmental stages of Dendroctonus armandi. The expression of the two genes was induced by JH analog (JHA) methoprene and 20E, and their functions were investigated by RNA interference. DaKr-h1 and DaBr-C were predominantly expressed in the heads of larvae and were significantly downregulated during the molting stage. In contrast, the DaKr-h1 transcript level was highest in the adult anterior midgut. DaBr-C was mainly expressed in female adults, with the highest transcript levels in the ovaries. In the larval and pupal stages, both JHA and 20E significantly induced DaKr-h1, but only 20E significantly induced DaBr-C, indicating the importance of hormones in metamorphosis. DaKr-h1 knockdown in larvae upregulated DaBr-C expression, resulting in precocious metamorphosis from larvae to pupae and the formation of miniature pupae. DaKr-h1 knockdown in pupae suppressed DaBr-C expression, increased emergence, caused abnormal morphology, and caused the formation of small-winged adults. These results suggest that DaKr-h1 is required for the metamorphosis of D. armandi. Our findings provide insight into the roles of DaKr-h1 and DaBr-C in JH-induced transcriptional repression and highlight DaKr-h1 as a potential target for metamorphosis suppression in D. armandi.

Identification and Functional Characterization of Antifreeze Protein and Its Mutants in Dendroctonus armandi (Coleoptera: Curculionidae: Scolytinae) Larvae Under Cold Stress

Dendroctonus armandi (Tsai and Li) (Coleoptera: Curculionidae: Scolytinae) is considered to be the most destructive forest pest in the Qinling and Bashan Mountains of China. Low winter temperatures limit insect's populations, distribution, activity, and development. Insects have developed different strategies such as freeze-tolerance and freeze-avoidance to survive in low temperature conditions. In the present study, we used gene cloning, real-time polymerase chain reaction (PCR), RNA interference (RNAi), and heterologous expression to study the function of the D. armandi antifreeze protein gene (DaAFP). We cloned the 800 bp full-length cDNA encoding 228 amino acids of DaAFP and analyzed its structure using bioinformatics analysis. The DaAFP amino acid sequence exhibited 24-86% similarity with other insect species. The expression of DaAFP was high in January and in the larvae, head, and midgut of D. armandi. In addition, the expression of DaAFP increased with decreasing temperature and increasing exposure time. RNAi analysis also demonstrated that AFP plays an important role in the cold tolerance of overwintering larvae. The thermal hysteresis and antifreeze activity assay of DaAFP and its mutants indicated that the more regular the DaAFP threonine-cystine-threonine (TXT) motif, the stronger the antifreeze activity. These results suggest that DaAFP plays an essential role as a biological cryoprotectant in overwintering D. armandi larvae and provides a theoretical basis for new pest control methods.

Antennal transcriptome analysis of chemosensory genes in the cowpea beetle, Callosobruchus maculatus (F.)

Olfaction, one of the most important sensory systems governing insect behavior, is a possible target for pest management. Therefore, in this study, we analyzed the antennal transcriptome of the cowpea beetle, Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae: Bruchinae), which is a major pest of stored pulses and legumes. The de novo antennal RNA-seq assembly results identified 17 odorant, 2 gustatory, and 10 ionotropic receptors, 1 sensory neuron membrane protein, and 12 odorant-binding and 7 chemosensory proteins. Moreover, differential gene expression analysis of virgin male and female antennal samples followed by qRT-PCR revealed 1 upregulated and 4 downregulated odorant receptors in males. We also performed homology searches using the coding sequences built from previously proposed amino acid sequences derived from genomic data and identified additional chemosensory-related genes.

Involvement of chemosensory proteins in host plant searching in the bird cherry-oat aphid

Chemosensory systems are considered to play an important role in host plant selection in herbivorous insects. However, few studies have focused on chemosensory proteins (CSPs) for aphid host-location mechanisms. The roles of CSPs in searching for different Poaceae species (wheat, barley, triticale, maize and sorghum) were tested in Rhopalosiphum padi, an important cereal pest. The olfactometer assays showed that R. padi responds to plant odors. Seven R. padi CSP genes were identified. Influence of aphid morph, tissue and starvation state on expression patterns of CSPs was evaluated. Expression levels of CSP1, CSP4, CSP5 and CSP6 in winged aphids were significantly higher than those in wingless ones. Transcription levels of four genes (CSP1, CSP4, CSP5 and CSP6) were relatively higher in the head with antennae, and the four genes tended to be upregulated following starvation. Silencing of three CSPs (CSP4, CSP5 and CSP6) altered aphid host-location behavior in response to the five different host plants tested. Three volatile compounds of host plants (octanal, [E]-2-hexenol and linalool) have significant attraction to winged R. padi according to the four-arm olfactometer tests. Molecular docking predicted hydrogen bonding sites which played key roles in the binding of CSP4, CSP5 and CSP6 with volatile compounds. Knockdown of CSP4 or CSP5 significantly decreased the staying time of R. padi in the arms with octanal. However, knockdown of CSP6 could not affect the response of R. padi to octanal. These results bring evidence for the involvement of three CSPs in R. padi host-location behavior.

RNA Interference-Based Forest Protection Products (FPPs) Against Wood-Boring Coleopterans: Hope or Hype?

