Transcripts of the nicotinic acetylcholine receptor subunit gene Pxylα6 with premature stop codons are associated with spinosad resistance in diamondback moth, Plutella xylostella

Insecticide resistance and characteristics of mutations related to target site insensitivity of diamondback moths in Taiwan

Diamondback moth (DBM, Plutella xylostella) is the most significant pest of cruciferous vegetables as they rapidly develop high-level resistance to many insecticides. Monitoring DBM susceptibility and target-site mutation frequency is essential for pest control. In this study, 10 insecticides were tested on 11 field populations. Frequencies of target-site mutations (including para, ace1, Rdl1, RyR1, and nAChRα6 genes) were estimated by pyrosequencing. Insecticides registered after 2007 for DBM control in Taiwan, i.e., spinetoram, chlorantraniliprole, chlorfenapyr, metaflumizone, and flubendiamide, showed >80% mortality toward several populations; Bacillus thurigiensis, emamectin benzoate, and chlorfluazuron showed medium to low efficacy in all populations; and tolfenpyrad and mevinphos were highly ineffective. Susceptibility to insecticides varied substantially among populations: eight out of nine populations were highly susceptible to spinetoram, but only one was susceptible to flubendiamide. Target-site mutations related to organophosphates, pyrethroids, fipronil, and diamides were detected in all populations, but there were few spinosad and spinetoram mutations. Our three-year field study demonstrated rapid efficacy loss for all insecticides tested, particularly for more toxic insecticides. Skipped-generation selection of a field DBM strain to emamectin benzoate, metaflumizone, chlorantraniliprole, and flubendiamide revealed that mortality rates dropped from 60 to 80% to <10% after 6 generations. Next-generation sequencing was performed to identify possible target gene mutations. A resistance management program that considers the instability of resistance to some chemicals and pertinent data on resistance mechanisms should be established. Identifying compounds to overcome high-frequency field DBM point mutations could be beneficial for pest control.

Monitoring of the nAChRsα6 G275A spinetoram resistance allele in Drosophila melanogaster populations from New York vineyards

BACKGROUND

Spinosyns are a group of naturally occurring and semi-synthetic insecticides with widespread utility in agriculture, including organic production systems. One example is spinetoram (Delegate), which is the only registered insecticide in New York State (for control of Drosophila melanogaster in vineyards) to which vinegar flies have not yet evolved high levels of resistance. However, low levels of resistance have been found in vineyard populations of D. melanogaster, and a highly resistant strain was obtained after only five selections (in the laboratory). We identified the nAChR α6 mutation (G275A) responsible for the resistance and developed a rapid, high-throughput assay for resistance.

RESULTS

Surveys of collections made in 2023 show low levels of the resistance allele in four populations. A correlation was observed between vineyard use of spinetoram and frequency of the resistance allele, but not between county-wide use of spinosyns and frequency of the resistance allele.

CONCLUSIONS

One of the sites we monitored was previously surveyed in 2019 and the frequency of the resistance allele detected in 2023 had increased. Implications of these findings to resistance management of D. melanogaster are discussed. © 2024 Society of Chemical Industry.

Alternative Splicing and Nonsense-Mediated Decay of a Zebrafish GABA Receptor Subunit Transcript

The superfamily of Cys-loop ionotropic neurotransmitter receptors includes those that detect GABA, glutamate, glycine, and acetylcholine. There is ample evidence that many Cys-loop receptor subunit genes include alternatively spliced exons. In this study, we report a novel example of alternative splicing (AS): we show that the 68-bp exon 3 in the zebrafish gabrr2b gene-which codes for the ρ2b GABAAR subunit-is an alternative cassette exon. Skipping of gabrr2b exon 3 results in a downstream frame shift and a premature termination codon (PTC). We provide evidence in larval zebrafish that transcripts containing the PTC are subject to degradation through nonsense-mediated decay. We also compile reports of AS of homologous exons in other Cys-loop receptor genes in multiple species. Our data add to a large body of research demonstrating that exon 3 in Cys-loop receptor genes is a conserved site for AS, the effects of which can vary from novel splice-isoform generation to downregulation of gene expression through transcript degradation.

The Molecular Resistance Mechanisms of European Earwigs from Apple Orchards Subjected to Different Management Strategies

To date, apple orchards are among the most treated crops in Europe with up to 35 chemical treatments per year. Combining control methods that reduce the number of pesticide treatments is essential for agriculture and more respectful of the environment, and the use of predatory insects such as earwigs may be valuable to achieve this goal. European earwigs, Forficula auricularia (Dermaptera: Forficulidae) are considered beneficial insects in apple orchards where they can feed on many pests like aphids. The aim of this study was to investigate the potential impact of orchards' insecticide treatments on resistance-associated molecular processes in natural populations of earwigs. Because very few molecular data are presently available on earwigs, our first goal was to identify earwig resistance-associated genes and potential mutations. Using earwigs from organic, integrated pest management or conventional orchards, we identified mutations in acetylcholinesterase 2, α1 and β2 nicotinic acetylcholine receptors. In addition, the expression level of these targets and of some essential detoxification genes were monitored using RT-qPCR. Unexpectedly, earwigs collected in organic orchards showed the highest expression for acetylcholinesterase 2. Four cytochromes P450, one esterase and one glutathione S-transferases were over-expressed in earwigs exposed to various management strategies in orchards. This first study on resistance-associated genes in Forficula auricularia paves the way for future experimental studies aimed at better understanding the potential competition between natural enemies in apple orchards in order to optimize the efficiency of biocontrol.

