Overexpression of a cytochrome P450 and a UDP-glycosyltransferase is associated with imidacloprid resistance in the Colorado potato beetle, Leptinotarsa decemlineata

UDP-Glycosyltransferases from the UGT344 Family Are Involved in Sulfoxaflor Resistance in Aphis gossypii Glover

Simple Summary

The cotton aphid, Aphis gossypii Glover, is a notorious pest in cotton and cucurbit fields. The control of A. gossypii has typically relied on the application of chemical insecticides. Sulfoxaflor is the first commercially available sulfoximine insecticide, which exhibits great efficacy against sap-feeding insect pests and has been applied as an alternative insecticide for controlling of A. gossypii in China. Consequently, A. gossypii quickly developed resistance to this insecticide. Hence, in this study, to clarify the potential detoxifying roles of UGTs (one of the phase II detoxification enzymes) in resistance of A. gossypii against sulfoxaflor, the synergistic effects of two synergists (sulfinpyrazone and 5-nitrouracil) against sulfoxaflor were investigated using the susceptible and laboratory-established sulfoxaflor resistant strain (SulR), and the expression levels of 15 UGT genes were determined by qRT-PCR. Furthermore, the involvement of highly upregulated UGTs in sulfoxaflor-resistant strain was functionally tested by RNA interference (RNAi). Our results suggest that overexpression of UGTs contributes to sulfoxaflor resistance in A. gossypii, which should be useful for understanding sulfoxaflor resistance mechanisms.

Abstract

UDP-glycosyltransferases (UGTs) are major phase II detoxification enzymes that catalyze the transfer of glycosyl residues from activated nucleotide sugars to acceptor hydrophobic molecules and play very important roles in the biotransformation of various endogenous and exogenous compounds. Our previous studies demonstrated that UGTs participated in the detoxification of insecticides in Aphis gossypii. However, the potential roles of UGTs in A. gossypii resistance to sulfoxaflor are still unclear. In this study, two inhibitors of UGT enzymes, sulfinpyrazone and 5-nitrouracil, significantly increased the toxicity of sulfoxaflor to a resistant strain of A. gossypii, whereas there were no synergistic effects in the susceptible strain. Based on the transcriptome sequencing results, the expression levels of 15 UGTs were analyzed by quantitative real-time PCR, and we found that seven UGT genes were highly over-expressed in a sulfoxaflor-resistant strain compared to the susceptible strain, including UGT344B4, UGT344C5, UGT344A11, UGT344A14, and UGT344L2. Further suppressing the expression of UGT344B4, UGT344C5, and UGT344A11 by RNA interference significantly increased the sensitivity of resistant aphids to sulfoxaflor, indicating that the overexpression of UGT genes is potentially associated with sulfoxaflor resistance. These results could provide valuable information for further understanding the mechanisms of insecticide resistance.

Glycosyltransferases: the multifaceted enzymatic regulator in insects

Glycosyltransferases (GTs) catalyse the reaction of glyco-conjugation of various biomolecules by transferring the saccharide moieties from an activated nucleotide sugar to nucleophilic glycosyl acceptor. In insects, GTs show diverse temporal and site-specific expression patterns and thus play significant roles in forming the complex biomolecular structures that are necessary for insect survival, growth and development. Several insects exhibit GT-mediated detoxification as a key defence strategy against plant allelochemicals and xenobiotic compounds, as well as a mechanism for pesticide cross-resistance. Also, these enzymes act as crucial effectors and modulators in various developmental processes of insects such as eye development, UV shielding, cuticle formation, epithelial development and other specialized functions. Furthermore, many of the known insect GTs have been shown to play a fundamental role in other physiological processes like body pigmentation, cuticular tanning, chemosensation and stress response. This review provides a detailed overview of the multifaceted functionality of insect GTs and summarizes numerous case studies associated with it.

Disruption of the cytochrome P450 CYP6BQ7 gene reduces tolerance to plant toxicants in the red flour beetle, Tribolium castaneum

In insects, the cytochrome P450 CYP6B family plays key roles in the detoxification of toxic plant substances. However, the function of CYP6 family genes in degrading plant toxicants in Tribolium castaneum, an extremely destructive global storage pest, have yet to be elucidated. In this study, a T. castaneum CYP gene, TcCYP6BQ7, was characterized. TcCYP6BQ7 expression was significantly induced after exposure to essential oil of the plant Artemisia vulgaris (EOAV). Spatiotemporal expression profiling revealed that TcCYP6BQ7 expression was higher in larval and adult stages of T. castaneum than in other developmental stages, and that TcCYP6BQ7 was predominantly expressed in the brain and hemolymph from the late larval stage. TcCYP6BQ7 silencing by RNA interference increased larvae mortality in response to EOAV from 49.67% to 71.67%, suggesting that this gene is associated with plant toxicant detoxification. Combined results from this study indicate that the CYP6 family gene TcCYP6BQ7 likely plays a pivotal role in influencing the susceptibility of T. castaneum to plant toxicants. These findings may have implications for the development of novel therapeutics to control this agriculturally important pest.

Proteomic Analyses Detect Higher Expression of C-Type Lectins in Imidacloprid-Resistant Colorado Potato Beetle Leptinotarsa decemlineata Say

Simple Summary

Surveillance and determining the mechanisms of pesticide resistance are key components of resistance management. Mechanisms can be investigated using biochemical, genomic, proteomic and other modern analytical techniques. In the present study, proteomic analyses of Colorado potato beetle (CPB), one of the most adaptable insect pests to both plant toxins and synthetic insecticides, were applied to identify protein differences in insecticide-susceptible and resistant strains. Proteins identified in abdominal and midgut tissues based on separating by 2-dimensional (2-D) gels and mass spectrometry were associated with insect innate immunity. A database search found that the highest match was a C-type lectin (CTL), which is a component in the insect’s innate immune system. The 2-D gel spot identified as a CTL was greater in the insecticide-resistant CPB strain, but the CTL spot size was increased by exposure to imidacloprid in the susceptible strain. This is a novel finding, which suggests that CTLs and insect immunity may respond to certain toxins as well as to pathogens. There may also be a potential application for pest management if insect immunity is targeted.

Abstract

The Colorado potato beetle (CPB) is one of the most adaptable insect pests to both plant toxins and synthetic insecticides. Resistance in CPB is reported for over 50 classes of insecticides, and mechanisms of insecticide-resistance include enhanced detoxification enzymes, ABC transporters and target site mutations. Adaptation to insecticides is also associated with changes in behaviour, energy metabolism and other physiological processes seemingly unrelated to resistance but partially explained through genomic analyses. In the present study, in place of genomics, we applied 2-dimensional (2-D) gel and mass spectrometry to investigate protein differences in abdominal and midgut tissue of insecticide-susceptible (S) and -resistant (R) CPB. The proteomic analyses measured constitutive differences in several proteins, but the highest match was identified as a C-type lectin (CTL), a component of innate immunity in insects. The constitutive expression of the CTL was greater in the multi-resistant (LI) strain, and the same spot was measured in both midgut and abdominal tissue. Exposure to the neonicotinoid insecticide, imidacloprid, increased the CTL spot found in the midgut but not in the abdominal tissue of the laboratory (Lab) strain. No increase in protein levels in the midgut tissue was observed in the LI or a field strain (NB) tolerant to neonicotinoids. With the exception of biopesticides, such as Bacillus thuringiensis (Bt), no previous studies have documented differences in the immune response by CTLs in insects exposed to synthetic insecticides or the fitness costs associated with expression levels of immune-related genes in insecticide-resistant strains. This study demonstrates again how CPB has been successful at adapting to insecticides, plant defenses as well as pathogens.

