Overexpression of two α-esterase genes mediates metabolic resistance to malathion in the oriental fruit fly, Bactrocera dorsalis (Hendel)

Two carboxyl/choline esterase (CCE) genes, TuCCE11 and TuCCE34, are related to abamectin resistance in Tetranychus urticae Koch

Tetranychus urticae is a widespread agricultural pest that exhibits highly intractable resistance to multiple acaricides, particularly abamectin. The rapid and complex evolution of insecticide resistance, along with the molecular mechanisms of abamectin resistance in T. urticae, are not fully elucidated, representing a significant impediment to the effective management of pest resistance. Carboxyl/choline esterase (CCE) is a member of the serine hydrolases involved in the initial phase of pesticide detoxification metabolism. In this study, 51 T. urticae CCE genes were identified based on the transcriptome and genome data of T. urticae strains from China. The transcript expressions of some CCE genes were significantly upregulated in resistant populations in comparison to the susceptible strain. Moreover, transcript levels were observed to increase in T. urticae following exposure to abamectin. Additionally, RNA interference (RNAi)-mediated silencing of TuCCE11 and TuCCE34 increased susceptibility to abamectin in T. urticae. The molecular docking analysis revealed that the higher binding energy of abamectin to TuCCE11 and TuCCE34 was -9.01 and -9.00 kcal/mol, respectively. In conclusion, the results of the present study indicate that TuCCE11 and TuCCE34 may play a role in the development of abamectin resistance in T. urticae.

Increased expression of an isoform of the long non-coding RNA, lnc37707, is associated with malathion resistance in Bactrocera dorsalis

The oriental fruit fly, Bactrocera dorsalis (Hendel) is an invasive pest threatening global fruit industries. Field populations of B. dorsalis exhibit complex insecticide resistance, hindering pest control efforts and exacerbating damage. Long non-coding RNAs (lncRNAs) are critical regulators of multiple bioprocess in insects, including insecticide resistance, and have potentials as novel target for pest management. Here, the candidate lncRNAs associated with malathion resistance in B. dorsalis were identified through RNA-seq. One of the isoforms of lnc37707, designated as lnc37707.10, was significantly enriched in the detoxification tissues of malathion-resistant (MR) strain. A specific fragment of lnc37707.10 (sflnc37707) was strongly associated with malathion resistance, and silencing sflnc37707 increased the susceptibility, whereas overexpressing it decreased susceptibility to malathion. Silencing sflnc37707 resulted in the down-regulation of 248 genes, but none of them included the four adjacent genes as its potential target. Instead, pathway analysis revealed significant enrichment of down-regulated genes involved in drug and xenobiotics metabolism, including P450s and GSTs. Bioinformatic analysis suggested a potential regulatory role of miRNA in the function of lnc37707. Further combining silence or overexpression sflnc37707 with miRNA mimic treatment identified that BdGSTd10 (an important gene involved in malathion resistance) and miR-1000 was strongly linked to lnc37707.10. Finally, a ceRNA (competing endogenous RNA) regulatory axis was proposed, where lnc37707.10 might indirectly modulate BdGSTd10 by sponging miR-1000 to regulate the malathion resistance in B. dorsalis. These findings provide a new insight into insecticide resistance and a potential lncRNA target for the sustainable pest management.

Detoxification of Chlorfenapyr by a Parkin-GSTd2 Module in Bactrocera dorsalis (Diptera: Tephritidae)

The oriental fruit fly, Bactrocera dorsalis (Hendel), is a highly invasive and destructive pest. Chlorfenapyr is a widely used insecticide that disrupts mitochondrial activity. The Parkin protein plays conserved roles in maintaining mitochondrial homeostasis, but the role of Parkin in response to chlorfenapyr remains largely unknown. Here, we report that BdParkin is required for chlorfenapyr detoxification, and dsRNA targeting BdParkin improves the insecticidal efficacy of chlorfenapyr. Among the genes whose expression levels are affected by BdParkin RNAi, knock-down of the glutathione S-transferase gene BdGSTd2 increases the insecticidal efficacy of chlorfenapyr. Molecular docking reveals potential interactions between BdGSTd2 and tralopyril, an insecticidal metabolite of chlorfenapyr. These results suggest that BdParkin could impact the response of B. dorsalis to chlorfenapyr through metabolic processes regulated by BdGSTd2. Our findings could offer new insights into how insects detoxify chlorfenapyr and provide molecular targets for developing a sustainable management strategy for B. dorsalis.

Characterization of Four Carboxylesterases Involved in Detoxification of β-Cypermethrin, λ-Cyhalothrin, and Malathion in Helicoverpa armigera

Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) is a globally devastating pest and has evolved with varying levels of resistance to a broader spectrum of insecticides. CarEs are major enzymes involved in insecticide detoxification and metabolic resistance. Four CarE genes were identified and cloned from H. armigera, and the expression pattern analyses indicated that they were predominantly expressed in the detoxifying tissues and larval feeding periods. The insecticide inductive assays further suggested that these CarE genes with expressions were significantly upregulated after beta-cypermethrin, lambda-cyhalothrin, malathion, and chlorpyrifos exposures. The purified recombinant proteins of both CarE016B and CarE016C by bacterial expression exhibited significantly higher catalytic efficiency toward α-naphthyl acetate and also showed relatively stronger binding with both β-cypermethrin and λ-cyhalothrin compared to the CarE006C and CarE015A. In vitro metabolism assay with HPLC suggests that CarE016B and CarE016C have the ability to metabolize β-cypermethrin, λ-cyhalothrin, and malathion with varying hydrolase activities. GC-MS/MS analysis identified 3-phenoxybenzaldehyde (3-PBAld) as the metabolite of both β-cypermethrin and λ-cyhalothrin. Homology modeling and molecular docking analyses further indicate that these three types of insecticides could be anchored into the active pocket of both CarEs. Collectively, these results demonstrate that both CarE016B and CarE016C play a critical role in the detoxification of pyrethroid and organophosphate insecticides in H. armigera. This study provides the foundations for a comprehensive understanding of the role of the CarEs family in insecticide detoxification and resistance in H. armigera.

SlCarE054 in Spodoptera litura (Lepidoptera: Noctuidae) showed direct metabolic activity to β-cypermethrin with stereoselectivity

Carboxylesterases (CarEs) is an important detoxification enzyme system in phase Ⅰ participating in insecticides resistance. In our previous study, SlCarE054, a CarEs gene from lepidoptera class, was screened out to be upregulated in a pyrethroids and organophosphates resistant population. Its overexpression was verified in two field-collected populations of Spodoptera litura (Lepidoptera: Noctuidae) resistant to pyrethroids and organophosphates by qRT-PCR. Spatiotemporal expression results showed that SlCarE054 was highly expressed in the pupae stage and the digestive tissue midgut. To further explore its role in pyrethroids and organophosphates resistance, its metabolism activity to insecticides was determined by UPLC. Its recombinant protein showed significant metabolism activity to cyhalothrin and fenvalerate, but not to phoxim or chlorpyrifos. The metabolic activity of SlCarE054 to β-cypermethrin showed stereoselectivity, with higher metabolic activity to θ-cypermethrin than the enantiomer α-cypermethrin. The metabolite of β-cypermethrin was identified as 3-phenoxybenzaldehyde. Further modelling and docking analysis indicated that β-cypermethrin, cyhalothrin and fenvalerate could bind with the catalytic triad of the 3D structure of SlCarE054. The interaction of β-cypermethrin with SlCarE054 also showed the lowest binding energy. Our work provides evidence that SlCarE054 play roles in β-cypermethrin resistance in S. litura.

Overexpression of an Integument Esterase Gene LbEST-inte4 Infers the Malathion Detoxification in Liposcelis bostrychophila (Psocoptera: Liposcelididae)

Liposcelis bostrychophila, commonly known as booklouse, is an important stored-product pest worldwide. Studies have demonstrated that booklices have developed resistance to several insecticides. In this study, an integument esterase gene, LbEST-inte4, with upregulated expression, was characterized in L. bostrychophila. Knockdown of LbEST-inte4 resulted in a substantial increase in the booklice susceptibility to malathion. Overexpression of LbEST-inte4 in Drosophila melanogaster significantly enhanced its malathion tolerance. Molecular modeling and docking analysis suggested potential interactions between LbEST-inte4 and malathion. When overexpressed LbEST-inte4 in Sf9 cells, a notable elevation in esterase activity and malathion tolerance was observed. HPLC analysis indicated that the LbEST-inte4 enzyme could effectively degrade malathion. Taken together, the upregulated LbEST-inte4 appears to contribute to malathion tolerance in L. bostrychophila by facilitating the depletion of malathion. This study elucidates the molecular mechanism underlying malathion detoxification and provides the foundations for the development of effective prevention and control measures against psocids.