Immunodetection of Truncated Forms of the α6 Subunit of the nAChR in the Brain of Spinosad Resistant Ceratitis capitata Phenotypes

The α6 subunit of the nicotinic acetylcholine receptor (nAChR) has been proposed as the target for spinosad in insects. Point mutations that result in premature stop codons in the α6 gene of Ceratitis capitata flies have been previously associated with spinosad resistance, but it is unknown if these transcripts are translated and if so, what is the location of the putative truncated proteins. In this work, we produced a specific antibody against C. capitata α6 (Ccα6) and validated it by ELISA, Western blotting and immunofluorescence assays in brain tissues. The antibody detects both wild-type and truncated forms of Ccα6 in vivo, and the protein is located in the cell membrane of the brain of wild-type spinosad sensitive flies. On the contrary, the shortened transcripts present in resistant flies generate putative truncated proteins that, for the most part, fail to reach their final destination in the membrane of the cells and remain in the cytoplasm. The differences observed in the locations of wild-type and truncated α6 proteins are proposed to determine the susceptibility or resistance to spinosad.

The knockout of the nicotinic acetylcholine receptor subunit gene α1 (nAChR α1) through CRISPR/Cas9 technology exposes its involvement in the resistance of Spodoptera exigua to insecticides

Insect nicotinic acetylcholine receptors (nAChRs) are the directed targets of many insecticides. However, there have been no reports on the molecular characterization of the nAChR gene family or the causal association between nAChR α1 and resistance to insecticides in S. exigua, which is a significant agricultural pest. In this study, we identified a total of 9 candidate nAChR subunits in S. exigua, namely nAChR α1-α7 and nAChR β1-β2. For functional validation roles of Seα1 in insecticide resistance of S. exigua, we introduced a ∼ 1041-bp deletion of the Seα1 gene in a homozygous mutant strain (Seα1-KO) by CRISPR/Cas9 genome editing system, resulting in a premature truncation of the Seα1 protein and the subsequent loss of functional transmembrane (TM) 3 and TM4 elements. Compared with WH-S strain (wild-type strain), the Seα1-KO strain exhibited 2.62-folds resistant to trifluoropyrimidine, 8.3-folds resistant to dimehypo, and 5.28-folds resistant to dinotefuran, but no significant change in susceptibility to emamectin benzoate, spinetoram, lambda-cyhalothrin, permethrin and chlorpyrifos. Thus, this study has laid a solid foundation for investigating the role of nAChRs in S. exigua, and provides evidence for the crucial involvement of the α1 subunit in the mechanism of trifluoropyrimidine, dimehypo, and dinotefuran in S. exigua. Moreover, it provides a reference for the value of Seα1 subunit and its homologues in other species as insecticide targets.

Selection and characterization of spinetoram resistance in field collected Drosophila melanogaster

Insecticides are commonly employed in vineyards to control vinegar flies and limit sour rot disease. Widespread resistance to available insecticides is having a negative impact on managing Drosophila melanogaster populations, rendering control of sour rot more difficult. An insecticide registered for use in vineyards to which resistance is not yet widespread (at least in New York and Missouri) is spinetoram. Spinetoram targets the nicotinic acetylcholine receptor α6, and mutations in α6 have been associated with resistance in some insects. Our goals were to select for a spinetoram resistant strain of D. melanogaster (starting with field collected populations), characterize the resistance, and identify the mutation responsible. After five selections a strain (SpinR) with >190-fold resistance was obtained. Resistance could not be overcome by insecticide synergists, suggesting an altered target site was involved. We cloned and sequenced the α6 allele from the spinetoram resistant strain and identified a mutation causing a glycine to alanine change at amino acid 301 (equivalent position to the G275E mutation found in some spinosad/spinetoram resistant insects). This mutation was found at low levels in field populations, but increased with each selection until it became homozygous in SpinR. We discuss how the identification of the spinetoram resistance mutation can be used for resistance management.