Association between overexpression of cytochrome P450 genes and deltamethrin resistance in Rhipicephalus microplus

Cytochrome P450 monooxygenases mediated metabolic detoxification has been recognized as one of the mechanisms involved in resistance to pyrethroids, which is a class of pesticides that includes acaricides such as deltamethrin. Several cytochrome P450 (CYP) genes were identified in arthropod pests which are upregulated in response to exposure to pesticides used as acaricides. However, to date, limited information is available with respect to CYP genes and their response to acaricide exposure in ticks. We cloned and sequenced four CYP genes, the CYP41, CYP3006G8, CYP319A1 and CYP4W1 from reference susceptible IVRI-I strain of Rhipicephalus microplus. The expression pattern of the genes was investigated using qPCR in reference susceptible IVRI-I, pyrethroid-resistant IVRI-IV and multi-acaricide resistant IVRI-V strains. The effect of a single exposure of deltamethrin, at a concentration of 2600 μg/mL and 299.7 μg/mL on IVRI-IV and IVRI-V strains, respectively, on the expression of the four CYP genes was evaluated. In IVRI-IV strain, the CYP41 gene was highly overexpressed (FC 8.72) while CYP3006G8 was underexpressed with FC of 0.06. All the four genes were overexpressed in IVRI-V strain. After exposure to deltamethrin, the CYP3006G8 transcript levels were significantly upregulated at all time intervals in both resistant strains with the highest FC of 11.62 at 12 h in IVRI-IV and 13.38 at 3 h in IVRI-V. Our results suggest that the constitutive overexpression of CYP41 and deltamethrin induced upregulation of CYP3006G8 contribute to the development of pyrethroid resistance, specifically deltamethrin, in these two reference strains.

Transcriptomics-Based Approach Identifies Spinosad-Associated Targets in the Colorado Potato Beetle, Leptinotarsa decemlineata

Simple Summary

The Colorado potato beetle Leptinotarsa decemlineata is a potato pest that can cause substantial damages to potato crops worldwide. Multiple approaches have been leveraged to control this pest including the use of a variety of insecticides. Resistance to different insecticides aimed at controlling this insect has been reported and much work has been conducted in recent years to elucidate the underlying molecular changes associated with insecticide resistance in L. decemlineata. However, information is sparse regarding the molecular impact associated with spinosad treatment in this insect pest. The current study thus explores transcriptional changes associated with spinosad response in L. decemlineata exposed to this compound using high-throughput sequencing. Results presented show multiple transcripts of interest that exhibit differential expression in spinosad-treated L. decemlineata and provide a preliminary footprint of transcripts affected by this insecticide in this potato pest. Select targets identified in this signature should be further explored in follow-up studies to better characterize their contribution, if any, in the process of spinosad resistance.

Abstract

The Colorado potato beetle Leptinotarsa decemlineata is an insect pest that threatens potato crops globally. The primary method to control its damage on potato plants is the use of insecticides, including imidacloprid, chlorantraniliprole and spinosad. However, insecticide resistance has been frequently observed in Colorado potato beetles. The molecular targets and the basis of resistance to imidacloprid and chlorantraniliprole have both been previously quantified. This work was undertaken with the overarching goal of better characterizing the molecular changes associated with spinosad exposure in this insect pest. Next-generation sequencing was conducted to identify transcripts that were differentially expressed between Colorado potato beetles exposed to spinosad versus control insects. Results showed several transcripts that exhibit different expression levels between the two conditions, including ones coding for venom carboxylesterase-6, chitinase 10, juvenile hormone esterase and multidrug resistance-associated protein 4. In addition, several microRNAs, such as miR-12-3p and miR-750-3p, were also modulated in the investigated conditions. Overall, this work reveals a molecular footprint underlying spinosad response in Colorado potato beetles and provides novel leads that could be targeted as part of RNAi-based approaches to control this insect pest.

Genome-Wide Identification and Expression Analysis of Udp-Glucuronosyltransferases in the Whitefly Bemisia Tabaci (Gennadius) (HemipterA: Aleyrodidae)

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is an important agricultural pest worldwide. Uridine diphosphate (UDP)-glucuronosyltransferases (UGTs) are one of the largest and most ubiquitous groups of proteins. Because of their role in detoxification, insect UGTs are attracting increasing attention. In this study, we identified and analyzed UGT genes in B. tabaci MEAM1 to investigate their potential roles in host adaptation and reproductive capacity. Based on phylogenetic and structural analyses, we identified 76 UGT genes in the B. tabaci MEAM1 genome. RNA-seq and real-time quantitative PCR (RT-qPCR) revealed differential expression patterns of these genes at different developmental stages and in association with four host plants (cabbage, cucumber, cotton and tomato). RNA interference results of selected UGTs showed that, when UGT352A1, UGT352B1, and UGT354A1 were respectively silenced by feeding on dsRNA, the fecundity of B. tabaci MEAM1 was reduced, suggesting that the expressions of these three UGT genes in this species may be associated with host-related fecundity. Together, our results provide detailed UGTs data in B.tabaci and help guide future studies on the mechanisms of host adaptation by B.tabaci.

Functional analysis of cytochrome P450 genes linked with acetamiprid resistance in melon aphid, Aphis gossypii

Cytochrome P450 monooxygenases (P450s) are highly conserved multifunctional enzymes that play crucial roles in insecticide resistance development. In this study, the molecular mechanisms of P450s in acetamiprid resistance development to melon aphid, Aphis gossypii was investigated. Acetamiprid resistant (32.64-fold resistance) population (Ace-R) of A. gossypii was established by continuous selection with acetamiprid for 24 generations. Quantitative Real Time PCR was carried out to analyze the expression of P450 genes in both acetamiprid resistant (Ace-R) and susceptible (Ace-S) strains. Result showed that nine genes (CYP6CY14, CYP6DC1, CYP6CZ1, CYP6DD1, CYP6CY5, CYP6CY9, CYP6DA1, CYP6CY18, and CYP6CY16) of CYP3 clade, four genes (CYP302A1, CYP315A1, CYP301A1, and CYP314A1) of CYP2 clade, two genes (CYP4CK1, CYP4G51) of CYP4 clade and three genes (CYP306A1, CYP305E1, CYP307A1) of mitochondrial clade (Mito clad) were significantly up-regulated, in Ace-R compared to Ace-S strain. Whilst CYP4CJ2 gene from (CYP4 clade) was significantly down-regulated in Ace-R strain. Furthermore, RNA interference-mediated knockdown of CYP6CY14, CYP6DC1, and CYP6CZ1 genes significantly increased the sensitivity of Ace-R strain to acetamiprid. Taken together, this study showed that P450 genes especially CYP6CY14, CYP6DC1 and CYP6CZ1 are potentially involved in acetamiprid resistance development in A. gossypii. This study could be useful to understand the molecular basis of acetamiprid resistance mechanism in A. gossypii.