Carboxylesterase and Cytochrome P450 Confer Metabolic Resistance Simultaneously to Azoxystrobin and Some Other Fungicides in Botrytis cinerea

Plant pathogens have frequently shown multidrug resistance (MDR) in the field, often linked to efflux and sometimes metabolism of fungicides. To investigate the potential role of metabolic resistance in B. cinerea strains showing MDR, the azoxystrobin-sensitive strain B05.10 and -resistant strain Bc242 were treated with azoxystrobin. The degradation half-life of azoxystrobin in Bc242 (9.63 days) was shorter than that in B05.10 (28.88 days). Azoxystrobin acid, identified as a metabolite, exhibited significantly lower inhibition rates on colony and conidia (9.34 and 11.98%, respectively) than azoxystrobin. Bc242 exhibited higher expression levels of 34 cytochrome P450s (P450s) and 11 carboxylesterase genes (CarEs) compared to B05.10 according to RNA-seq analysis. The expression of P450 genes Bcin_02g01260 and Bcin_12g06380, along with the CarEs Bcin_12g06360 in Saccharomyces cerevisiae, resulted in reduced sensitivity to various fungicides, including azoxystrobin, kresoxim-methyl, pyraclostrobin, trifloxystrobin, iprodione, and carbendazim. Thus, the mechanism of B. cinerea MDR is linked to metabolism mediated by the CarE and P450 genes.

Functional analysis of an overexpressed glutathione S-transferase BdGSTd5 involved in malathion and malaoxon detoxification in Bactrocera dorsalis

Glutathione S-transferases (GSTs) are one of the three detoxification enzyme families. The constitutive and inducible overexpression of GSTs genes plays an important role in insecticide resistance. Previous study showed that malathion resistance was polygenic, and elevated GSTs activity was one of the important factor participating in malathion resistance of Bactrocera dorsalis (Hendel), a serious economic pest worldwide. BdGSTd5 overexpression was inducible upon exposure to malathion. However, the involvement of BdGSTd5 in malathion resistance has not been clarified. In this study, we found that BdGSTd5 sequence harbored the conserved region of delta class GSTs, which were overexpressed in malathion resistant strain of B. dorsalis compared to malathion susceptible strain. The highest mRNA expression level of BdGSTd5 was found in 1-day-old adult, and the levels decreased with aging. The dsBdGSTd5 injection effectively silenced (73.4% reduction) the expression of BdGSTd5 and caused significant increase in susceptibility to malathion with a cumulative mortality increasing of 13.5% at 72 h post malathion treatment (p < 0.05). Cytotoxicity assay demonstrated that BdGSTd5 was capable of malathion detoxification. Molecular docking analysis further indicated the interactive potential of BdGSTd5 with malathion and its toxic oxide malaoxon. The recombinant BdGSTd5 exhibited glutathione-conjugating activity toward 1-chloro-2, 4-dinitrobenzene and malathion and malaoxon metabolic capacity with significant reduction (p < 0.05) of the peak areas by 90.0% and 73.1%, respectively. Taken together, the overexpressed BdGSTd5 contributes to malathion metabolism and resistance, which detoxify the malathion in B. dorsalis via directly depleting malathion and malaoxon.

Molecular identification of carboxylesterase genes and their potential roles in the insecticides susceptibility of Grapholita molesta

Grapholita molesta is one of the most damaging pests worldwide in stone and pome fruits. Application of chemical pesticides is still the main method to control this pest, which results in resistance to several types of insecticides. Carboxylesterase (CarE) is one of the important enzymes involved in the detoxification metabolism and tolerance of xenobiotics and insecticides. However, the roles of CarEs in insecticides susceptibility of G. molesta are still unclear. In the present study, the enzyme activity of CarEs and the mRNA expression of six CarE genes were consistently elevated after treatment with three insecticides (emamectin benzoate, lambda-cyhalothrin, and chlorantraniliprole). According to spatio-temporal expression profiles, six CarE genes expressed differently in different developmental stages, and highly expressed in some detoxification metabolic organs. RNAi-mediated knockdown of these six CarE genes indicated that the susceptibility of G. molesta to all these three insecticides were obviously raised after GmCarE9, GmCarE14, GmCarE16, and GmCarE22 knockdown, respectively. Overall, these results demonstrated that GmCarE9, GmCarE14, GmCarE16, and GmCarE22 play a role in the susceptibility of G. molesta to emamectin benzoate, lambda-cyhalothrin, and chlorantraniliprole treatment. This study expands our understanding of CarEs in insects, that the same CarE gene could participate in the susceptibility to different insecticides.

Identification, expression profiles and involvement in insecticides tolerance and detoxification of carboxylesterase genes in Bactrocera dorsalis

Carboxylesterases (CarEs) are a multifunctional superfamily of enzymes and play an important role in detoxification of various insecticides in insects. The oriental fruit fly, Bactrocera dorsalis, is one of the most destructive agricultural pests and has developed different degrees of resistance to organophosphates in field. However, the involvement of BdCarEs in tolerance or resistance to other alternative insecticides are still unclear. In the present study, 33 BdCarEs genes were identified based on the genome database of B. dorsalis. Phylogenetic analysis demonstrated that they were classified into nine clades, with abundance of α-esterases. Meanwhile, the sequence characterization and the chromosome distribution were also analyzed. The spatiotemporal expression analysis of BdCarEs genes suggested that the diversity of potential function in different physiological processes. With the exception of BdCarE21, all BdCarEs genes responded to at least one insecticide exposure, and BdCarE20 was found to be up-regulated after exposure to all five tested insecticides individually. Eight BdCarEs genes were overexpressed in MR strain when compared to that in SS strain. Subsequently, knockdown the expression of representative BdCarEs genes significantly increased the susceptibility of the oriental fruit fly to corresponding insecticides, which indicated that the tested BdCarEs genes contributed to one or multiple insecticide detoxification. These findings provide valuable insights into the potential role in respond to tolerance or resistance to insecticides with different mode of action, and will facilitate development of efficiency management strategy for B. dorsalis.

Comparative expression profiles of carboxylesterase orthologous CXE14 in two closely related tea geometrid species, Ectropis obliqua Prout and Ectropis grisescens Warren

Insect carboxylesterases (CXEs) can be expressed in multiple tissues and play crucial roles in detoxifying xenobiotic insecticides and degrading olfactory cues. Therefore, they have been considered as an important target for development of eco-friendly insect pest management strategies. Despite extensive investigation in most insect species, limited information on CXEs in sibling moth species is currently available. The Ectropis obliqua Prout and Ectropis grisescens Warren are two closely related tea geometrid species, which share the same host of tea plant but differ in geographical distribution, sex pheromone composition, and symbiotic bacteria abundance, providing an excellent mode species for studies of functional diversity of orthologous CXEs. In this study, we focused on EoblCXE14 due to its previously reported non-chemosensory organs-biased expression. First, the EoblCXE14 orthologous gene EgriCXE14 was cloned and sequence characteristics analysis showed that they share a conserved motif and phylogenetic relationship. Quantitative real-time polymerase chain reaction (qRT-PCR) was then used to compare the expression profiles between two Ectropis spp. The results showed that EoblCXE14 was predominately expressed in E. obliqua larvae, whereas EgriCXE14 was abundant in E. grisescens at multiple developmental stages. Interestingly, both orthologous CXEs were highly expressed in larval midgut, but the expression level of EoblCXE14 in E. obliqua midgut was significantly higher than that of EgriCXE14 in E. grisescens midgut. In addition, the potential effect of symbiotic bacteria Wolbachia on the CXE14 was examined. This study is the first to provide comparative expression profiles of orthologous CXE genes in two sibling geometrid moth species and the results will help further elucidate CXEs functions and identify a potential target for tea geometrid pest control.

Chromosome-level genome assembly reveals potential epigenetic mechanisms of the thermal tolerance in the oriental fruit fly, Bactrocera dorsalis

The oriental fruit fly, Bactrocera dorsalis (Hendel), has very strong ecological adaptability and phenotypic plasticity. Here, the genome of B. dorsalis was assembled into 549.45 Mb sequences with a contig N50 length of 12.81 Mb. Among, 95.67 % assembled genome sequences were anchored on six chromosomes with an N50 length of 94.63 Mb. According to the basic characteristics of the sex chromosomes of Tephritidae, the X chromosome of B. dorsalis was identified. Significant gene expansions were detected in several important gene families related to adaptability. In particular, we annotated 50 histone modification enzymes (HMEs) in this genome. A comparative transcriptome analysis indicated that 12 HME genes were differentially expressed in two thermo-tolerant strains (heat and cold). Interestingly, four and seven of the 12 HME genes responded to heat shock or cold hardening, respectively. These evidences suggested that the histone modification as an epigenetic modification may be involved in the thermal tolerance of B. dorsalis, but with different regulation mechanisms in thermal acclimation and hardening. The high quality genome of B. dorsalis provides an invaluable resource for further functional genomic study. Moreover, comparative genomic analysis will shed insights on revealing the mechanisms of adaptive evolution in this fly.