Transposable elements and xenobiotic resistance

Transposable elements or TEs are well known drivers of adaptive change in plants and animals but their role in insecticide resistance remains poorly documented. This review examines the potential role of transposons in resistance and identifies key areas where our understanding remains unclear. Despite well-known model systems such as upregulation of Drosophila Cyp6g1, many putative examples lack functional validation. The potential types of transposon-associated changes that could lead to resistance are reviewed, including changes in up-regulation, message stability, loss of function and alternative splicing. Where potential mechanisms appear absent from the resistance literature examples are drawn from other areas of biology. Finally, ways are suggested in which transgenic expression could be used to validate the biological significance of TE insertion. In the absence of such functional expression studies many examples of the association of TEs and resistance genes therefore remain as correlations.

CRISPR-mediated knockout of nicotinic acetylcholine receptor (nAChR) α6 subunit confers high levels of resistance to spinosyns in Spodoptera frugiperda

Spinosyn insecticides (spinosad and spinetoram) have been widely used to control a number of agricultural pests including the fall armyworm, Spodoptera frugiperda. Mutations of the nicotinic acetylcholine receptor α6 subunit (nAChRα6) have been reported to confer high levels of resistance to spinosyns in several insect pests. Here we used CRISPR-mediated gene knockout to determine the involvement of S. frugiperda nAChRα6 (Sfα6) in spinosyns susceptibility. A Sfα6 knockout strain of S. frugiperda (Sfα6-KO) was established using dual single guide RNA (sgRNA) directed large fragment deletion with the CRISPR/Cas9 system. Sfα6-KO showed high levels of resistance to spinosad (307-fold) and spinetoram (517-fold) compared with the progenitor strain YJ-19, while no resistance was observed to emamectin benzoate, indoxacarb, chlorfenapyr, chlorantraniliprole and broflanilide. Genetic analyses confirmed that spinosad resistance in Sfα6-KO was autosomal, incompletely recessive and tightly linked to the edited deletion mutation of Sfα6. Our results provided in vivo functional evidence for Sfα6 as the major target of spinosyns against S. frugiperda, and demonstrated that disruption of Sfα6 causes high level resistance to spinosyns. Although no mutations of Sfα6 have yet been reported in any field populations of S. frugiperda, it is critical to develop F₁ screens and/or DNA-based methods to detect and monitor the mutant allele frequencies of Sfα6 across global populations of S. frugiperda.

Knockin of the G275E mutation of the nicotinic acetylcholine receptor (nAChR) α6 confers high levels of resistance to spinosyns in Spodoptera exigua

Spinosyns, including spinosad and spinetoram, act on the insect central nervous system, gradually paralyzing or destroying the target insect. Spinosad resistance is associated with loss-of-function mutations in the nicotinic acetylcholine receptor (nAChR) α6 subunit in a number of agricultural pests. Using gene editing, nAChR α6 has been verified as a target for spinosyns in five insect species. Recently, a point mutation (G275E) in exon 9 of nAChR α6 was identified in spinosad-resistant strains of Thrips palmi and Tuta absoluta. To date, no in vivo functional evidence has been obtained to support that this mutation is involved in spinosyn resistance in lepidopteran pests. In this study, the G275E mutation was introduced into the nAChR of Spodoptera exigua using clustered regularly interspaced short palindromic repeats (CRISPR) / CRISPR-associated protein 9 (Cas9) gene-editing technology. Reverse transcriptase-polymerase chain reaction and sequencing confirmed that this mutation was present in exon 9 of the nAChR transcripts in the edited 275E strain. The results of bioassays showed that the 275E strain was highly resistant to spinosad (230-fold) and spinetoram (792-fold) compared to the unedited background strain, directly confirming that the G275E mutation of the nAChR α6 subunit confers high levels of spinosyn resistance in S. exigua. Inheritance analysis showed that the resistance trait is autosomal and incompletely recessive. This study employs a reverse genetics approach to validate the functional role played by the G275E mutation in nAChR α6 of S. exigua in spinosyns resistance and provides another example of the use of CRISPR/Cas9 gene-editing technology to confirm the role played by candidate target site mutations in insecticide resistance.

Nicotinic acetylcholine receptor modulator insecticides act on diverse receptor subtypes with distinct subunit compositions

Insect nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels mainly expressed in the central nervous system of insects. They are the directed targets of many insecticides, including neonicotinoids, which are the most widely used insecticides in the world. However, the development of resistance in pests and the negative impacts on bee pollinators affect the application of insecticides and have created a demand for alternatives. Thus, it is very important to understand the mode of action of these insecticides, which is not fully understood at the molecular level. In this study, we systematically examined the susceptibility of ten Drosophila melanogaster nAChR subunit mutants to eleven insecticides acting on nAChRs. Our results showed that there are several subtypes of nAChRs with distinct subunit compositions that are responsible for the toxicity of different insecticides. At least three of them are the major molecular targets of seven structurally similar neonicotinoids in vivo. Moreover, spinosyns may act exclusively on the α6 homomeric pentamers but not any other nAChRs. Behavioral assays using thermogenetic tools further confirmed the bioassay results and supported the idea that receptor activation rather than inhibition leads to the insecticidal effects of neonicotinoids. The present findings reveal native nAChR subunit interactions with various insecticides and have important implications for the management of resistance and the development of novel insecticides targeting these important ion channels.