Population expansion and genomic adaptation to agricultural environments of the soybean looper, Chrysodeixis includens

Evolutionary studies of insect pests improve our ability to anticipate problems in agricultural ecosystems, such as pest outbreaks, control failures, or expansions of the host range. Here, we investigated the mechanisms underlying the evolutionary processes behind the recent census size expansion and local adaptation of Chrysodeixis includens. First, we sequenced mitochondrial markers to conduct a phylogeographic investigation of C. includens historical processes. Then, we combined a de novo genotyping-by-sequencing approach with a study of agricultural landscapes to uncover recent processes of adaptation. Primarily, we found low genetic diversity across all markers and clear indications of a recent demographic expansion. We also found a lack of significant isolation by distance (IBD), and weak or absent genetic structure considering geographic locations. However, we did find initial signs of population differentiation that were associated with host plant types (i.e., soybean and cotton). Agricultural landscape attributes, including soybean crops, were significantly associated with putative markers under positive selection. Moreover, positive selection associated with host differentiation was putatively linked to digestive enzymes. This study showed how landscape composition and host plants can affect the evolutionary process of agricultural pest insects such as C. includens.

The genome of pest Rhynchophorus ferrugineus reveals gene families important at the plant-beetle interface

The red palm weevil, Rhynchophorus ferrugineus, infests palm plantations, leading to large financial losses and soil erosion. Pest-host interactions are poorly understood in R. ferrugineus, but the analysis of genetic diversity and pest origins will help advance efforts to eradicate this pest. We sequenced the genome of R. ferrugineus using a combination of paired-end Illumina sequencing (150 bp), Oxford Nanopore long reads, 10X Genomics and synteny analysis to produce an assembly with a scaffold N50 of ~60 Mb. Structural variations showed duplication of detoxifying and insecticide resistance genes (e.g., glutathione S-transferase, P450, Rdl). Furthermore, the evolution of gene families identified those under positive selection including one glycosyl hydrolase (GH16) gene family, which appears to result from horizontal gene transfer. This genome will be a valuable resource to understand insect evolution and behavior and to allow the genetic modification of key genes that will help control this pest.

A Day in the Life: Identification of Developmentally Regulated MicroRNAs in the Colorado Potato Beetle (Leptinotarsa decemlineata; Coleoptera: Chrysomelidae)

The Colorado potato beetle (Leptinotarsa decemlineata (Say)) is an important pest of the cultivated potato (Solanum tuberosum (L.) [Solanales: Solanaceae]). With its broad resistance toward commonly used insecticides, it is clear that more sophisticated control strategies are needed. Due to their importance in insect development, microRNAs (miRNAs) represent a potential tool to employ in insect control strategies. However, most studies conducted in this area have focused on model species with well-annotated genomes. In this study, next-generation sequencing was used to catalogue the miRNAs produced by L. decemlineata across all eight stages of its development, from eggs to adults. For most stages, the length of miRNAs peaked between 21 and 22 nt, though it was considerably longer for the egg stage (26 nt). Global profiling of miRNAs revealed three distinct developmental clusters: 1) egg stage; 2) early stage (first, second, and third instar); and 3) late stage (fourth instar, prepupae, pupae, and adult). We identified 86 conserved miRNAs and 33 bonafide novel miRNAs, including stage-specific miRNAs and those not previously identified in L. decemlineata. Most of the conserved miRNAs were found in multiple developmental stages, whereas the novel miRNAs were often stage specific with the bulk identified in the egg stage. The identified miRNAs have a myriad of putative functions, including growth, reproduction, and insecticide resistance. We discuss the putative roles of some of the most notable miRNAs in the regulation of L. decemlineata development, as well as the potential applications of this research in Colorado potato beetle management.

UDP-glycosyltransferases contribute to spirotetramat resistance in Aphis gossypii Glover

Uridine diphosphate (UDP)-glycosyltransferases (UGTs) catalyze the conjugation of small lipophilic endogenous and exogenous compounds with sugars to produce water-soluble glycosides, playing an important role in insect endobiotic regulation and xenobiotic detoxification. In this study, two UGT-inhibitors, sulfinpyrazone and 5-nitrouracil, significantly increased spirotetramat toxicity against third instar nymphs of resistant Aphis gossypii, whereas there were no synergistic effects in apterous adult aphids, suggesting UGT involvement in spirotetramat resistance in cotton aphids. Furthermore, the UHPLC-MS/MS was employed to determine the content of spirotetramat and its four metabolites (S-enol, S-glu, S-mono, S-keto) in the honeydew of resistant cotton aphids under spirotetramat treatment. No residual spirotetramat was detected in the honeydew, while its four metabolites were detected at a S-enol: S-glu: S-mono: S-keto ratio of 69.30: 6.54: 1.44: 1.00. Therefore, glycoxidation plays a major role in spirotetramat inactivation and excretion in resistant aphids. Compared with the susceptible strain, the transcriptional levels of UGT344M2 were significantly upregulated in nymphs and adults of the resistant strain. RNA interference of UGT344M2 dramatically increased spirotetramat toxicity in nymphs, but no such effect were found in the resistant adult aphids. Overall, UGT-mediated glycoxidation were found to be involved in spirotetramat resistance. The suppression of UGT344M2 significantly increased the sensitivity of resistant nymphs to spirotetramat, suggesting that UGT344M2 upregulation might be associated with spirotetramat detoxification. This study provides an overview of the involvement of metabolic factors, UGTs, in the development of spirotetramat resistance.

Overexpression of UDP-glycosyltransferase potentially involved in insecticide resistance in Aphis gossypii Glover collected from Bt cotton fields in China

BACKGROUND

The cotton aphid Aphis gossypii Glover is one of the most destructive insect pests. It has evolved resistance to numerous insecticides around the world due to the application of insecticides. Uridine diphosphate (UDP)-glycosyltransferases (UGTs) have been reported to potentially facilitate the detoxification process of imidacloprid and thiamethoxam in A. gossypii.

RESULTS

In this study, the field populations of A. gossypii developed different levels of resistance to multiple insecticides. A UGT inhibitor, 5-nitrouracil, dramatically increased the toxicity of acetamiprid in resistant populations, moderately increased the toxicity of sulfoxaflor in the imidacloprid susceptible (IMI_S) population, and populations from Yuncheng in Shanxi Province (SXYC) and Jingzhou in Hubei Province (HBJZ), and increased the toxicity of bifenthrin in the IMI_S and HBJZ populations, but there was no synergism on omethoate or carbosulfan. Quantitative real-time PCR analysis revealed that UGT344B4 and UGT344C7 were overexpressed in all field populations, and UGT344N4 was overexpressed in the SDBZ and HBZJ populations. Furthermore, the suppression of UGT344B4 or UGT344C7 by RNA interference significantly increased the susceptibility to bifenthrin in the IMI_S population and the susceptibility to sulfoxaflor in the SXYC population.

CONCLUSION

These results suggested that UGTs are potentially involved in the detoxification of neonicotinoid, sulfoximine, and pyrethroid insecticides in A. gossypii. Furthermore, the overexpression of UGTs could be associated with insecticide resistance in field populations of A. gossypii. The results might be helpful for the management of insecticide resistance in field populations of A. gossypii. © 2019 Society of Chemical Industry.