Molecular Mechanisms Underlying Metabolic Resistance to Cyflumetofen and Bifenthrin in Tetranychus urticae Koch on Cowpea

Tetranychus urticae Koch (T. urticae) is one of the most tremendous herbivores due to its polyphagous characteristics, and is resistant to most acaricides. In this study, enzyme-linked immunosorbent assay (ELISA), transcriptome sequencing (RNA-seq) and quantitative real-time PCR (qRT-PCR) were carried out to analyze the mechanisms of T. urticae metabolic resistance to cyflumetofen and bifenthrin on cowpea. The enzyme activity of UDP-glucuronosyltransferases (UGTs) and carboxylesterases (CarEs) in the cyflumetofen-resistant (R_cfm) strain significantly decreased, while that of cytochrome P450 monooxygenases (P450s) significantly increased. Meanwhile, the activities of glutathione-S-transferases (GSTs), CarEs and P450s in the bifenthrin-resistant (R_bft) strain were significantly higher than those in the susceptible strain (Lab_SS). According to the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) analyses, in the R_cfm mite strain, two carboxyl/cholinesterase (CCE) genes and two P450 genes were upregulated and one gene was downregulated, namely CYP392E7; in the R_bft mite strain, eleven CCE, nine UGT, two P450, four GST and three ABC genes were upregulated, while four CCE and three P450 genes were downregulated. Additionally, 94 differentially expressed genes (DEGs) were common to the two resistant groups. Specifically, TuCCE46 and TuCCE70 were upregulated in both resistant groups. Furthermore, the qRT-PCR validation data were consistent with those from the transcriptome sequencing analysis. Specifically, TuCCE46 (3.37-fold) was significantly upregulated in the R_cfm strain, while in the R_bft strain, TeturUGT22 (5.29-fold), teturUGT58p (1.74-fold), CYP392A11 (2.89-fold) and TuGSTd15 (5.12-fold) were significantly upregulated and TuCCE01 (0.13-fold) and CYP392A2p (0.07-fold) were significantly downregulated. Our study indicates that TuCCE46 might play the most important role in resistance to cyflumetofen, and TuCCE01, teturUGT58p, teturUGT22, CYP392A11, TuGSTd15, TuGSTm09 and TuABCG-13 were prominent in the resistance to bifenthrin. These findings provide further insight into the critical genes involved in the metabolic resistance of T. urticae to cyflumetofen and bifenthrin.

CYP4G100 contributes to desiccation resistance by mediating cuticular hydrocarbon synthesis in Bactrocera dorsalis

The oriental fruit fly Bactrocera dorsalis (Hendel) is expanding its distribution to higher latitudes. Our goal in this study was to understand how B. dorsalis adapts to higher latitude environments that are more arid than tropical regions. Cuticular hydrocarbons (CHCs) on the surface of the epicuticle in insects act as a hydrophobic barrier against water loss. The essential decarbonylation reaction in CHC synthesis is catalysed by CYP4G, a cytochrome P450 subfamily protein. Hence, in B. dorsalis it is necessary to clarify the function of the CYP4G gene and its role in desiccation resistance. CYP4G100 was identified in the B. dorsalis genome. The complete open reading frame (ORF) encodes a CYP4 family protein (552 amino acid residues) that has the CYP4G-specific insertion. This CYP4G gene was highly expressed in adults, especially in the oenocyte-rich peripheral fat body. The gene can be induced by desiccation treatment, suggesting its role in CHC synthesis and waterproofing. Silencing of CYP4G100 resulted in a decrease of CHC levels and the accumulation of triglycerides. It also increased water loss and resulted in higher desiccation susceptibility. CYP4G100 is involved in hydrocarbon synthesis and contributes to cuticle waterproofing to help B. dorsalis resist desiccation in arid environments.

The metabolic resistance of Nilaparvata lugens to chlorpyrifos is mainly driven by the carboxylesterase CarE17

The involvement of carboxylesterases (CarEs) in resistance to chlorpyrifos has been confirmed by the synergism analysis in Nilaparvata lugens. However, the function of specific CarE gene in chlorpyrifos resistance and the transcriptional regulatory mechanism are obscure. Herein, the expression patterns of 29 CarE genes in the susceptible and chlorpyrifos-resistant strains were analyzed. Among them, CarE3, CarE17 and CarE19 were overexpressed in the resistant strain, and knockdown of either CarE gene by RNA interference significantly increased the susceptibility to chlorpyrifos. Remarkably, knockdown of CarE17 reduced the enzymatic activity of CarE by 88.63 % and showed a much greater effect on increasing chlorpyrifos toxicity than silencing other two CarE genes. Overexpression of CarE17 in Drosophila melanogaster decreased the toxicity of chlorpyrifos to transgenic fruit flies. Furthermore, the region between - 205 to + 256 of CarE17 promoter sequence showed the highest promoter activity, and 16 transcription factors (TFs) were predicted from this region. Among these TFs, Lim1β and C15 were overexpressed in the resistant strain. Knockdown of either TF resulted in reduced CarE17 expression and a decrease in resistance of N. lugens to chlorpyrifos. These results indicate that the constitutive overexpression of Lim1β and C15 induces CarE17 expression thus conferring chlorpyrifos resistance in N. lugens.

Field Tests of Three Alternative Insecticides with Protein Bait for the Development of an Insecticide Rotation Program to Control Melon Flies, Zeugodacus cucurbitae (Coquillett) (Diptera: Tephritidae)

High levels of resistance to the spinosad-based insecticidal protein bait GF-120 have been detected in some populations of melon fly, Zeugodacus cucurbitae (Coquillett) (Diptera: Tephritidae), in Hawaii in 2017. To provide cucurbit farmers in Hawaii with alternative insecticides, we field-tested the effectiveness of Agri-Mek SC (a.i., abamectin), Mustang Maxx (a.i., zeta-cypermethrin), and Malathion 5EC (a.i., malathion), added to a protein bait spray (Nu-Lure Insect Bait). The insecticide and protein bait combinations were applied to the roosting plants of Z. cucurbitae around the perimeter of the cucurbit fields at one-week intervals. When individually tested, all three insecticides in combination with protein bait significantly reduced or suppressed the numbers of female flies caught in torula yeast traps. A two-week rotation of weekly applications of the three insecticides and GF-120 significantly reduced Z. cucurbitae numbers on a commercial zucchini farm on Maui. The percentage of marketable fruits harvested increased from 51% to 98% after implementing the insecticide rotation. Our findings will be used to provide cucurbit farmers with additional products to control Z. cucurbitae. The future focus will be on educating cucurbit farmers to use the insecticide rotation strategy to prevent or delay resistance development.

Contribution of multiple overexpressed carboxylesterase genes to indoxacarb resistance in Spodoptera litura

BACKGROUND

As an important family of detoxification enzymes, carboxylesterases (CarEs) have important roles in the development of insecticide resistance in almost all agricultural pests. Previous studies have suggested that enhancement of CarE activity is an important mechanism mediating indoxacarb resistance in Spodoptera litura, and several CarE genes have been found to be overexpressed in indoxacarb-resistant strains. However, the functions of these CarE genes in indoxacarb resistance needs to be further investigated.

RESULTS

The synergist triphenyl phosphate effectively reduced the resistance of S. litura to indoxacarb, suggesting an involvement of CarEs in indoxacarb resistance. Among seven identified S. litura CarE genes (hereafter SlituCOE), six were overexpressed in two indoxacarb-resistant strains, but there were no significant differences in gene copy number. Knockdown of SlituCOE009 and SlituCOE050 enhanced indoxacarb sensitivity in both susceptible and resistant strains, whereas knockdown of SlituCOE090, SlituCOE093 and SlituCOE074 enhanced indoxacarb sensitivity in only the resistant strain. Knockdown of the sixth gene, SlituCOE073, did not have any effect. Furthermore, simultaneous knockdown of the five SlituCOE genes had a greater effect on increasing indoxacarb sensitivity than silencing them individually. By contrast, overexpression of the five SlituCOE genes individually in Drosophila melanogaster significantly decreased the toxicity of indoxacarb to transgenic fruit flies. Furthermore, modeling and docking analysis indicated that the catalytic pockets of SlituCOE009 and SlituCOE074 were ideally shaped for indoxacarb and N-decarbomethoxylated metabolite (DCJW), but the binding affinity for DCJW was stronger than for indoxacarb.

CONCLUSION

This study reveals that multiple overexpressed CarE genes are involved in indoxacarb resistance in S. litura.

Metabolic-based insecticide resistance mechanism and ecofriendly approaches for controlling of beet armyworm Spodoptera exigua: a review

The beet army worm, Spodoptera exigua, is a widely distributed polyphagous pest of economically important crops worldwide. The management of this pest insect continues to face many challenges. Despite synthetic chemicals posing a serious threat to the environment, these remain the conventional approach for controlling S. exigua in the field. An over-reliance on chemical control has not only led to selection for resistance to insecticides and to a reduction of natural enemies, but has also polluted various components of ecosystem. Given these increasing pressures on the ecosystem, there is a need to implement integrated pest management (IPM) approaches exploiting a wider range of tools (biotechnological approaches, microbial control, biological control, cultural control, and use of host plant resistance) for an alternative to chemical control. The IPM approach can not only reduce the hazard of chemical residues in the environment and associated health problems, but may also provide best strategies to control insect pests. This review synthesizes published information on insecticide resistance of S. exigua and explores alternative IPM approaches to control S. exigua.