Neonicotinoids and spinosyns account for approximately 24% and 3% of the world market value of insecticides, respectively. However, the negative effects of neonicotinoids on pollinators have led to the development of novel insecticides, such as sulfoxaflor, flupyradifurone and triflumezopyrim. Although all act via insect nicotinic acetylcholine receptors, their modes of action are not fully understood. Our work shows that these insecticides act on diverse receptor subtypes with distinct subunit compositions. This finding could lead to the development of more selective insecticides to control pests with minimal effects on beneficial insects.

Population structure and insecticide resistance status of Tuta absoluta populations from Turkey

BACKGROUND

Tuta absoluta is a devastating pest in tomato production areas worldwide. After its first introduction to Turkey in 2009, it quickly became the major pest of tomato-growing areas. Although some biocontrol agents have been used, especially in greenhouses, the main control of T. absoluta relies heavily on chemical insecticides. However, failure in chemical control has often been reported due to resistance development. In this study, we investigated (i) the population structure of 22 T. absoluta populations across Turkey by analysing haplotypes, based on the cytochrome oxidase subunit I gene; (ii) the efficacy of three registered insecticides from different classes (metaflumizone, chlorantraniliprole and spinosad) in real field-greenhouse conditions; and (iii) the geographic distribution of target-site mutations associated with insecticide resistance.

RESULTS

The efficacy of spinosad was higher than that of chlorantraniliprole and metaflumizone in the greenhouse trials, as documented by the mortality rates obtained, up to 14 days post application. Known resistance mutations in ryanodine receptors (RyR) (i.e. the I4790M/K and G4946E), nicotinic acetylcholine receptors (G275E), acetylcholinesterases (A201S) and voltage-gated sodium channels (F1845Y and V1848I) were found at various frequencies across the populations genotyped. The I4790K diamide resistance mutation in the RyR has been reported for the first time in T. absoluta populations. Although a total of eight haplotypes were found, the overall mean genetic distance was lower than 0.001, indicating the high genetic homogeneity among Turkish T. absoluta populations.

CONCLUSION

The results will contribute to design area-wide resistance management programs in T. absoluta control in Turkey. However, more monitoring studies are needed to implement evidence-based insecticide resistance management strategies in the frame of integrated pest management. © 2021 Society of Chemical Industry.

New insights on the effects of spinosad on the development of Helicoverpa armigera

Helicoverpa armigera (cotton bollworm) is one of the most destructive pests worldwide. Due to resistance to Bacillus thuringiensis and conventional insecticides, an effective management strategy to control this pest is urgently needed. Spinosad, a natural pesticide, is considered an alternative; however, the mechanism underlying the developmental effects of sublethal spinosad exposure remains elusive. In this study, the mechanism was examined using an insect model of H. armigera. Results confirmed that exposure to sublethal spinosad led to reduced larval wet weight, delayed larval developmental period, caused difficulty in molting, and deformed pupae. Further investigation demonstrated that exposure to sublethal spinosad caused a significant decrease in 20E titer and increase in JH titer, thereby leading to the discordance between 20E and JH titers, and consequently alteration in the expression levels of HR3 and Kr-h1. These results suggested that sublethal spinosad caused hormonal disorders in larvae, which directly affect insect development. Our study serves as a reference and basis for the toxicity evaluation of spinosad on molting and pupation in insect metamorphosis, which may contribute to identifying targets for effective control of cotton bollworm.

Transposon-mediated insertional mutagenesis unmasks recessive insecticide resistance in the aphid Myzus persicae

The evolution of resistance to insecticides threatens the sustainable control of many of the world's most damaging insect crop pests and disease vectors. To effectively combat resistance, it is important to understand its underlying genetic architecture, including the type and number of genetic variants affecting resistance and their interactions with each other and the environment. While significant progress has been made in characterizing the individual genes or mutations leading to resistance, our understanding of how genetic variants interact to influence its phenotypic expression remains poor. Here, we uncover a mechanism of insecticide resistance resulting from transposon-mediated insertional mutagenesis of a genetically dominant but insecticide-susceptible allele that enables the adaptive potential of a previously unavailable recessive resistance allele to be unlocked. Specifically, we identify clones of the aphid pest Myzus persicae that carry a resistant allele of the essential voltage-gated sodium channel (VGSC) gene with the recessive M918T and L1014F resistance mutations, in combination with an allele lacking these mutations but carrying a Mutator-like element transposon insertion that disrupts the coding sequence of the VGSC. This results in the down-regulation of the dominant susceptible allele and monoallelic expression of the recessive resistant allele, rendering the clones resistant to the insecticide bifenthrin. These findings are a powerful example of how transposable elements can provide a source of evolutionary potential that can be revealed by environmental and genetic perturbation, with applied implications for the control of highly damaging insect pests.