Functional analysis of UGT201D3 associated with abamectin resistance in Tetranychus cinnabarinus (Boisduval)

Uridine diphosphate (UDP)-glycosyltransferases (UGTs) are widely distributed within living organisms and share roles in biotransformation of various lipophilic endo- and xenobiotics with activated UDP sugars. In this study, it was found that the activity of UGTs in abamectin-resistant (AbR) strain was significantly higher (2.35-fold) than that in susceptible strain (SS) of Tetranychus cinnabarinus. Further analysis showed that 5-nitrouracil, the inhibitor of UGTs, could enhance the lethal effect of abamectin on mites. From the previous microarray results, we found an UGT gene (UGT201D3) overexpressed in AbR strain. Quantitative PCR analysis showed that UGT201D3 was highly expressed and more inducible with abamectin exposure in the AbR strain. After silencing the transcription of UGT201D3, the activity of UGTs was decreased and the susceptibility to abamectin was increased in AbR strain whereas it was not in SS. Furthermore, UGT201D3 gene was then successfully expressed in Escherichia coli. The recombinant UGT201D3 exhibited α-naphthol activity (2.81 ± 0.43 nmol/mg protein/min), and the enzyme activity could be inhibited by abamectin (inhibitory concentration at 50%: 57.50 ± 3.54 μmol/L). High-performance liquid chromatography analysis demonstrated that the recombinant UGT201D3 could effectively deplete abamectin (15.77% ± 3.72%) incubating with 150 μg protein for 6 h. These results provided direct evidence that UGT201D3 was involved in abamectin resistance in T. cinnabarinus.

Changes in emergence phenology, fatty acid composition, and xenobiotic-metabolizing enzyme expression is associated with increased insecticide resistance in the Colorado potato beetle

The Colorado potato beetle (Leptinotarsa decemlineata) is a major agricultural pest of solanaceous crops. An effective management strategy employed by agricultural producers to control this pest species is the use of systemic insecticides. Recent emphasis has been placed on the use of neonicotinoid insecticides. Despite efforts to curb resistance development through integrated pest management approaches, resistance to neonicotinoids in L. decemlineata populations continues to increase. One contributing factor may be alterations in insect fatty acids, which have multiple metabolic functions and are associated with the synthesis of xenobiotic-metabolizing enzymes to mitigate the effects of insecticide exposure. In this study, we analyzed the fatty acid composition of L. decemlineata populations collected from an organic production field and from a commercially managed field to determine if fatty acid composition varied between the two populations. We demonstrate that a population of L. decemlineata that has a history of systemic neonicotinoid exposure (commercially managed) has a different lipid composition and differential expression of known metabolic detoxification mechanisms relative to a population that has not been exposed to neonicotinoids (organically managed). The fatty acid data indicated an upregulation of Δ⁶ desaturase in the commercially managed L. decemlineata population and suggest a role for eicosanoids and associated metabolic enzymes as potential modulators of insecticide resistance. We further observed a pattern of delayed emergence within the commercially managed population compared with the organically managed population. Variations in emergence timing together with specific fatty acid regulation may significantly influence the capacity of L. decemlineata to develop insecticide resistance.

The ABCB Multidrug Resistance Proteins Do Not Contribute to Ivermectin Detoxification in the Colorado Potato Beetle, Leptinotarsa decemlineata (Say)

The Colorado potato beetle, Leptinotarsa decemlineata (Say), is a significant agricultural pest that has developed resistance to many insecticides that are used to control it. Investigating the mechanisms of insecticide detoxification in this pest is important for ensuring its continued control, since they may be contributors to such resistance. Multidrug resistance (MDR) genes that code for the ABCB transmembrane efflux transporters are one potential source of insecticide detoxification activity that have not been thoroughly examined in L. decemlineata. In this study, we annotated the ABCB genes found in the L. decemlineata genome and then characterized the expression profiles across midgut, nerve, and Malpighian tubule tissues of the three full transporters identified. To investigate if these genes are involved in defense against the macrocyclic lactone insecticide ivermectin in this insect, each gene was silenced using RNA interference or MDR protein activity was inhibited using a chemical inhibitor, verapamil, before challenging the insects with a dose of ivermectin. Survival of the insects did not significantly change due to gene silencing or protein inhibition, suggesting that MDR transporters do not significantly contribute to defense against ivermectin in L. decemlineata.

Role of several cytochrome P450s in the resistance and cross-resistance against imidacloprid and acetamiprid of Bemisia tabaci (Hemiptera: Aleyrodidae) MEAM1 cryptic species in Xinjiang, China

Neonicotinoids are commonly used for the control of the whitefly Bemisia tabaci in cotton field. Laboratory test and field experiments have found that whitefly has a high risk of developing resistance and cross-resistance to the pesticide. Over expression of cytochrome P450 is one of the main mechanism that controls pesticide resistance in many insects. In this study we use MEAM1 whitefly, the dominant cryptic species of B. tabaci in Xinjiang cotton field, to investigate the possible resistance and cross-resistance mechanism controlled by cytochrome P450 enzymes. The P450 enzyme activity was higher in both selected strains of imidacloprid and acetamipird than that of susceptible strain. Synergism test showed that piperonyl butoxide (PBO) distinctly increased the control efficiency of imidacloprid and acetamiprid to the two resistance selected strains. Four out of 13 cytochrome genes, CYP4CS3, CYP6CX5, CYP6DW2 and CYP6CM1 were significantly up-regulated in the two selected strains based on real-time fluorescence quantitative PCR results. Other 3 genes, CYP6CX2, CYP6CX4 and CYP6DW3 were only highly expressed in the acetamiprid selected strain instead of the susceptible strain and imidacloprid selected strain. CYP6CM1 showed the highest expression level among all the 13 tested genes. No functional mutation of CYP6CM1 was found by sequence analysis. The possible role of these genes involving the resistance and cross-resistance of the whitefly MEAM1 cryptic species against neonicotinoids was discussed.

Differential expression of transcripts with potential relevance to chlorantraniliprole response in the Colorado potato beetle, Leptinotarsa decemlineata

The Colorado potato beetle (Leptinotarsa decemlineata [Say]) is an insect pest that can significantly harm potato plants worldwide. Control of this insect relies heavily on chemical insecticides such as chlorantraniliprole. Nevertheless, the complete molecular signature associated with response to this compound is lacking in L. decemlineata. In this study, amplification and quantification by qRT-PCR (quantitative reverse transcription-polymerase chain reaction) of targets relevant to chlorantraniliprole were undertaken in insects exposed to this chemical. This approach showed modulation of numerous cytochrome P450s, such as CYP350D1 and CYP4Q3, as well as upregulation of microRNAs (miRNAs), including miR-1-3p and miR-305-5p, in chlorantraniliprole-exposed insects. Functional assessment of transcript targets predicted to be regulated by these miRNAs was performed and revealed their likely impact on transcriptional regulation. RNAi-based targeting of CYP350D1 notably provided preliminary evidence of its underlying implication for chlorantraniliprole response in L. decemlineata. Overall, this study strengthens the current knowledge of the molecular changes linked to chlorantraniliprole response in L. decemlineata and provides novel targets with potential relevance to chlorantraniliprole susceptibility in this insect pest of global relevance.

Identification of candidate ATP-binding cassette transporter gene family members in Diaphorina citri (Hemiptera: Psyllidae) via adult tissues transcriptome analysis

The ATP-binding cassette (ABC) transporters exist in all living organisms and play major roles in various biological functions by transporting a wide variety of substrates across membranes. The functions of ABC transporters in drug resistance have been extensively studied in vertebrates; however, they are rarely characterized in agricultural pests. The Asian citrus psyllid, Diaphorina citri, is one of the most damaging pests of the Citrus genus because of its transmission of Huanglongbing, also known as Yellow Dragon disease. In this study, the next-generation sequencing technique was applied to research the ABC transporters of D. citri. Fifty-three ABC transporter genes were found in the RNA-Seq data, and among these ABC transporters, 4, 4, 5, 2, 1, 4, 18 and 15 ABC proteins belonged to the ABCA-ABCH subfamilies, respectively. Different expression profiles of 52 genes between imidacloprid-resistant and imidacloprid-susceptible strains were studied by qRT-PCR; 5 ABCGs and 4 ABCHs were significantly upregulated in the imidacloprid-resistant strain. In addition, five of the nine upregulated genes were widely expressed in adult tissues in spatial expression analysis. The results suggest that these genes may play key roles in this phenotype. In general, this study contributed to our current understanding of D. citri resistance to insecticides.