Transcriptome-Based Identification of a Functional Fasciola hepatica Carboxylesterase B

Bioinformatics analysis of the complete transcriptome of Fasciola hepatica, identified a total of ten putative carboxylesterase transcripts, including a 3146 bp mRNA transcript coding a 2205 bp open reading frame that translates into a protein of 735 amino acids, resulting in a predicted protein mass of 83.5 kDa and a putative carboxylesterase B enzyme. The gene coding for this enzyme was found in two reported F. hepatica complete genomes stretching 23,230 bp, containing two exons of 1282 and 1864 bp, respectively, as well as a 20,084 bp intron between the exons. The enzymatic activity was experimentally assayed on F. hepatica protein extracts by SDS-PAGE zymograms using synthetic chromogenic substrates, confirming both the theoretical molecular weight and carboxylesterase enzymatic activity. Further bioinformatics predicted that this enzyme is an integral component of the cellular membrane that should be active as a 167 kDa homodimer complex and polyacrylamide gel electrophoresis (PAGE) zymograms experiments confirmed the analysis. Additional bioinformatics analysis showed that DNA sequences that code for this particular enzyme are highly conserved in other parasitic trematodes, although they are labeled hypothetical proteins.

Two single mutations in carboxylesterase 001C improve fenvalerate hydrolase activity in Helicoverpa armigera

Carboxylesterases (CarEs) usually play critical roles in the detoxification of toxic chemicals and therefore may be involved in insecticide resistance in agricultural pests. Previous work has shown that CarE 001C from Helicoverpa armigera was able to metabolize the isomers of cypermethrin and fenvalerate. In this study, seven mutants of CarE 001C with single amino acid substitution were produced and expressed in the Escherichia coli. Enzyme kinetic analysis indicated that all seven mutations dramatically reduced enzymatic activities toward the generic substrate α-naphthyl acetate, but in vitro metabolism assay showed that two of the mutations, H423I and R322L, significantly improved hydrolase activities toward fenvalerate, with their recorded specific activities being 3.5 and 5.1 nM·s^-1·mg ^-1 proteins, respectively. Further, thermostability assay showed that the stability of one mutant enzyme was enhanced. This study will help us better understand the potential of CarEs in insecticide detoxification and resistance in H. armigera.

Gene selection for studying frugivore-plant interactions: a review and an example using Queensland fruit fly in tomato

Fruit production is negatively affected by a wide range of frugivorous insects, among them tephritid fruit flies are one of the most important. As a replacement for pesticide-based controls, enhancing natural fruit resistance through biotechnology approaches is a poorly researched but promising alternative. The use of quantitative reverse transcription PCR (RT-qPCR) is an approach to studying gene expression which has been widely used in studying plant resistance to pathogens and non-frugivorous insect herbivores, and offers a starting point for fruit fly studies. In this paper, we develop a gene selection pipe-line for known induced-defense genes in tomato fruit, Solanum lycopersicum, and putative detoxification genes in Queensland fruit fly, Bactrocera tryoni, as a basis for future RT-qPCR research. The pipeline started with a literature review on plant/herbivore and plant/pathogen molecular interactions. With respect to the fly, this was then followed by the identification of gene families known to be associated with insect resistance to toxins, and then individual genes through reference to annotated B. tryoni transcriptomes and gene identity matching with related species. In contrast for tomato, a much better studied species, individual defense genes could be identified directly through literature research. For B. tryoni, gene selection was then further refined through gene expression studies. Ultimately 28 putative detoxification genes from cytochrome P450 (P450), carboxylesterase (CarE), glutathione S-transferases (GST), and ATP binding cassette transporters (ABC) gene families were identified for B. tryoni, and 15 induced defense genes from receptor-like kinase (RLK), D-mannose/L-galactose, mitogen-activated protein kinase (MAPK), lipoxygenase (LOX), gamma-aminobutyric acid (GABA) pathways and polyphenol oxidase (PPO), proteinase inhibitors (PI) and resistance (R) gene families were identified from tomato fruit. The developed gene selection process for B. tryoni can be applied to other herbivorous and frugivorous insect pests so long as the minimum necessary genomic information, an annotated transcriptome, is available.

The adipokinetic hormone signaling system regulates the sensitivity of Bactrocera dorsalis to malathion

The neuropeptide adipokinetic hormone (AKH) binds to the AKH receptor (AKHR) to regulate carbohydrate and lipid metabolism. It also participates in the insect anti-stress response. We used RT-qPCR to detect the expression levels of 39 neuropeptides in malathion-susceptible (MS) and malathion-resistant (MR) strains of Bactrocera dorsalis. AKH and AKHR were highly expressed in the MR strain. Using a malathion bioassay and RNA interference (RNAi), we demonstrated that AKHR is involved in the susceptibility of B. dorsalis to malathion. We found significantly reduced expression of two detoxification enzyme genes (glutathione-S-transferase, GST and α-esterase, CarE) after AKHR RNAi. Based on our previous data, GSTd10 and CarE6 participate the direct metabolism of malathion in this fly, which is also verified by a malathion metabolism assay by HPLC using the crude enzymes in the current study. These results suggest that AKHR plays an important role in affecting malathion susceptibility via detoxification enzyme genes.

Enhanced hydrolysis of β-cypermethrin caused by deletions in the glycin-rich region of carboxylesterase 001G from Helicoverpa armigera

BACKGROUND

Carboxylesterase (CarE) is a major class of enzyme involved in the detoxification of toxic xenobiotics in various insect species. Previous work has shown that the carboxylesterase gene CarE001G found in Helicoverpa armigera is more active and can metabolize synthesized pyrethroids, such as β-cypermethrin, one of the commonly used commercial insecticides for lepidopteran pest control. In addition, CarE001G is very special as it has a very specific glycine-rich region located adjacent to its C-terminal. But whether mutations in this unique sequence can change the biochemistry and function of CarE001G are unknown.

RESULTS

In this study, four variants of CarE001G with different deletions in the glycine-rich region were obtained and functionally expressed in Escherichia coli. The recombinant proteins were purified and confirmed by Western blot and mass spectrometry analyses. These mutant enzymes showed high catalytic efficiency toward the model substrate α-naphthyl acetate. Inhibition study showed that β-cypermethrin had relatively strong inhibition on CarE activities. In vitro metabolism assay showed that the mutant enzymes significantly enhanced their metabolic activities toward β-cypermethrin with specific activities between 4.0 and 5.6 nmol L^-1 min^-1 mg^-1 protein. Molecular docking analyses consistently demonstrated that deletion mutations in the glycine-rich region may facilitate the anchoring of the β-cypermethrin molecule in the active binding pocket of the mutant enzymes.

CONCLUSION

The data show that deletion mutations can cause qualitative change in the capacity of CarEs in the detoxification of β-cypermethrin. This indicates that deletion mutations in the glycine-rich region may have the potential to cause synthesized pyrethroid (SP) resistance in H. armigera in the future. © 2020 Society of Chemical Industry.

NADPH-cytochrome P450 reductase potentially involved in indoxacarb resistance in Spodoptera litura

NADPH-cytochrome P450 reductase (CPR) plays a central role in the metabolism of insecticides. Numerous studies have shown that CPR is associated with insecticide resistance in insect. In this study, two transcripts of Spodoptera litura CPR (SlCPR-X1 and SlCPR-X2) were identified and cloned, and the deduced protein of SlCPR-X1 contains all the conserved CPR structural features (N-terminal membrane anchor, FMN, FAD and NADP binding domains, FAD binding motif, and catalytic residues). However, no N-terminal member anchor and a shorter FMN binding region have been identified in the deduced protein of SlCPR-X2. The specific expression patterns showed that SlCPR-X1 and SlCPR-X2 were detected in all tested developmental stages and tissues, but highly expressed in third-, fourth-, and fifth-instar larvae, and in midgut and fat body. In addition, compared with the susceptible strain, SlCPR-X1 and SlCPR-X2 were up-regulated and more inducible when treated with indoxacarb in the indoxacarb-resistant strain. However, the relative expression, up-regulation and induction of SlCPR-X1 were all higher than those of SlCPR-X2 in the indoxacarb-resistant strain. Furthermore, RNA interference and baculovirus expression system combined with MTT cytotoxicity assay demonstrated that only SlCPR-X1 with the N-terminal membrane anchor as the major CPR potentially involved in S. litura indoxacarb resistance. The outcome of this study further expands our understanding of the important role of insect CPR in xenobiotics detoxification and resistance development, and CPR could be a potential target for insecticide resistance management mediated by RNAi or CRISPR/Cas.

De Novo Transcriptomic Analyses Revealed Some Detoxification Genes and Related Pathways Responsive to Noposion Yihaogong® 5% EC (Lambda-Cyhalothrin 5%) Exposure in Spodoptera frugiperda Third-Instar Larvae

Simple Summary

Insect pest resistance to synthetic insecticides is a major problem that limits efficient management and thus decreases productivity for farmers and increases the use of harmful materials that pollute the environment and endanger humans and beneficial organisms. A major approach for resistance management is understanding how insect pest field populations develop resistance at molecular levels. To provide a comprehensive insight into the resistance mechanisms of Spodoptera frugiperda larvae to lambda-cyhalothrin 5%, we investigated the molecular basis of resistance mechanism in field collected population of fall armyworm (Spodoptera frugiperda) to lambda-cyhalothrin 5% insecticide, a pyrethroid insecticide by using de novo transcriptomics analysis. We found that resistance to lambda-cyhalothrin 5% can be metabolic by increasing the levels of detoxifying enzymes such as P450, GST and UGT and related genes to insecticide resistance in the field population. The obtained transcriptome information provides large gene resources available for further studying the resistance development of Spodoptera frugiperda to pesticides. The DGE data provide comprehensive insights into the gene expression profiles of fall armyworm (Spodoptera frugiperda) to lambda-cyhalothrin 5% and will facilitate the study of the role of each gene in lambda-cyhalothrin resistance development.