Mutation (G275E) of nAChR subunit Foα6 associated with spinetoram resistance in Australian western flower thrips, Frankliniella occidentalis (Pergande)

Western flower thrips, Frankliniella occidentalis is an economically important agricultural pest. It causes damage by feeding and oviposition or indirectly by plant virus transmission. Australian F. occidentalis are resistant to many insecticides including spinosad and the related chemical spinetoram. Spinetoram resistance to F. occidentalis has been recently reported in three different Australian States, however, mechanisms conferring that resistance have not been investigated. To identify the mechanisms underlying resistance to spinetoram in F. occidentalis, we sequenced the genomic region of nicotinic acetylcholine receptor Foα6 in number of spinosad and spinetoram resistant field-populations. We found that a single nucleotide substitution (G to A) in exon 9 of the α6 subunit was present in resistant strains (G275E) and absent from susceptible. By examining field populations we consider the G275E mutation is the major cause of resistance to spinetoram in Australian F. occidentalis. We developed a real-time PCR diagnostic assay to quickly identify resistant alleles in field-populations. The method was used to test spinetoram resistant F. occidentalis collected from Australian cotton during the 2018-2019. Results show thrips tested carried the G275E mutation and the resistance allele was unusually widely distributed. The wide distribution of G275E mutation was not expected because spinetoram is not extensively used in Australian cotton. We speculate resistance may relate to extensive chemical use in crops nearby such as horticulture where thrips are often targeted for control. Our molecular diagnostic assay can provide timely and precise resistance frequency information that can support sustainable chemical use including spinetoram based IPM.

Peroxiredoxin alleviates the fitness costs of imidacloprid resistance in an insect pest of rice

Chemical insecticides have been heavily employed as the most effective measure for control of agricultural and medical pests, but evolution of resistance by pests threatens the sustainability of this approach. Resistance-conferring mutations sometimes impose fitness costs, which may drive subsequent evolution of compensatory modifier mutations alleviating the costs of resistance. However, how modifier mutations evolve and function to overcome the fitness cost of resistance still remains unknown. Here we show that overexpression of P450s not only confers imidacloprid resistance in the brown planthopper, Nilaparvata lugens, the most voracious pest of rice, but also leads to elevated production of reactive oxygen species (ROS) through metabolism of imidacloprid and host plant compounds. The inevitable production of ROS incurs a fitness cost to the pest, which drives the increase or fixation of the compensatory modifier allele T65549 within the promoter region of N. lugens peroxiredoxin (NlPrx) in the pest populations. T65549 allele in turn upregulates the expression of NlPrx and thus increases resistant individuals' ability to clear the cost-incurring ROS of any source. The frequent involvement of P450s in insecticide resistance and their capacity to produce ROS while metabolizing their substrates suggest that peroxiredoxin or other ROS-scavenging genes may be among the common modifier genes for alleviating the fitness cost of insecticide resistance.

Identification of the Aedes aegypti nAChR gene family and molecular target of spinosad

BACKGROUND

Spinosad is an insecticide with unique mode of action (MOA) of disrupting nicotinic acetylcholine receptor and is efficacious against many insect species. Mutations in the nicotinic acetylcholine receptor (nAChR) α6 subunit have been identified that are associated with levels of spinosad resistance, but the molecular characterization of the nAChR gene family and a causative association between nAChR α6 and resistance to spinosad in Aedes aegypti, a primary vector of many arboviruses, have not yet been reported.

RESULTS

In this study, we identified 10 candidate nAChR subunits in Ae. Aegypti, nAChRα1-α9 and nAChRβ1, showing similarly orthologous relationships with Anopheles gambiae. With the application of the CRISPR/Cas9 genome editing system, we introduced a 32-bp deletion at the 5' end of the Aaeα6 (Ae. aegypti nAChR α6) gene in a homozygous mutant strain (Aaeα6-KO). The mutation produced two successive pre-mature stop codons, resulting in loss of function in the target receptor. The Aaeα6-KO mutant strain exhibited a 320-fold level of resistance to spinosad compared with wildtype. A recessive mode of inheritance for spinosad resistance was found in the Aaeα6-KO strain.

CONCLUSION

CRISPR/Cas9 introduced truncated Aaeα6 receptor in Ae. aegypti resulted in an increased level of resistance to spinosad, suggesting that the conserved nAChR α6 subunit is the target for spinosad insecticide. © 2020 Society of Chemical Industry.