Characterization of UDP-Glucuronosyltransferases and the Potential Contribution to Nicotine Tolerance in Myzus persicae

Uridine diphosphate (UDP)-glycosyltransferases (UGTs) are major phase II detoxification enzymes involved in glycosylation of lipophilic endobiotics and xenobiotics, including phytoalexins. Nicotine, one of the most abundant secondary plant metabolites in tobacco, is highly toxic to herbivorous insects. Plant-herbivore competition is the major impetus for the evolution of large superfamilies of UGTs and other detoxification enzymes. However, UGT functions in green peach aphid (Myzus persicae) adaptation are unknown. In this study, we show that UGT inhibitors (sulfinpyrazone and 5-nitrouracil) significantly increased nicotine toxicity in M. persicae nicotianae, suggesting that UGTs may be involved in nicotine tolerance. In total, 101 UGT transcripts identified in the M. persicae genome/transcriptome were renamed according to the UGT Nomenclature Committee guidelines and grouped into 11 families, UGT329, UGT330, UGT339, UGT341-UGT345, and UGT348-UGT350, with UGT344 containing the most (57). Ten UGTs (UGT330A3, UGT339A2, UGT341A6, UGT342B3, UGT343C3, UGT344D5, UGT344D8, UGT348A3, UGT349A3, and UGT350A3) were highly expressed in M. persicae nicotianae compared to M. persicae sensu stricto. Knockdown of four UGTs (UGT330A3, UGT344D5, UGT348A3, and UGT349A3) significantly increased M. persicae nicotianae sensitivity to nicotine, suggesting that UGT expression in this subspecies may be associated with nicotine tolerance and thus host adaptation. This study reveals possible UGTs relevant to nicotine adaptation in tobacco-consuming M. persicae nicotianae, and the findings will facilitate further validation of the roles of these UGTs in nicotine tolerance.

Differential expression of genes in greenbug (Schizaphis graminum Rondani) treated by imidacloprid and RNA interference

BACKGROUND

Insecticides act as toxins, inhibitors of digestion and deterrents, and affect the expression of many genes in insects. To assess key genes associated with the detoxification or regulation of imidacloprid in greenbug, Schizaphis graminum (Rondani), the transcriptome and digital gene expression (DGE) profile were analyzed using Illumina sequencing.

RESULTS

In total, 48 763 494 clean reads were obtained by sequencing. Expression profile analysis showed that 2782 unigenes were differently expressed between the imidacloprid treatment and control groups. After exposure to imidacloprid, the expression levels of 1846 unigenes were upregulated and 936 were downregulated in comparison with controls. Expression patterns of the top 20 highly expressed genes show that they could be involved in the detoxification of imidacloprid. Silencing of multidrug resistance-associated gene (MRA), GATA-binding gene (GAT) and takeout-like precursor gene (TLP) resulted in increasing susceptibility to imidacloprid.

CONCLUSIONS

The differentially expressed genes in S. graminum have potential regulatory or detoxification roles in response to imidacloprid. These results should be useful in understanding the molecular mechanisms of greenbug adaption to imidacloprid. © 2018 Society of Chemical Industry.

Substrate specificity and promiscuity of horizontally transferred UDP-glycosyltransferases in the generalist herbivore Tetranychus urticae

Uridine diphosphate (UDP)-glycosyltransferases (UGTs) catalyze the addition of UDP-sugars to small hydrophobic molecules, turning them into more water-soluble metabolites. While their role in detoxification is well documented for vertebrates, arthropod UGTs have only recently been linked to the detoxification and sequestration of plant toxins and insecticides. The two-spotted spider mite Tetranychus urticae is a generalist herbivore notorious for rapidly developing resistance to insecticides and acaricides. We identified a set of eight UGT genes that were overexpressed in mites upon long-term acclimation or adaptation to a new host plant and/or in mite strains highly resistant to acaricides. Functional expression revealed that they were all catalytically active and that the majority preferred UDP-glucose as activated donor for glycosylation of model substrates. A high-throughput substrate screening of both plant secondary metabolites and pesticides revealed patterns of both substrate specificity and promiscuity. We further selected nine enzyme-substrate combinations for more comprehensive analysis and determined steady-state kinetic parameters. Among others, plant metabolites such as capsaicin and several flavonoids were shown to be glycosylated. The acaricide abamectin was also glycosylated by two UGTs and one of these was also overexpressed in an abamectin resistant strain. Our study corroborates the potential role of T. urticae UGTs in detoxification of both synthetic and natural xenobiotic compounds and paves the way for rapid substrate screening of arthropod UGTs.

Global transcriptome profiling and functional analysis reveal that tissue-specific constitutive overexpression of cytochrome P450s confers tolerance to imidacloprid in palm weevils in date palm fields

Background

Cytochrome P450-dependent monooxygenases (P450s), constituting one of the largest and oldest gene superfamilies found in many organisms from bacteria to humans, play a vital role in the detoxification and inactivation of endogenous toxic compounds. The use of various insecticides has increased over the last two decades, and insects have developed resistance to most of these compounds through the detoxifying function of P450s. In this study, we focused on the red palm weevil (RPW), Rhynchophorus ferrugineus, the most devastating pest of palm trees worldwide, and demonstrated through functional analysis that upregulation of P450 gene expression has evolved as an adaptation to insecticide stress arising from exposure to the neonicotinoid-class systematic insecticide imidacloprid.

Results

Based on the RPW global transcriptome analysis, we identified 101 putative P450 genes, including 77 likely encoding protein coding genes with ubiquitous expression. A phylogenetic analysis revealed extensive functional and species-specific diversification of RPW P450s, indicating that multiple CYPs actively participated in the detoxification process. We identified highly conserved paralogs of insect P450s that likely play a role in the development of resistance to imidacloprid: Drosophila Cyp6g1 (CYP6345J1) and Bemisia tabaci CYP4C64 (CYP4LE1). We performed a toxicity bioassay and evaluated the induction of P450s, followed by the identification of overexpressed P450s, including CYP9Z82, CYP6fra5, CYP6NR1, CYP6345J1 and CYP4BD4, which confer cross-resistance to imidacloprid. In addition, under imidacloprid insecticide stress in a date palm field, we observed increased expression of various P450 genes, with CYP9Z82, CYP4BD4, CYP6NR1 and CYP6345J1 being the most upregulated detoxification genes in RPWs. Expression profiling and cluster analysis revealed P450 genes with multiple patterns of induction and differential expression. Furthermore, we used RNA interference to knock down the overexpressed P450s, after which a toxicity bioassay and quantitative expression analysis revealed likely candidates involved in metabolic resistance against imidacloprid in RPW. Ingestion of double-stranded RNA (dsRNA) successfully knocked down the expression of CYP9Z82, CYP6NR1 and CYP345J1 and demonstrated that silencing of CYP345J1 and CYP6NR1 significantly decreased the survival rate of adult RPWs treated with imidacloprid, indicating that overexpression of these two P450s may play an important role in developing tolerance to imidacloprid in a date palm field.