Abstract

The fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is a polyphagous, invasive insect pest which causes significant losses in important crops wherever it has spread. The use of pesticides in agriculture is a key tool in the management of many important crop pests, including S. frugiperda, but continued use of insecticides has selected for various types of resistance, including enzyme systems that provide enhanced mechanisms of detoxification. In the present study, we analyzed the de novo transcriptome of S. frugiperda larvae exposed to Noposion Yihaogong^® 5% emulsifiable concentrate (EC) insecticide focusing on detoxification genes and related pathways. Results showed that a total of 1819 differentially expressed genes (DEGs) were identified in larvae after being treated with Noposion Yihaogong^® 5% EC insecticide, of which 863 were up- and 956 down-regulated. Majority of these differentially expressed genes were identified in numerous Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, including metabolism of xenobiotics and drug metabolism. Furthermore, many of S. frugiperda genes involved in detoxification pathways influenced by lambda-cyhalothrin stress support their predicted role by further co-expression network analysis. Our RT-qPCR results were consistent with the DEG’s data of transcriptome analysis. The comprehensive transcriptome sequence resource attained through this study enriches the genomic platform of S. frugiperda, and the identified DEGs may enable greater molecular underpinnings behind the insecticide-resistance mechanism caused by lambda-cyhalothrin.

Digestion-related proteins in the tobacco hornworm, Manduca sexta

Food digestion is vital for the survival and prosperity of insects. Research on insect digestive enzymes yields knowledge of their structure and function, and potential targets of antifeedants to control agricultural pests. While such enzymes from pest species are more relevant for inhibitor screening, a systematic analysis of their counterparts in a model insect has broader impacts. In this context, we identified a set of 122 digestive enzyme genes from the genome of Manduca sexta, a lepidopteran model related to some major agricultural pests. These genes encode hydrolases of proteins (85), lipids (20), carbohydrates (16), and nucleic acids (1). Gut serine proteases (62) and their noncatalytic homologs (11) in the S1A subfamily are encoded by abundant transcripts whose levels correlate well with larval feeding stages. Aminopeptidases (10), carboxypeptidases (10), and other proteases (3) also participate in dietary protein digestion. A large group of 11 lipases as well as 9 esterases are probably responsible for digesting lipids in diets. The repertoire of carbohydrate hydrolases (16) is relatively small, including two amylases, three maltases, two sucrases, two α-glucosidases, and others. Lysozymes, peptidoglycan amidases, and β-1,3-glucanase may hydrolyze peptidoglycans and glucans to harvest energy and defend the host from microbes on plant leaves. One alkaline nuclease is associated with larval feeding, which is likely responsible for hydrolyzing denatured DNA and RNA undergoing autolysis at a high pH of midgut. Proteomic analysis of the ectoperitrophic fluid from feeding larvae validated at least 131 or 89% of the digestive enzymes and their homologs. In summary, this study provides for the first time a holistic view of the digestion-related proteins in a lepidopteran model insect and clues for comparative research in lepidopteran pests and beyond.

Functional Analyses of House Fly Carboxylesterases Involved in Insecticide Resistance

Carboxylesterase-mediated metabolism is one of major mechanisms involved in insecticide resistance. Our previous study has identified multiple carboxylesterase genes with their expression levels were significantly upregulated in pyrethroid resistant house flies. To further explore their metabolic functions, we used insect Spodoptera frugiperda (Sf9) cells to express these carboxylesterases in vitro and measure their hydrolytic activities toward esterase substrates. Our results indicated that these carboxylesterases can efficiently hydrolyze α-naphthyl acetate rather than β- naphthyl acetate. A cell based MTT cytotoxicity assay indicated that carboxylesterase-expressing cells show enhanced tolerance to permethrin, suggesting important roles of these carboxylesterases in metabolizing permethrin and thereby protecting cells from permethrin treatments. The metabolic functions of carboxylesterases were further verified by conducting in vitro metabolism studies toward permethrin and its potential metabolites 3-phenoxybenzyl alcohol and 3-phenoxybenzaldehyde, which not only suggested the potential metabolic pathway of permethrin in insects, but also important roles of these candidate carboxylesterases in metabolizing permethrin and conferring resistance in house flies. Homology modeling and docking were finally conducted to reflect interactions between permethrin ligand and carboxylesterase proteins, visually confirming the metabolic functions of carboxylesterases to insecticides in house flies.

Functional Characterization of an α-Esterase Gene Associated with Malathion Detoxification in Bradysia odoriphaga

Carboxylesterases (CarEs) are a multigene superfamily of metabolic enzymes involved in metabolic detoxification of xenobiotics. In this study, an α-esterase gene (BoαE1) was identified from Bradysia odoriphaga. Phylogenetic analysis classified BoαE1 into the α-esterase clade. Developmental expression analysis indicated that BoαE1 was significantly expressed in the second to fourth larval stages. Tissue-specific expression analysis indicated that BoαE1 was highly expressed in the larval midgut. After exposure to LC₃₀ of malathion, the CarE activity of B. odoriphaga was induced and the transcriptional level of BoαE1 was significantly up-regulated. Silencing of BoαE1 significantly increased the susceptibility of B. odoriphaga larvae to malathion. Inhibition assays in vitro indicated that malathion significantly inhibited BoαE1 activity. GC-MS assay showed that BoαE1 possesses hydrolase activity toward malathion and participates in the detoxification of malathion. These results strongly suggest that BoαE1 plays a crucial role in detoxification of malathion in B. odoriphaga.

Functional Characterization of Two Carboxylesterase Genes Involved in Pyrethroid Detoxification in Helicoverpa armigera

Insect carboxylesterases are major enzymes involved in metabolism of xenobiotics including insecticides. Two carboxylesterase genes, CarE001A and CarE001H, were cloned from the destructive agricultural pest Helicoverpa armigera. Quantitative real-time polymerase chain reaction showed that CarE001A and CarE001H were predominantly expressed in fat body and midgut, respectively; developmental expression analyses found that the expression levels of both CarEs were significantly higher in fifth-instar larvae than in other life stages. Recombinant CarE001A and CarE001H expressed in the Escherichia coli exhibited high enzymatic activity toward α-naphthyl acetate. Inhibition assays showed that organophosphates had strong inhibition on CarEs activity compared to pyrethroids. Metabolism assays indicated that CarE001A and CarE001H were able to metabolize β-cypermethrin and λ-cyhalothrin. Homology modeling and molecular docking analyses demonstrated that β-cypermethrin could fit nicely into the active pocket of both carboxylesterases. These results suggested that CarE001A and CarE001H could play important roles in the detoxification of pyrehtroids in H. armigera.

Divergent molecular evolution in glutathione S-transferase conferring malathion resistance in the oriental fruit fly, Bactrocera dorsalis (Hendel)

Insect glutathione S-transferases (GSTs) are important in insecticide detoxification and Insect-specific GSTs, Epsilon and Delta, have largely expanded in insects. In this study, we functionally expressed and characterized an epsilon class GST gene (BdGSTe8), predominant in the adult Malpighian tubules of Bactrocera dorsalis. This gene may be associated with malathion resistance based on transcriptional studies of resistant and susceptible strains. RNA interference-mediated knockdown of this gene significantly recovered malathion susceptibility in the adults of a malathion-resistant strain, and overexpression of BdGSTe8 enhanced resistance in transgenic Drosophila. Analysis of BdGSTe8 polymorphism showed that several point mutations may be associated with metabolic resistance to malathion. A cytotoxicity assay in Escherichia coli indicated that both of the recombinant BdGSTe8 proteins may play a functional role in protecting cells from toxicity. The allele of BdGSTe8-B conferred higher levels of malathion detoxification capability. Liquid chromatography and ultra-performance liquid chromatography-tandem mass spectrometry analysis showed that the BdGSTe8-A allele did not metabolize malathion directly. However, the BdGSTe8-B allele was involved in the direct metabolism of malathion, which was caused by a mutation in V128A. Further analysis of the sequence suggests that BdGSTe8 evolved rapidly. It maybe play the role of a backup gene and could become a new gene in the future in order to retain the ability of detoxification of malathion, which was driven by positive selection. These results suggest that divergent molecular evolution in BdGSTe8 has played a role in metabolic resistance to malathion in B. dorsalis.

Recent research status of Bactrocera dorsalis: Insights from resistance mechanisms and population structure

Bactrocera dorsalis (Hendel) is considered to be a highly invasive and destructive agricultural pest due to its strong dispersal and adaptive capacity. Rapid development of insecticide resistance poses a serious threat to the sustainable control of this pest. Here, the resistance mechanisms and invasion pathways of this fly are outlined for a better understanding of the resistance-gene flow pattern and invasion routes. We believe this microreview will provide a glimpse of the native regions, spread and management of resistance, and guide future work on these important topics.