Association Between Susceptibility of Thrips palmi to Spinetoram and Frequency of G275E Mutation Provides Basis for Molecular Quantification of Field-Evolved Resistance

Putative mechanisms underlying spinosyn resistance have been identified in controlled studies on many species; however, mechanisms underlying field-evolved resistance and the development of a molecular diagnostic method for monitoring field resistance have lagged behind. Here, we examined levels of resistance of melon thrips, Thrips palmi Karny (Thysanoptera:Thripidae), to spinetoram as well as target site mutations in field populations across China to identify potential mechanisms and useful molecular markers for diagnostic and quantifying purposes. In resistant populations, we identified the G275E mutation, which has previously been linked to spinosyns resistance, and F314V mutation, both located in the α6 subunit of the nicotinic acetylcholine receptor. There was a strong correlation between levels of spinetoram resistance and allele frequency of G275E mutation in field-collected populations (r2 = 0.84) and those reared under laboratory conditions for two to five generations (r2 = 0.91). LC50 ranged from 0.12 to 0.66 mg/liter in populations without G275E mutation, whereas it ranged from 33.12 to 39.91 mg/liter in most populations with a G275E mutation frequency more than 90%. Our results indicate that the field-evolved resistance of T. palmi to spinetoram in China is mainly conferred by the G275E mutation. The frequency of the G275E mutation provides a useful diagnostic for quantifying resistance levels in field populations of T. palmi.

A De Novo Transcriptomics Approach Reveals Genes Involved in Thrips Tabaci Resistance to Spinosad

The onion thrip, Thrips tabaci (Thysanoptera: Thripidae) is a major polyphagous pest that attacks a wide range of economically important crops, especially Allium species. The thrip's damage can result in yield loss of up to 60% in onions (Allium cepa). In the past few decades, thrip resistance to insecticides with various modes of actions have been documented. These include resistance to spinosad, a major active compound used against thrips, which was reported from Israel. Little is known about the molecular mechanisms underlying spinosad resistance in T. tabaci. We attempted to characterize the mechanisms involved in resistance to spinosad using quantitative transcriptomics. Susceptible (LC50 = 0.6 ppm) and resistant (LC50 = 23,258 ppm) thrip populations were collected from Israel. An additional resistant population (LC50 = 117 ppm) was selected in the laboratory from the susceptible population. De novo transcriptome analysis on the resistant and susceptible population was conducted to identify differently expressed genes (DGEs) that might be involved in the resistance against spinosad. In this analysis, 25,552 unigenes were sequenced, assembled, and functionally annotated, and more than 1500 DGEs were identified. The expression levels of candidate genes, which included cytochrome P450 and vittelogenin, were validated using quantitative RT-PCR. The cytochrome P450 expression gradually increased with the increase of the resistance. Higher expression levels of vitellogenin in the resistant populations were correlated with higher fecundity, suggesting a positive effect of the resistance on resistant populations. This research provides a novel genetic resource for onion thrips and a comprehensive molecular examination of resistant populations to spinosad. Those resources are important for future studies concerning thrips and resistance in insect pests regarding agriculture.

Functional validation of nicotinic acetylcholine receptor (nAChR) α6 as a target of spinosyns in Spodoptera exigua utilizing the CRISPR/Cas9 system

BACKGROUND

The beet armyworm, Spodoptera exigua, is a serious agricultural pest that is primarily controlled using chemical insecticides. Recently, resistance to the insecticide spinosad has been described in S. exigua field populations. To date, there has been no functional evidence proving the involvement of the nicotinic acetylcholine receptor (nAChR) α6 mutation in spinosad resistance in S. exigua.

RESULTS

In this study, using the CRISPR/Cas9 genome-editing system, a homozygous strain (Seα6-KO) with approximately 1760-bp deletion within Seα6 in S. exigua causing a premature truncation of Seα6 was successfully constructed. Insecticide bioassays showed that Seα6-KO exhibited 373-fold higher resistance to spinosad and 850-fold higher resistance to spinetoram compared to WH-S strain with the same genetic background but showed no significant change in susceptibility to emamectin benzoate and chlorantraniliprole. Genetic analysis revealed that Seα6-KO is inherited as an incompletely recessive trait.

CONCLUSION

The results clearly demonstrated the functional role of Seα6 in resistance to spinosyn insecticides and provide an example of using genome editing to verify a target premature truncation associated with resistance.

Evolutionary trade-offs of insecticide resistance - The fitness costs associated with target-site mutations in the nAChR of Drosophila melanogaster

The evolution of resistance to drugs and pesticides poses a major threat to human health and food security. Neonicotinoids are highly effective insecticides used to control agricultural pests. They target the insect nicotinic acetylcholine receptor and mutations of the receptor that confer resistance have been slow to develop, with only one field‐evolved mutation being reported to date. This is an arginine‐to‐threonine substitution at position 81 of the nAChR_β1 subunit in neonicotinoid‐resistant aphids. To validate the role of R81T in neonicotinoid resistance and to test whether it may confer any significant fitness costs to insects, CRISPR/Cas9 was used to introduce an analogous mutation in the genome of Drosophila melanogaster. Flies carrying R81T showed an increased tolerance (resistance) to neonicotinoid insecticides, accompanied by a significant reduction in fitness. In comparison, flies carrying a deletion of the whole nAChR_α6 subunit, the target site of spinosyns, showed an increased tolerance to this class of insecticides but presented almost no fitness deficits.