Conclusion

Our study provides useful background information on imidacloprid-specific induction and overexpression of P450s, which may enable the development of diagnostic tools/markers for monitoring the spread of insecticide resistant RPWs. The observed trend of increasing tolerance to imidacloprid in the date palm field therefore indicated that strategies for resistance management are urgently needed.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5837-4) contains supplementary material, which is available to authorized users.

Identification of detoxification genes in imidacloprid-resistant Asian citrus psyllid (Hemiptera: Lividae) and their expression patterns under stress of eight insecticides

BACKGROUND

The Asian citrus psyllid, Diaphorina citri, is one of the major pests in citrus-growing areas around the world. The application of insecticides is the most effective method to reduce the population of D. citri. However, D. citri has developed resistance to multiple classes of insecticides. Understanding resistance mechanisms is crucial to the management of D. citri. In this study, molecular assays were performed to characterize imidacloprid resistance mechanisms.

RESULTS

Based on the D. citri transcriptome database and other known insect resistance genes, 16 cytochrome P450, eight glutathione-S-transferase and six esterase genes were selected for cloning and sequencing. The gene expression analysis of 30 detoxification genes demonstrated that the relative expression of CYP4g15, CYP303A1, CYP4C62, CYP6BD5, GSTS1 and EST-6 were moderately high (>5-fold increase) in the imidacloprid-resistant strain. Feeding of double-stranded RNA (dsRNA) reduced the expression of the six genes (46.7%-72.1%) and resulted in significant adult mortality (65.62%-82.76%). We also determined the ability of different insecticides to induce the six selected genes. The expression of CYP4C62 and GSTS1 genes were the most significantly upregulated in adults treated with all insecticides, except for chlorfenapyr. In chlorfenapyr-treated D. citri, expression of CYP4g15 and CYP303A1 were the most highly induced.

CONCLUSION

Overexpressed detoxification genes were associated with imidacloprid resistance, as confirmed by RNA interference feeding tests. The induction of the six selected genes when exposed to different insecticides supported the hypothesis that they were involved in the metabolism of the tested insecticides. © 2018 Society of Chemical Industry.

Knockdown of cytochrome P450 CYP6 family genes increases susceptibility to carbamates and pyrethroids in the migratory locust, Locusta migratoria

Insect cytochrome P450 monooxygenase (CYP) plays a key role in the detoxification of insecticides. In this study, four cDNA sequences of CYP6 genes were identified and characterized. Transcription levels of LmCYP6HC1 and LmCYP6HCL1 were high in first- and fourth-instar nymph stages, respectively. LmCYP6HN1 was primarily expressed in the egg to third-instar nymph stages, while LmCYP6HQ1 was predominantly expressed in the stages from fourth-instar nymph to the adult. The four CYP6 genes were predominantly distributed in the antenna, brain, fat body, integument, and hemolymph. Piperonyl butoxide exposure inhibited total CYP activity and synergized the toxicity of carbamates and pyrethroids. Knockdown of LmCYP6HL1, LmCYP6HN1, and LmCYP6HQ1 increased nymph mortality following exposure to carbaryl, and silencing of LmCYP6HC1, LmCYP6HL1, LmCYP6HN1, and LmCYP6HQ1 comprehensively raised nymph mortality following exposure to fluvalinate. Knockdown of LmCYP6HL1 or LmCYP6HN1 significantly increased nymph mortality following exposure to cypermethrin or fenvalerate, respectively. These results suggest that the CYP6 family plays a key role in determining the susceptibility of Locusta migratoria to both carbamates and pyrethroids.

Identification of a novel cytochrome P450 CYP3356A1 linked with insecticide detoxification in Bradysia odoriphaga

BACKGROUND

Cytochrome P450 monooxygenases play an important role in the metabolic detoxification of insecticides in insect pests. However, little is known about the role of a specific P450 gene and its responses to insecticide exposure in Bradysia odoriphaga, a major pest in Chinese chive production.

RESULTS

In this study, a novel P450 gene, CYP3356A1, was cloned from Bradysia odoriphaga. The full-length cDNA sequence of CYP3356A1 is 2153 bp and its open reading frame (ORF) encodes 508 amino acids. Quantitative real time PCR(qRT-PCR) analyses in different tissues showed that CYP3356A1 expression was the highest in the Malpighian tubule. Moreover, among the different developmental stages of the insect, the highest expression of CYP3356A1 was found in fourth-instar larvae. Expression of CYP3356A1 was upregulated by treatment with imidacloprid, thiamethoxam, and β-cypermethrin at median lethal concentrations (LC₅₀ ). RNA interference (RNAi)-mediated silencing of CYP3356A1 significantly increased mortality by 36.90%, 25.17%, and 36.73 when fourth-instar B. odoriphaga larvae were exposed to imidacloprid, thiamethoxam, and β-cypermethrin, respectively, at the LC₅₀ dose.

CONCLUSION

These results demonstrate that CYP3356A1 is related to the detoxification of imidacloprid, thiamethoxam, and β-cypermethrin in B. odoriphaga. Moreover, the study also increased our understanding of the molecular mechanisms of insecticide detoxification in this pest insect. © 2018 Society of Chemical Industry.

UDP-glycosyltransferase genes and their association and mutations associated with pyrethroid resistance in Anopheles sinensis (Diptera: Culicidae)

BACKGROUND

UDP-glycosyltransferase (UGT) is an important biotransformation superfamily of enzymes. They catalyze the transfer of glycosyl residues from activated nucleotide sugars to acceptor hydrophobic molecules, and function in several physiological processes, including detoxification, olfaction, cuticle formation, pigmentation. The diversity, classification, scaffold location, characteristics, phylogenetics, and evolution of the superfamily of genes at whole genome level, and their association and mutations associated with pyrethroid resistance are still little known.

METHODS

The present study identified UGT genes in Anopheles sinensis genome, classified UGT genes in An. sinensis, Anopheles gambiae, Aedes aegypti and Drosophila melanogaster genomes, and analysed the scaffold location, characteristics, phylogenetics, and evolution of An. sinensis UGT genes using bioinformatics methods. The present study also identified the UGTs associated with pyrethroid resistance using three field pyrethroid-resistant populations with RNA-seq and RT-qPCR, and the mutations associated with pyrethroid resistance with genome re-sequencing in An. sinensis.

RESULTS

There are 30 putative UGTs in An. sinensis genome, which are classified into 12 families (UGT301, UGT302, UGT306, UGT308, UGT309, UGT310, UGT313, UGT314, UGT315, UGT36, UGT49, UGT50) and further into 23 sub-families. The UGT308 is significantly expanded in gene number compared with other families. A total of 119 UGTs from An. sinensis, An. gambiae, Aedes aegypti and Drosophila melanogaster genomes are classified into 19 families, of which seven are specific for three mosquito species and seven are specific for Drosophila melanogaster. The UGT308 and UGT302 are proposed to main families involved in pyrethroid resistance. The AsUGT308D3 is proposed to be the essential UGT gene for the participation in biotransformation in pyrethroid detoxification process, which is possibly regulated by eight SNPs in its 3' flanking region. The UGT302A3 is also associated with pyrethroid resistance, and four amino acid mutations in its coding sequences might enhance its catalytic activity and further result in higher insecticide resistance.