Forest insects are emerging in large extension in response to ongoing climatic changes, penetrating geographic barriers, utilizing novel hosts, and influencing many hectares of conifer forests worldwide. Current management strategies have been unable to keep pace with forest insect population outbreaks, and therefore novel and aggressive management strategies are urgently required to manage forest insects. RNA interference (RNAi), a Noble Prize-winning discovery, is an emerging approach that can be used for forest protection. The RNAi pathway is triggered by dsRNA molecules, which, in turn, silences genes and disrupts protein function, ultimately causing the death of the targeted insect. RNAi is very effective against pest insects; however, its proficiency varies significantly among insect species, tissues, and genes. The coleopteran forest insects are susceptible to RNAi and can be the initial target, but we lack practical means of delivery, particularly in systems with long-lived, endophagous insects such as the Emerald ash borer, Asian longhorn beetles, and bark beetles. The widespread use of RNAi in forest pest management has major challenges, including its efficiency, target gene selection, dsRNA design, lack of reliable dsRNA delivery methods, non-target and off-target effects, and potential resistance development in wood-boring pest populations. This review focuses on recent innovations in RNAi delivery that can be deployed against forest pests, such as cationic liposome-assisted (lipids), nanoparticle-enabled (polymers or peptides), symbiont-mediated (fungi, bacteria, and viruses), and plant-mediated deliveries (trunk injection, root absorption). Our findings guide future risk analysis of dsRNA-based forest protection products (FPPs) and risk assessment frameworks incorporating sequence complementarity-based analysis for off-target predictions. This review also points out barriers to further developing RNAi for forest pest management and suggests future directions of research that will build the future use of RNAi against wood-boring coleopterans.

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.

Genomic content of chemosensory genes correlates with host range in wood-boring beetles (Dendroctonus ponderosae, Agrilus planipennis, and Anoplophora glabripennis)

Background

Olfaction and gustation underlie behaviors that are crucial for insect fitness, such as host and mate selection. The detection of semiochemicals is mediated via proteins from large and rapidly evolving chemosensory gene families; however, the links between a species’ ecology and the diversification of these genes remain poorly understood. Hence, we annotated the chemosensory genes from genomes of select wood-boring coleopterans, and compared the gene repertoires from stenophagous species with those from polyphagous species.

Results

We annotated 86 odorant receptors (ORs), 60 gustatory receptors (GRs), 57 ionotropic receptors (IRs), 4 sensory neuron membrane proteins (SNMPs), 36 odorant binding proteins (OBPs), and 11 chemosensory proteins (CSPs) in the mountain pine beetle (Dendroctonus ponderosae), and 47 ORs, 30 GRs, 31 IRs, 4 SNMPs, 12 OBPs, and 14 CSPs in the emerald ash borer (Agrilus planipennis). Four SNMPs and 17 CSPs were annotated in the polyphagous wood-borer Anoplophora glabripennis. The gene repertoires in the stenophagous D. ponderosae and A. planipennis are reduced compared with those in the polyphagous A. glabripennis and T. castaneum, which is largely manifested through small gene lineage expansions and entire lineage losses. Alternative splicing of GR genes was limited in D. ponderosae and apparently absent in A. planipennis, which also seems to have lost one carbon dioxide receptor (GR1). A. planipennis has two SNMPs, which are related to SNMP3 in T. castaneum. D. ponderosae has two alternatively spliced OBP genes, a novel OBP “tetramer”, and as many as eleven IR75 members. Simple orthology was generally rare in beetles; however, we found one clade with orthologues of putative bitter-taste GRs (named the “GR215 clade”), and conservation of IR60a from Drosophila melanogaster.

Conclusions

Our genome annotations represent important quantitative and qualitative improvements of the original datasets derived from transcriptomes of D. ponderosae and A. planipennis, facilitating evolutionary analysis of chemosensory genes in the Coleoptera where only a few genomes were previously annotated. Our analysis suggests a correlation between chemosensory gene content and host specificity in beetles. Future studies should include additional species to consolidate this correlation, and functionally characterize identified proteins as an important step towards improved control of these pests.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-6054-x) contains supplementary material, which is available to authorized users.

Identification, Expression Patterns and RNA Interference of Aquaporins in Dendroctonus armandi (Coleoptera: Scolytinae) Larvae During Overwintering

The ability to survive annual temperature minima could be a key determinant of distribution limits for insects under global climate change. Recent studies have suggested that insect aquaporins are indispensable for cellular water management under conditions that lead to dehydration and cold stress. Aquaporins are integral membrane water channel proteins in the major intrinsic protein superfamily and promote selected solutes and the movement of water across biological membranes. We cloned and characterized nine full-length aquaporins from Dendroctonus armandi (DaAqps), the most destructive forest pest in the Qinling Mountains of Shaanxi Province, China. Eight of the DaAqps belong to three classical aquaporin grades, including the Drosophila integral protein, the Pyrocoelia rufa integral protein, the entomoglyceroporins and one that belongs to the unorthodox grade of aquaporin 12-like channels. The DaAqps were increasingly expressed during different developmental stages and in different larval tissues, and expression peaked in mid-winter. They were tested under cold conditions for different lengths of time, and the expression of almost all DaAqps was down regulated with decreasing temperatures and long-term exposure to cold conditions. However, when the lowest temperatures were reached, the levels were immediately upregulated. These genes indicate that cold tolerance can improve through mortality responses at low temperatures after RNA interference of DaAqps. In our study, we analyzed the molecular response, expression patterns, and RNA interference of DaAqps and clarified the crucial role of protective compounds (aquaporins) underlying D. armandi cold tolerance and provide a new pest control method.

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.