The Transcription Factor MafB Regulates the Susceptibility of Bactrocera dorsalis to Abamectin via GSTz2

Pesticide resistance is a serious problem that poses a major challenge to pest control. One of the most potent resistance mechanisms is the overexpression of genes coding for detoxification enzymes. The expression of detoxification genes is regulated by a series of transcription factors. Previous studies have revealed that the increased expression of detoxification genes contributes to the insecticide tolerance of Bactrocera dorsalis. Our objective was thus to identify the transcription factors involved in this process. Temporal expression profiles showed that the transcription factor MafB and detoxification genes were expressed highly in the fat body. Further analysis showed that the expression of MafB, GSTz2, and CYP473A3 was induced by abamectin. Disruption of the MafB transcription factor through RNA interference decreased the transcript levels of GSTz2 and CYP473A3 and increased the susceptibility to abamectin significantly. Direct silencing of the expression of GSTz2 also increased susceptibility to abamectin, while CYP473A3 did not. In conclusion, these results suggest that the expression of GSTz2 and CYP473A3 was regulated by the transcription factor MafB, and the up-regulation of GSTz2 via MafB decreased the susceptibility of B. dorsalis to abamectin.

Downregulation of carboxylesterase contributes to cyflumetofen resistance in Tetranychus cinnabarinus (Boisduval)

BACKGROUND

Increased expression or point mutations of carboxyl/cholinesterases (CCEs) have been involved in many cases of insecticide and acaricide resistance. However, it has been only rarely documented that downregulation of CCE genes is associated with resistance, although many insecticides and acaricides need hydrolytic activation in vivo. Previously, expression analysis of a laboratory-selected cyflumetofen-resistant strain of Tetranychus cinnabarinus indicated that resistance was associated with increased expression of a CCE gene of TcCCE04, but also the downregulation of two CCE genes, TcCCE12 and TcCCE23.

RESULTS

Synergism experiments revealed the importance of ester hydrolysis in cyflumetofen toxicity, because treatment with S,S,S-tributylphosphorotrithioate (DEF) caused strong inhibition of cyflumetofen hydrolysis, in both the susceptible and resistant strains. Moreover, silencing expression of TcCCE12 and TcCCE23 via RNAi further decreased the susceptibility of mites to cyflumetofen significantly, suggesting that downregulated CCE genes could be involved in cyflumetofen resistance. In addition, it was shown that recombinant TcCCE12 protein could hydrolyze cyflumetofen effectively.

CONCLUSION

Decreased esterase activity via downregulation of specific CCE genes most likely contributes to cyflumetofen resistance by decreased activation of cyflumetofen to its active metabolite. Mixtures of cyflumetofen and esterase-inhibition acaricides (e.g. organophosphates or carbamates) should be avoided in field applications. © 2019 Society of Chemical Industry.

Latrophilin mediates insecticides susceptibility and fecundity through two carboxylesterases, esterase4 and esterase6, in Tribolium castaneum

Latrophilin (LPH) is known as an adhesion G-protein-coupled receptor which involved in multiple physiological processes in organisms. Previous studies showed that lph not only involved the susceptibility to anticholinesterase insecticides but also affected fecundity in Tribolium castaneum. However, its regulatory mechanisms in these biological processes are still not clear. Here, we identified two potential downstream carboxylesterase (cce) genes of Tclph, esterase4 and esterase6, and further characterized their interactions with Tclph. After treatment of T. castaneum larvae with carbofuran or dichlorvos insecticides, the transcript levels of Tcest4 and Tcest6 were significantly induced from 12 to 72 h. RNAi against Tcest4 or Tcest6 led to the higher mortality compared with the controls after the insecticides treatment, suggesting that these two genes play a vital role in detoxification of insecticides in T. castaneum. Furthermore, with insecticides exposure to Tclph knockdown beetles, the expression of Tcest4 was upregulated but Tcest6 was downregulated, indicating that beetles existed a compensatory response against the insecticides. Additionally, RNAi of Tcest6 resulted in 43% reductions in female egg laying and completely inhibited egg hatching, which showed the similar phenotype as that of Tclph knockdown. These results indicated that Tclph affected fecundity by positively regulating Tcest6 expression. Our findings will provide a new insight into the molecular mechanisms of Tclph involved in physiological functions in T. castaneum.

Two delta class glutathione S-transferases involved in the detoxification of malathion in Bactrocera dorsalis (Hendel)

BACKGROUND

The oriental fruit fly Bactrocera dorsalis (Hendel), a widespread agricultural pest, has evolved resistance to many insecticides, including organophosphorus compounds. Glutathione S-transferases (GSTs) are involved in xenobiotic detoxification and insecticide resistance in many insects. However, the role of delta class GSTs in detoxifying malathion in B. dorsalis is unknown. Here, we evaluated the roles of two delta class GSTs in malathion detoxification in this species.

RESULTS

Two delta class GSTs genes, BdGSTd1 and BdGSTd10, were characterized in B. dorsalis. They were highly expressed in 5-day-old adults, as well as in midgut and Malpighian tubules. Upon malathion exposure, the two genes were upregulated by 2.63- and 2.85-fold, respectively. Injection of double-stranded RNA targeting BdGSTd1 or BdGSTd10 significantly reduced their mRNA levels in adults and also significantly increased adult susceptibility to malathion. The expression of these two GSTs in Escherichia coli helped the host to endure malathion stress at a concentration of 10 µg mL^-1 according to a Cell Counting Kit-8 assay. High-performance liquid chromatography analyses indicated that malathion could be significantly depleted by the two delta GSTs. The role of BdGSTd10 in malathion sequestration was also discussed.

CONCLUSION

BdGSTd1 and BdGSTd10 play important roles in the detoxification of malathion in B. dorsalis. © 2019 Society of Chemical Industry.

Identification and biochemical characterization of carboxylesterase 001G associated with insecticide detoxification in Helicoverpa armigera

Carboxylesterases (CarEs) are a major class of detoxification enzymes involved in insecticide resistance in various insect species. In this study, a novel CarE 001G was isolated from the cotton bollworm Helicoverpa armigera, one of the most destructive agricultural insect pests. The open reading frame of 001G has 2244 nucleotides and putatively encodes 747 amino acid residues. The deduced CarE possessed the highly conserved catalytic triads(Ser-Glu-His) and pentapeptide motifs (Gly-X-Ser-X-Gly), suggesting 001G is biologically active. The truncated 001G was successfully expressed in Escherichia coli, and the recombinant proteins were purified and tested. The enzyme kinetic assay showed the purified proteins could catalyze two model substrates, α-naphthyl acetate and β-naphthyl acetate, with a kcat of 8.8 and 2.3 s^-1, a Km of 9.6 and 16.2 μM, respectively. The inhibition study with pyrethroid, organophosphate and neonicotinoid insecticides showed different inhibition profile against the purified CarE. The HPLC assay demonstrated that the purified proteins were able to metabolize β-cypermethrin, λ-cyhalothrin and fenvalerate insecticides, exhibiting respective specific activities of 1.7, 1.4 and 0.5 nM/min/mg protein. However, the purified proteins were not able to metabolize the chlorpyrifos, parathion-methyl, paraoxon-ethyl and imidacloprid. The modeling and docking analyses consistently demonstrated that the pyrethroid molecule fits snugly into the catalytic pocket of the CarE 001G. Collectively, our results suggest that 001G may play a role in pyrethroids detoxification in H. armigera.

The organophosphorus pesticide dimethoate decreases cell viability and induces changes in different biochemical parameters of rat pancreatic stellate cells

In the present study we employed cultured pancreatic stellate cells to study the effect of the organophosphorus insecticide dimethoate on pancreatic cell physiology. Esterase activity, cell viability, reactive oxygen species generation and Ca2+ mobilization were examined. Our results show that dimethoate (0.1, 1 and 10 μM) induced a concentration-dependent inhibition of cholinesterase enzymatic activity at all concentrations tested. A drop in carboxylesterase activity was noted in the presence of 10 μM dimethoate. In the presence of the pesticide a decrease in cell viability was detected. The clearer effect could be observed when the cells had been incubated during 96 h in the presence of dimethoate. The pesticide induced a slight but statistically significant increase in the production of reactive oxygen species in the mitochondria. Incubation of cells with dimethoate, in the presence of Ca2+ in the extracellular medium, led to a slow and progressive increase in [Ca2+]c towards an elevated value over the prestimulation level. A similar behavior was observed in the absence of extracellular Ca2+, indicating that dimethoate releases Ca2+ from the intracellular stores. Our results suggest that dimethoate might alter intracellular pathways that are critical for pancreatic physiology, creating a situation potentially leading to dysfunction in the exocrine pancreas.

Functional analysis of four upregulated carboxylesterase genes associated with fenpropathrin resistance in Tetranychus cinnabarinus (Boisduval)

BACKGROUND

Carboxylesterases (CarEs) are important in pesticide resistance. Four overexpressed CarE genes with inducible character were screened out in fenpropathrin-resistant Tetranychus cinnabarinus, but their functional roles remained to be further analyzed by RNAi and protein expression.