CONCLUSIONS

This study provides the diversity, phylogenetics and evolution of UGT genes, and potential UGT members and mutations involved in pyrethroid resistance in An. sinensis, and lays an important basis for the better understanding and further research on UGT function in defense against insecticide stress.

UDP-Glycosyltransferase Genes in the Striped Rice Stem Borer, Chilo suppressalis (Walker), and Their Contribution to Chlorantraniliprole Resistance

Uridine diphosphate glycosyltransferases (UGTs) are multifunctional detoxification enzymes, which are involved in metabolizing various chemicals and contribute to the development of insecticide resistance. However, the possible roles of UGTs in chlorantraniliprole resistance in Chilo suppressalis have rarely been studied in detail. Based on genome data, 24 UGT genes in C. suppressalis belonging to 11 families were identified, which were designated by the UGT nomenclature committee. Synergism assay data suggested that UGTs are potentially involved in chlorantraniliprole resistance in C. suppressalis.CsUGT40AL1 and CsUGT33AG3 were significantly overexpressed in the chlorantraniliprole resistant strain (12.36- and 5.34-fold, respectively). The two UGTs were highly expressed in the larval Malpighian tubules, fat body, and midgut; however, expression was lowest in the head. Injection of individual dsRNAs reduced the expression of the two target genes (by 69.34% and 48.74%, respectively) and caused significant higher larval mortality (81.33% and 54.67%, respectively). Overexpression of CsUGT40AL1 and CsUGT33AG3 was potentially involved in chlorantraniliprole resistance in C. suppressalis, as confirmed by the RNAi assay. Our findings suggest that overexpression of UGTs may contribute to chlorantraniliprole resistance in C. suppressalis.

UDP-Glycosyltransferases are involved in imidacloprid resistance in the Asian citrus psyllid, Diaphorina citri (Hemiptera: Lividae)

UDP-glycosyltransferases (UGTs), as phase II detoxification enzymes, are widely distributed within living organisms and play vital roles in the biotransformation of endobiotics and xenobiotics in insects. Insects increase the expression of detoxification enzymes to cope with the stress of xenobiotics, including insecticides. However, the roles of UGTs in insecticide resistance are still seldom reported. In this study, two UGT inhibitors, namely, 5-nitrouracil and sulfinpyrazone, were found to synergistically increase the toxicity of imidacloprid in the resistant population of Diaphorina citri. Based on transcriptome data, a total of 17 putative UGTs were identified. Quantitative real-time PCR showed that fourteen of the 17 UGT genes were overexpressed in the resistant population relative to the susceptible population. Using RNA interference technology to knockdown six UGT genes, the results suggested that silencing the selected UGT375A1, UGT383A1, UGT383B1, and UGT384A1 genes dramatically increased the toxicity of imidacloprid in the resistant population. However, silencing the UGT362B1 and UGT379A1 genes did not result in a significant increase in the toxicity of imidacloprid in the resistant population. These findings revealed that some upregulated UGT genes were involved in imidacloprid resistance in D. citri. These results shed some light upon and further our understanding of the mechanisms of insecticide resistance in insects.

An effective pest management approach in potato to combat insect pests and herbicide

Insect pests and weeds incur significant yield losses to potato crop worldwide. The increasing crop losses provide impetus for the development of pest management strategy that is equally effective against insect pests and weeds. In the present study, a molecular approach was used to develop transgenic potato lines (cv. Marabel) effective against Colorado potato beetle (Leptinotarsa decemlineata Say), potato tuber moth (Phthorimaea operculella Z.) and Basta® application. Agrobacterium tumefaciens strain EHA105 harboring binary vector pTF101.1 containing cry1Ac gene under the control of 35S and AoPR1 promoters was used to infect leaf discs and internodal explants. Phosphinothrincin was used at optimal concentration (2 mg/l) for the screening of primary transformants. The standard molecular assays exhibited gene integration and expression in putative transgenics. Real-time data revealed up to ninefold high cry1Ac transcript levels, whereas cry protein amount was estimated to 0.4 ppm in primary transformants. The analysis of first tuber progeny showed proper integration cry1Ac and bar genes in subsequent progeny. The transgenic plants also showed tolerance to the application of Basta®. The efficacy of cry1Ac was evaluated by allowing larvae of Colorado potato beetle (CPB) and potato tuber moth (PTM) to feed on transgenic plants. Results revealed appreciable mortality levels of different larval instars of CPB (20-100%) and PTM (50-100%). Overall, our results exhibit the potential of these transgenic lines to be used in a potato breeding program with the purpose to control insect pests and weeds.

Transcriptomic analysis reveals similarities in genetic activation of detoxification mechanisms resulting from imidacloprid and chlorothalonil exposure

The Colorado potato beetle, Leptinotarsa decemlineata (Say), is an agricultural pest of commercial potatoes in parts of North America, Europe, and Asia. Plant protection strategies within this geographic range employ a variety of pesticides to combat not only the insect, but also plant pathogens. Previous research has shown that field populations of Leptinotarsa decemlineata have a chronological history of resistance development to a suite of insecticides, including the Group 4A neonicotinoids. The aim of this study is to contextualize the transcriptomic response of Leptinotarsa decemlineata when exposed to the neonicotinoid insecticide imidacloprid, or the fungicides boscalid or chlorothalonil, in order to determine whether these compounds induce similar detoxification mechanisms. We found that chlorothalonil and imidacloprid induced similar patterns of transcript expression, including the up-regulation of a cytochrome p450 and a UDP-glucuronosyltransferase transcript, which belong to protein families associated with xenobiotic metabolism. Further, transcriptomic responses varied among individuals within the same treatment group, suggesting individual insects’ responses vary within a population and may cope with chemical stressors in a variety of manners.

Agricultural fungicides inadvertently influence the fitness of Colorado potato beetles, Leptinotarsa decemlineata, and their susceptibility to insecticides

The Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), is an agricultural pest of solanaceous crops which has developed insecticide resistance at an alarming rate. Up to this point, little consideration has been given to unintended, or inadvertent effects that non-insecticide xenobiotics may have on insecticide susceptibility in L. decemlineata. Fungicides, such as chlorothalonil and boscalid, are often used to control fungal pathogens in potato fields and are applied at regular intervals when L. decemlineata populations are present in the crop. In order to determine whether fungicide use may be associated with elevated levels of insecticide resistance in L. decemlineata, we examined phenotypic responses in L. decemlineata to the fungicides chlorothalonil and boscalid. Using enzymatic and transcript abundance investigations, we also examined modes of molecular detoxification in response to both insecticide (imidacloprid) and fungicide (boscalid and chlorothalonil) application to more specifically determine if fungicides and insecticides induce similar metabolic detoxification mechanisms. Both chlorothalonil and boscalid exposure induced a phenotypic, enzymatic and transcript response in L. decemlineata which correlates with known mechanisms of insecticide resistance.

Detoxification mechanisms involved in ivermectin resistance in the cattle tick, Rhipicephalus (Boophilus) microplus

The cattle tick Rhipicephalus microplus is one of the most important ectoparasites with great sanitary and economic impact for cattle rearing worldwide. Ivermectin is commonly used to control tick populations, but its use over the last 30 years has led to the development of resistant populations of R. microplus, and a concomitant loss of efficacy. In this context, we aimed to determine the metabolic mechanisms that contribute to ivermectin resistance in a resistant strain of this species. We performed lethal time bioassays with inhibitors of detoxifying enzymes and xenobiotic transporters (four detoxification pathways) using two strains of ticks: a susceptible strain, Mozo, and a resistant strain, Juarez. We used four inhibitors to test the involvement of different families of proteins responsible for detoxification of ivermectin, namely cytochrome P450, esterases, glutathione-S-transferase, and ATP Binding Cassette Transporters. We calculated the synergistic factor for each inhibitor and strain. To different degrees, all tested inhibitors altered the mortality rates in the strain Juarez, indicating that multiple mechanisms are responsible for the resistant phenotype. Detoxification mechanisms mediated by ABC transporters were observed to be the most important. Esterases, glutathione-S-transferases, and cytochrome-oxidases played less important roles in detoxification.