Disruption of nicotinic acetylcholine receptor α6 mediated by CRISPR/Cas9 confers resistance to spinosyns in Plutella xylostella

BACKGROUND

The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system provides some advantages over other reverse genetic techniques to investigate the causal relationship between insecticide resistance phenotype and candidate gene. Several studies published to date point to the nicotinic acetylcholine receptor (nAChR) α6 subunit strongly associated with spinosyns resistance in insects, including Plutella xylostella. However, reverse genetic verification of the P. xylostella nAChRα6 has not yet been achieved via an in vivo approach.

RESULTS

Here, we successfully constructed a homozygous strain (Pxα6-KO) with a 2-nt deletion mutation of nAChRα6 by CRISPR/Cas9 coupled with non-homologous end joining approach in P. xylostella. The manipulated mutation results in a frame shift in the open reading frame of transcripts, which produces a predicted protein truncated in the TM3-TM4 loop region. When compared to the background strain IPP-S, the knockout strain Pxα6-KO exhibited 229- and 1462-fold resistance to spinosad and spinetoram, respectively, but no or limited (resistance ratios <3-fold) effects on the toxicities of imidacloprid, abamectin, β-cypermethrin, indoxacarb, metaflumizone and chlorantraniliprole. Furthermore, the mode of inheritance of the acquired spinetoram resistance was autosomal recessive and significantly linked with the 2-nt deletion mutation of nAChRα6 in the Pxα6-KO strain.

CONCLUSION

In vivo functional investigation demonstrates the causality of the Pxα6 truncating mutation with high levels of resistance to spinosyns in P. xylostella. Our results suggest the Pxα6-KO strain underlies an autosomal, recessive mode of inheritance for spinetoram resistance, and reinforces the association of this gene to the mode of action of spinosyns. © 2019 Society of Chemical Industry.

Activities of Select Enzymes Involved in Insecticide Resistance in Spinosad-Resistant and -Susceptible Strains of Musca domestica (Diptera: Muscidae)

A Musca domestica L. strain collected from Pakistan has recently been shown to be resistant to spinosad; however, there is scarce information about the mechanism of resistance. For this reason, we explored whether a metabolic-based mechanism was responsible by analyzing the activities of the metabolic detoxifying enzymes, carboxylesterases, glutathione S-transferases, and mixed-function oxidases, in both a spinosad-selected (Spin-SEL) strain of M. domestica and a susceptible counterpart (Lab-susceptible). The results revealed that both strains were statistically at par in terms of enzyme activities. The activity of carboxylesterases in the Lab-susceptible strain was 78.17 ± 3.06 in comparison to 79.16 ± 3.31 nmol min-1 mg-1 in the Spin-SEL strain. The activity of mixed-function oxidases was 51.58 ± 4.20 in the Lab-susceptible strain, whereas 54.33 ± 4.08 pmol min-1 mg-1 was recorded in the Spin-SEL strain. The activity of glutathione S-transferases was 86.50 ± 4.59 (Lab-susceptible) and 90.33 ± 2.81 nmol min-1 mg-1 (Spin-SEL). These results revealed that the studied enzymes might not be responsible for spinosad resistance in the studied strain of M. domestica. Therefore, studies should be extended to find out other possible mechanisms of spinosad resistance.

In silico identification and assessment of insecticide target sites in the genome of the small hive beetle, Aethina tumida

BACKGROUND

The small hive beetle, Aethina tumida, is a rapidly emerging global pest of honey bee colonies. Small hive beetle infestation can be extremely destructive, which may cause honey bees to abscond and render colony infrastructure unusable. Due to the impacts small hive beetles have on honey bees, a wide variety of physical, cultural, and chemical control measures have been implemented to manage small hive beetle infestations. The use of insecticides to control small hive beetle populations is an emerging management tactic. Currently, very little genomic information exists on insecticide target sites in the small hive beetle. Therefore, the objective of this study is to utilize focused in silico comparative genomics approaches to identify and assess the potential insecticide sensitivity of the major insecticide target sites in the small hive beetle genome.

RESULTS

No previously described resistance mutations were identified in any orthologs of insecticide target sites. Alternative exon use and A-to-I RNA editing were absent in AtumSC1. The ryanodine receptor in small hive beetle (Atum_Ryr) was highly conserved and no previously described resistance mutations were identified. A total of 12 nAChR subunits were identified with similar alternative exon use in other insects. Alternative exon use and critical structural features of the GABA-gated chloride channel subunits (Atum_RDL, Atum_GRD, and Atum_LCCH3) were conserved. Five splice variants were found for the glutamate-gated chloride channel subunit. Exon 3c of Atum_GluCl may be a beetle-specific alternative exon. The co-occurrence of exons 9a and 9b in the pH-sensitive chloride channel (Atum_pHCl) is a unique combination that introduces sites of post-translational modification. The repertoire and alternative exon use for histamine-gated chloride channels (Atum-HisCl), octopamine (Atum_OctR) and tyramine receptors (Atum_TAR) were conserved.