RESULTS

Feeding a single double-stranded (ds)RNA of each of four genes led to gene-specific downregulation of mRNA, decreased esterase activity and diminished resistance in T. cinnabarinus. More interestingly, feeding four dsRNAs simultaneously led to a more significant decrease in enzymatic activity and fold resistance than feeding a single dsRNA individually, suggesting that these CarE genes were involved in fenpropathrin-resistance and had cooperative roles. The gene CarE6 was regarded as the primary and representative candidate to be functionally expressed, because silencing of CarE6 led to the most significant decrease in resistance level. The activity of CarE6 protein was competitively inhibited by fenpropathrin. It could effectively decompose 41.7 ± 0.09% of fenpropathrin within 3 h, proving that CarE6 protein was capable of metabolizing fenpropathrin effectively in T. cinnabarinus.

CONCLUSION

The results confirm that four CarE genes are cooperatively involved in fenpropathrin resistance and the metabolic enzymes encoded by these overexpressed genes do indeed metabolize acaricide in resistant T. cinnabarinus in the evolution of acaricide resistance. © 2018 Society of Chemical Industry.

Effect of Sublethal Doses of Imidacloprid on the Biological Performance of Aphid Endoparasitoid Aphidius gifuensis (Hymenoptera: Aphidiidae) and Influence on Its Related Gene Expression

The integrated pest management (IPM) strategy was developed and used in combination with pesticides and beneficial biological control agents. To further develop IPM efficiency, it is important to evaluate the side effects of pesticides on biological control agents. Aphidius gifuensis is one of the most important aphid natural enemies and has been successfully used to control Myzys persicae and other aphid species. Imidacloprid (IMD) is a popular pesticide used worldwide and is highly toxic to non-target arthropods. Here, we investigated the short-term sublethal toxicity of IMD in Aphidius gifuensis and its impact on the biological performance and gene expression of this parasitoid. We found that sublethal IMD doses had a significant negative effect on the life history traits of female A. gifuensis, including shortening the lifespan and lowering parasitic capacity. Moreover, exposure to sublethal IMD also adversely affected the response of A. gifuensis to aphid-infested plant volatiles. Based on the transcriptome analysis, we found that the exposure to sublethal IMD doses significantly affected expression of genes involved in the central nervous system, energy metabolism, olfactory, and detoxification system of A. gifuensis. RT-qPCR also revealed that short term expose to sublethal IMD doses significantly induced the gene expression of genes related to the central nervous system (nAChRa7, nAChRa9, TbH, OAR1, NFR, TYR, and DAR1), olfactory system (OR28 and IR8a1), and detoxification system (CYP49p3, CYP6a2, and POD), while it suppressed the expression of genes involved in the central nervous system (nAChRa4 and nAChRb1), olfactory system (Orco1, IR8a2, and GR1), and detoxification system (GST2). Furthermore, exposure to sublethal doses of IMD also significantly increased the activities of CarEs and POD, whereas we observed no influence on the activities of CAT, GST, and SOD. Our results indicate that sublethal IMD doses might adversely affect the biological performance of A. gifuensis by altering gene expression related to the function of olfactory, nervous, energy metabolism, and detoxification systems. Thus, how the use of pesticides directly affect insect population should be considered when used in conjunction with natural pest parasitoids in IPM strategies.

NADPH-Cytochrome P450 Reductase Mediates the Resistance of Aphis (Toxoptera) citricidus (Kirkaldy) to Abamectin

NADPH-cytochrome P450 reductase (CPR) plays an essential role in the cytochrome P450 enzyme system, which aids in the metabolism of endogenous and exogenous compounds including the detoxification of insecticides. In this study, the CPR transcript in Aphis (Toxoptera) citricidus (Kirkaldy) was cloned, and the deduced amino acid sequence contained an N-terminal membrane anchor, three conserved binding domains (flavin mononucleotide, flavin adeninedinucleotide, and nicotinamide adenine dinucleotide phosphate), a flavin adeninedinucleotide-binding motif, and catalytic residues. Based on phylogenetic analysis, AcCPR was grouped in the hemipteran branch. AcCPR was ubiquitously expressed at all developmental stages and was most abundant in the adults and least abundant in third instar nymphs. Compared with other tested tissues of adults, the expression level of AcCPR was significantly high in the gut. Feeding double-stranded RNA of AcCPR reduced the AcCPR mRNA level and the activity of AcCPR in aphids, and the treated insects exhibited higher susceptibility to abamectin than the control group. Furthermore, the heterologous overexpression of AcCPR in Sf9 cells resulted in a greater viability than control cells when treated with abamectin. All results demonstrated that AcCPR may contribute to the resistance of A.citricidus to abamectin, and CPR may be a potential target for novel insecticide design or a new factor in the development of insecticide resistance.

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.

Functional characterization of BdB1, a well-conserved carboxylesterase among tephritid fruit flies associated with malathion resistance in Bactrocera dorsalis (Hendel)

There are many evidences that insect carboxylesterase possess important physiological roles in xenobiotic metabolism and are implicated in the detoxification of organophosphate (OP) insecticides. Despite the ongoing resistance development in the oriental fruit fly, Bactrocera dorsalis (Hendel), the molecular basis of carboxylesterase and its ability to confer OP resistance remain largely obscure. This study was initiated to provide a better understanding of carboxylesterase-mediated resistance mechanism in a tephritid pest fly. Here, we narrow this research gap by demonstrating a well-conserved esterase B1 gene, BdB1, mediates malathion resistance development via gene upregulation with the use of a laboratory selected malathion-resistant strain (MR) of B. dorsalis. No sequence mutation of BdB1 was detected between MR and the susceptible strain (MS) of B. dorsalis. BdB1 is predominantly expressed in the midgut, a key insect tissue for detoxification. As compared with transcripts in MS, BdB1 was significantly more abundant in multiple tissues in the MR. RNA interference (RNAi)-mediated knockdown of BdB1 significantly increased malathion susceptibility. Furthermore, heterologous expression along with cytotoxicity assay revealed BdB1 could probably have the function of malathion detoxification.

The alternative splicing of BdTai and its involvement in the development of Bactrocera dorsalis (Hendel)

Interest in insect metamorphosis has primarily focused on juvenile hormone (JH) and ecdysone. Compared to ecdysone signaling, the molecular action of JH is less well established because Methoprene-tolerant (Met) as the JH receptor has been identified until recently. In vitro studies have indicated that Met forms an active JH-dependent complex with one partner protein, Taiman (Tai). However, the related studies on Tai's role in insect metamorphosis are very limited. In this study, five Tai isoforms differing in C-terminal region are identified from the oriental fruit fly Bactrocera dorsalis, an important worldwide pest infesting fruits and vegetables. The spatiotemporal expression pattern analysis indicates that BdTai-A and BdTai-B are highly expressed in early larvae while BdTai-D is more abundant in middle-late larvae. Meanwhile, in vivo methoprene stress leads to dramatic expression pattern fluctuation of BdTai isoforms. The subsequent reverse genetic study reveal that all Tai isoforms (denoted as "Tai-core") depletion in larvae stage of B. dorsalis produce precocious larvae-pupae development, i.e. shortened pupation process and miniature pupae. Further knockdown of individual Tai isoform show that silence of BdTai-E causes the same phenotype as of BdTai-core RNAi. The current data suggest that BdTai-E is involved in transducing the JH signal that represses metamorphosis. Besides, isoforms should be considered when studying Tai functions.

Molecular characterisation of two α-esterase genes involving chlorpyrifos detoxification in the diamondback moth, Plutella xylostella

BACKGROUND

Carboxylesterases (CarEs) are involved in metabolic detoxification of dietary and environmental xenobiotics in insects. However, owing to the complexity of the protein family, the involvement of CarEs in insecticide metabolism in Plutella xylostella has not been fully elucidated. This study aimed to characterise two CarE genes and assess their potential roles in response to chlorpyrifos in P. xylostella.

RESULTS

Synergistic tests showed that triphenyl phosphate decreased the resistance of the third-instar larvae to chlorpyrifos. The treatment of the third-instar larvae with chlorpyrifos at the LC₃₀ dose led to a significant increase in CarE activity. Two CarE cDNAs (Pxae18 and Pxae28) were subsequently sequenced and characterised. Both genes were expressed predominantly in the larval midgut. Most importantly, two CarE genes showed significantly higher expression in the chlorpyrifos-resistant strain than in the susceptible strain. RNAi knockdown of Pxae18 and Pxae28 significantly increased the mortality to chlorpyrifos from 40% in the control to 73.8 and 63.3% respectively.

CONCLUSION

RNAi knockdown of Pxae18 and Pxae28 significantly inhibited detoxification ability and increased the mortality in P. xylostella. The results indicate that these two CarE genes play important roles in the detoxification of chlorpyrifos in P. xylostella. © 2016 Society of Chemical Industry.