Expression profile changes of cytochrome P450 genes between thiamethoxam susceptible and resistant strains of Aphis gossypii Glover

Cytochrome P450 monooxygenases represent a key detoxification mechanism in neonicotinoids resistance in Aphis gossypii Glover. Synergism analysis has indicates that P450s are involved in thiamethoxam resistance. In this study, expression changes in the transcripts of P450 genes were determined in thiamethoxam-susceptible and thiamethoxam-resistant strains. Nine P450 genes in CYP3 clade were significantly overexpressed in the resistant strain (especially CYP6CY14, which was increased 17.67-fold) compared with the susceptible strain. Transcripts of ecdysone synthesis-related P450 genes, including CYP302A1, CYP306A1, CYP307A1 and CYP315A1, were up-regulated in the resistant strain, which may accelerate molting hormone production. The ecdysone response genes (ecdysone receptor (EcR), ultra-spiracle (USP) and Broad-complex protein (Br-C)) were overexpressed in the resistant strain. RNA interference (RNAi) targeting CYP6CY14 significantly increased the sensitivity of the resistant aphid to thiamethoxam. The results of the present study indicate the possible involvement of these P450 genes in thiamethoxam resistance. Our findings may facilitate further work to validate the roles of these P450s in thiamethoxam resistance based on heterologous expression, and show that screening the expression changes in P450 genes can reveal the impact of thiamethoxam on ecdysone synthesis-related P450 genes. These results are useful for understanding the mechanism of thiamethoxam resistance and will contribute to the management of insecticide-resistant cotton aphids in China.

Contribution of cytochrome P450 monooxygenase CYP380C6 to spirotetramat resistance in Aphis gossypii Glover

The cytochrome P450 monooxygenases play a key role in detoxification mechanism for spirotetramat resistance in Aphis gossypii Glover. However, only one P450 genes (CYP6DA2), among thirty-five P450 genes identified from Aphis gossypii transcriptome database, has been reported to play important role in spirotetramat resistance in previous resistance level until now. In this study, after the confirmation of the rise of resistance level and important roles of P450s in spirotetramat resistance by the synergism analysis, the gene expression changes were determined for P450 genes in spirotetramat susceptible and resistant strains. Compared with the susceptible strain, CYP6CY4, CYP6CY14, CYP6CY18 and CYP6DC1 in CYP3 Clade were up-regulated in resistant nymphs, with the CYP6CY14, CYP6CY4, CYP6DC1, and CYP6CY18 increased to 2.54-, 1.51-, 1.31- and 1.29-fold, respectively. Eight genes in CYP3 Clade, three genes in CYP4 Clade and one gene in Mito Clade were down-regulated. In resistant adult aphids, CYP380C6 in CYP4 Clade, CYP353B1 in CYP2 Clade, and CYP307A1 in Mito Clade were up-regulated under spirotetramat stress, with the CYP380C6, CYP353B1 and CYP307A1 increased to 2.89-, 1.91-, and 1.38-fold, respectively. In contrast, the other P450 genes were almost down-regulated, especially these P450 genes in CYP3 Clade, CYP4 Clade and Mito Clade. RNA interference of CYP380C6 significantly increased the sensitivity of the resistant adults and nymphs to spirotetramat, while suppression of CYP6CY14 could not increase the toxicity of spirotetramat. These results indicate the possible involvement of the CYP380C6 genes in spirotetramat resistance at present very high resistance levels. Screening the expression changes of P450 genes under different spirotetramat resistance levels in the genome-scale will provide an overall view on the possible metabolic factors in the resistance development. The results may facilitate further work to validate the roles of P450 in spirotetramat resistance with heterologous expression.

Thiamethoxam Resistance in Aphis gossypii Glover Relies on Multiple UDP-Glucuronosyltransferases

Uridine diphosphate (UDP)-glycosyltransferases (UGTs) are major phase II enzymes that conjugate a variety of small lipophilic molecules with UDP sugars and alter them into more water-soluble metabolites. Therefore, glucosidation plays a major role in the inactivation and excretion of a great variety of both endogenous and exogenous compounds. In this study, two inhibitors of UGT enzymes, sulfinpyrazone and 5-nitrouracil, significantly increased the toxicity of thiamethoxam against the resistant strain of Aphis gossypii, which indicates that UGTs are involved in thiamethoxam resistance in the cotton aphid. Based on transcriptome data, 31 A. gossypii UGTs belonging to 11 families (UGT329, UGT330, UGT341, UGT342, UGT343, UGT344, UGT345, UGT348, UGT349, UGT350, and UGT351) were identified. Compared with the thiamethoxam-susceptible strain, the transcripts of 23 UGTs were elevated, and the transcripts of 13 UGTs (UGT344J2, UGT348A2, UGT344D4, UGT341A4, UGT343B2, UGT342B2, UGT350C3, UGT344N2, UGT344A14, UGT344B4, UGT351A4, UGT344A11, and UGT349A2) were increased by approximately 2.0-fold in the resistant cotton aphid. The suppression of selected UGTs significantly increased the insensitivity of resistant aphids to thiamethoxam, suggesting that the up-regulated UGTs might be associated with thiamethoxam tolerance. This study provides an overall view of the possible metabolic factor UGTs that are relevant to the development of insecticide resistance. The results might facilitate further work to validate the roles of these UGTs in thiamethoxam resistance.

Identification of glutathione S-transferases in Bemisia tabaci (Hemiptera: Aleyrodidae) and evidence that GSTd7 helps explain the difference in insecticide susceptibility between B. tabaci Middle East-Minor Asia 1 and Mediterranean

The Bemisia tabaci (Gennadius) (Hemiptera:Aleyrodidae) species complex includes invasive and destructive pests of field crops, and the sibling species MEAM1 and MED are its two most damaging members. Previous research indicated that the replacement of Middle East-Minor Asia 1 (MEAM1) by Mediterranean (MED) as the dominant B. tabaci species in China can be mainly attributed to MED's greater tolerance to insecticides. Glutathione S-transferases (GSTs) play important roles in the detoxification of hydrophobic toxic compounds. To increase our understanding of differences in insecticide resistance between B. tabaci MEAM1 and MED, we searched the genomic and transcriptomic databases and identified 23 putative GSTs in both B. tabaci MEAM1 and MED. Through measuring mRNA levels of 18 of the GSTs after B. tabaci MEAM1 and MED adults were exposed to the insecticide imidacloprid, we found that the expression levels were increased more in B. tabaci MED than in MEAM1 (in particular, the expression level of GST-d7 was increased by 4.39-fold relative to the control). Knockdown of GST-d7 in B. tabaci MED but not in B. tabaci MEAM1 resulted in a substantial increase in the mortality of imidacloprid-treated adults. These results indicate that differences in GST-d7 may help explain why insecticide tolerance is greater in B. tabaci MED than in B. tabaci MEAM1.