Characterization of a β-Adrenergic-Like Octopamine Receptor in the Oriental Fruit Fly, Bactrocera dorsalis (Hendel)

The biogenic amine octopamine plays a critical role in the regulation of many physiological processes in insects. Octopamine transmits its action through a set of specific G-protein coupled receptors (GPCRs), namely octopamine receptors. Here, we report on a β-adrenergic-like octopamine receptor gene (BdOctβR1) from the oriental fruit fly, Bactrocera dorsalis (Hendel), a destructive agricultural pest that occurs in North America and the Asia-Pacific region. As indicated by RT-qPCR, BdOctβR1 was highly expressed in the central nervous system (CNS) and Malpighian tubules (MT) in the adult flies, suggesting it may undertake important roles in neural signaling in the CNS as well as physiological functions in the MT of this fly. Furthermore, its ligand specificities were tested in a heterologous expression system where BdOctβR1 was expressed in HEK-293 cells. Based on cyclic AMP response assays, we found that BdOctβR1 could be activated by octopamine in a concentration-dependent manner, confirming that this receptor was functional, while tyramine and dopamine had much less potency than octopamine. Naphazoline possessed the highest agonistic activity among the tested agonists. In antagonistic assays, mianserin had the strongest activity and was followed by phentolamine and chlorpromazine. Furthermore, when the flies were kept under starvation, there was a corresponding increase in the transcript level of BdOctβR1, while high or low temperature stress could not induce significant expression changes. The above results suggest that BdOctβR1 may be involved in the regulation of feeding processes in Bactrocera dorsalis and may provide new potential insecticide leads targeting octopamine receptors.

Involvement of Three Esterase Genes from Panonychus citri (McGregor) in Fenpropathrin Resistance

The citrus red mite, Panonychus citri (McGregor), is a major citrus pest with a worldwide distribution and an extensive record of pesticide resistance. However, the underlying molecular mechanism associated with fenpropathrin resistance in this species have not yet been reported. In this study, synergist triphenyl phosphate (TPP) dramatically increased the toxicity of fenpropathrin, suggesting involvement of carboxylesterases (CarEs) in the metabolic detoxification of this insecticide. The subsequent spatiotemporal expression pattern analysis of PcE1, PcE7 and PcE9 showed that three CarEs genes were all over-expressed after insecticide exposure and higher transcripts levels were observed in different field resistant strains of P. citri. Heterologous expression combined with 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetra-zolium bromide (MTT) cytotoxicity assay in Spodoptera frugiperda (Sf9) cells revealed that PcE1-, PcE7- or PcE9-expressing cells showed significantly higher cytoprotective capability than parental Sf9 cells against fenpropathrin, demonstrating that PcEs probably detoxify fenpropathrin. Moreover, gene silencing through the method of leaf-mediated dsRNA feeding followed by insecticide bioassay increased the mortalities of fenpropathrin-treated mites by 31% (PcE1), 27% (PcE7) and 22% (PcE9), respectively, after individual PcE gene dsRNA treatment. In conclusion, this study provides evidence that PcE1, PcE7 and PcE9 are functional genes mediated in fenpropathrin resistance in P. citri and enrich molecular understanding of CarEs during the resistance development of the mite.

Characteristics of carboxylesterase genes and their expression-level between acaricide-susceptible and resistant Tetranychus cinnabarinus (Boisduval)

Carboxylesterases (CarEs) play important roles in metabolism and detoxification of dietary and environmental xenobiotics in insects and mites. On the basis of the Tetranychuscinnabarinus transcriptome dataset, 23 CarE genes (6 genes are full sequence and 17 genes are partial sequence) were identified. Synergist bioassay showed that CarEs were involved in acaricide detoxification and resistance in fenpropathrin- (FeR) and cyflumetofen-resistant (CyR) strains. In order to further reveal the relationship between CarE gene's expression and acaricide-resistance in T. cinnabarinus, we profiled their expression in susceptible (SS) and resistant strains (FeR, and CyR). There were 8 and 4 over-expressed carboxylesterase genes in FeR and CyR, respectively, from which the over-expressions were detected at mRNA level, but not DNA level. Pesticide induction experiment elucidated that 4 of 8 and 2 of 4 up-regulated genes were inducible with significance in FeR and CyR strains, respectively, but they could not be induced in SS strain, which indicated that these genes became more enhanced and effective to withstand the pesticides' stress in resistant T. cinnabarinus. Most expression-changed and all inducible genes possess the Abhydrolase_3 motif, which is a catalytic domain for hydrolyzing. As a whole, these findings in current study provide clues for further elucidating the function and regulation mechanism of these carboxylesterase genes in T. cinnabarinus' resistance formation.

Identification of a carboxylesterase associated with resistance to naled in Bactrocera dorsalis (Hendel)

Compared to other organophosphate-resistant and -susceptible (S) lines of Bactrocera dorsalis, the carboxylesterase (CBE) BdE5 in the naled-resistant (nal-r) line has been found to possess remarkable quantitative elevation. Our study attempts to identify the role of BdE5 in naled resistance, and we discovered several points of interest. Firstly, activity staining on native PAGE revealed that the percentage of flies with intensive BdE5 bands in the nal-r line was substantially higher than in the S line, indicating that the BdE5 band correlates with naled susceptibility. Secondly, in vitro and in vivo inhibition assays showed that BdE5 was inhibited by naled in both lines; under diagnostic doses of naled, the overall extent of inhibition on CBEs was much greater in the S line than in the nal-r line. Thirdly, NanoLC-nanoESi-MS/MS analysis used the NCBI database to identify and annotate BdE5 as an esterase FE4-like (XP_011200445.1) in B. dorsalis. Fourthly, rapid amplification of cDNA ends was used to obtain the 2012-bp full-length BdE5 cDNA, which contained an open reading frame of 1770bp and encoded a putative protein of 590 amino acid residues. Phylogenetic analysis revealed that BdE5 is a secreted β-esterase (E clade) closely related to CG6414 (NP_570089), a CBE in Drosophila melanogaster. Finally, our relative quantification real-time PCR data showed a significant elevation in transcript levels of the BdE5 gene in nal-r line. Our results confirmed that BdE5 is correlated with naled resistance and provides further understanding about the identification and molecular characteristics of BdE5 in B. dorsalis.

Functional characterization of an α-esterase gene involving malathion detoxification in Bactrocera dorsalis (Hendel)

Extensive use of insecticides in many orchards has prompted resistance development in the oriental fruit fly, Bactrocera dorsalis (Hendel). In this study, a laboratory selected strain of B. dorsalis (MR) with a 21-fold higher resistance to malathion was used to examine the resistance mechanisms to this organophosphate insecticide. Carboxylesterase (CarE) was found to be involved in malathion resistance in B. dorsalis from the synergism bioassay by CarE-specific inhibitor triphenylphosphate (TPP). Molecular studies further identified a previously uncharacterized α-esterase gene, BdCarE2, that may function in the development of malathion resistance in B. dorsalis via gene upregulation. This gene is predominantly expressed in the Malpighian tubules, a key insect tissue for detoxification. The transcript levels of BdCarE2 were also compared between the MR and a malathion-susceptible (MS) strain of B. dorsalis, and it was significantly more abundant in the MR strain. No sequence mutation or gene copy changes were detected between the two strains. Functional studies using RNA interference (RNAi)-mediated knockdown of BdCarE2 significantly increased the malathion susceptibility in the adult files. Furthermore, heterologous expression of BdCarE2 combined with cytotoxicity assay in Sf9 cells demonstrated that BdCarE2 could probably detoxify malathion. Taken together, the current study bring new molecular evidence supporting the involvement of CarE-mediated metabolism in resistance development against malathion in B. dorsalis and also provide bases on functional analysis of insect α-esterase associated with insecticide resistance.

Inheritance, Realized Heritability, and Biochemical Mechanisms of Malathion Resistance in Bactrocera dorsalis (Diptera: Tephritidae)

To better characterize the resistance development and therefore establish effective pest management strategies, this study was undertaken to investigate the inheritance mode and biochemical mechanisms of malathion resistance in the oriental fruit fly, Bactrocera dorsalis (Hendel), which is one of the most notorious pests in the world. After 22 generations of selection with malathion, the malathion-resistant (MR) strain of B. dorsalis developed a 34-fold resistance compared with a laboratory susceptible strain [malathion-susceptible (MS)]. Bioassay results showed that there was no significant difference between the LD50 values of malathion against the progenies from both reciprocal crosses (F(1)-SR and F(1)-RS). The degree of dominance values (D) was calculated as 0.39 and 0.32 for F(1)-RS and F(1)-SR, respectively. The logarithm dosage-probit mortality lines of the F(2) generation and progeny from the backcross showed no clear plateaus of mortality across a range of doses. In addition, Chi-square analysis revealed significant differences between the mortality data and the theoretical expectations. The realized heritability (h(2)) value was 0.16 in the laboratory-selected resistant strain of B. dorsalis. Enzymatic activities identified significant changes of carboxylesterases, cytochrome P450 (general oxidases), and glutathione S-transferases in MR compared with the MS strain of B. dorsalis. Taken together, this study revealed for the first time that malathion resistance in B. dorsalis follows an autosomal, incompletely dominant, and polygenic mode of inheritance and is closely associated with significantly elevated activities of three major detoxification enzymes.