CONCLUSIONS

The recently published small hive beetle genome likely serves as a reference for insecticide-susceptible versions of insecticide target sites. These comparative in silico studies are the first step in discovering targets that can be exploited for small hive beetle-specific control as well as tracking changes in the frequency of resistance alleles as part of a resistance monitoring program. Comparative toxicity alongside honey bees is required to verify these in silico predictions.

The evolution of multiple-insecticide resistance in UK populations of tomato leafminer, Tuta absoluta

BACKGROUND

The tomato leafminer, Tuta absoluta, is an economically important pest of tomatoes in Europe, Africa, Asia and South America. In the UK this species is controlled using an integrated pest management (IPM) programme which incorporates the insecticides spinosad and chlorantraniliprole. In response to UK grower concerns of loss of efficacy of these compounds at certain sites, insecticide bioassays were performed on five populations collected from four commercial glasshouses and potential mechanisms of resistance investigated.

RESULTS

We observed high levels of resistance to spinosad in four of the strains, and in two of these tolerance to chlorantraniliprole. Selection of one of these strains with chlorantraniliprole rapidly resulted in a line exhibiting potent resistance to this compound. Sequencing of messenger RNA encoding the nicotinic acetylcholine receptor (nAChR) α6 subunit, target of spinosad, revealed Taα6 transcripts in the spinosad-resistant strains that lack exon 4 and encode a highly truncated protein, or contain a triplet deletion in the predicted first transmembrane domain resulting in the loss of a highly conserved amino acid. Sequencing of the ryanodine receptor gene, encoding the target of diamide insecticides, of the chlorantraniliprole-selected line revealed an amino acid substitution (G4903V) that has been previously linked to diamide resistance in populations of T. absoluta in the Mediterranean and South America.

CONCLUSION

Taken together our results reveal emerging resistance in UK populations of T. absoluta to two of the most important insecticides used as part of IPM, with significant implications for the control of this species in the UK. © 2019 Society of Chemical Industry.

Identification and risk assessment of spinosad resistance in a California population of Drosophila suzukii

BACKGROUND

The bioinsecticide spinosad is among the most widely used insecticides for managing spotted-wing drosophila, Drosophila suzukii (Matsumura), and is critical for preventing fruit infestation in organic berry production. Recent reports, however, have raised concerns that the efficacy of this material is declining in fields near Watsonville, CA, a major hub of commercial berry production in the USA and the first location where D. suzukii was reported in North America.

RESULTS

In this study, we performed dose-response analyses on D. suzukii from commercial raspberry plantings near Watsonville as well as a second untreated site in California using a widely implemented bioassay protocol. We found that Watsonville flies exhibited spinosad LC₅₀ values 4.3-7.7 times higher than those from the untreated location and 11.6-22.4 times higher than previously reported susceptible baselines. Additionally, tolerance to spinosad continued to increase after additional selection for five generations, though this result was only statistically significant after prolonged exposure to residues.

CONCLUSIONS

These findings confirm that spinosad resistance is emerging in the Watsonville area and document the first known occurrence in the USA, presenting an urgent need for the development of alternative management strategies to control this pest. Additional work is needed to resolve the underlying molecular mechanism(s) that confers spinosad resistance in D. suzukii and assess the potential for this trait to spread into new populations. © 2018 Society of Chemical Industry.

Multiple mutations in the nicotinic acetylcholine receptor Ccα6 gene associated with resistance to spinosad in medfly

Spinosad is an insecticide widely used for the control of insect pest species, including Mediterranean fruit fly, Ceratitis capitata. Its target site is the α6 subunit of the nicotinic acetylcholine receptors, and different mutations in this subunit confer resistance to spinosad in diverse insect species. The insect α6 gene contains 12 exons, with mutually exclusive versions of exons 3 (3a, 3b) and 8 (8a, 8b, 8c). We report here the selection of a medfly strain highly resistant to spinosad, JW-100 s, and we identify three recessive Ccα6 mutant alleles in the JW-100 s population: (i) Ccα63aQ68* containing a point mutation that generates a premature stop codon on exon 3a (3aQ68*); (ii) Ccα63aAG>AT containing a point mutation in the 5' splicing site of exon 3a (3aAG > AT); and (iii) Ccα63aQ68*-K352* that contains the mutation 3aQ68* and another point mutation on exon 10 (K352*). Though our analysis of the susceptibility to spinosad in field populations indicates that resistance has not yet evolved, a better understanding of the mechanism of action of spinosad is essential to implement sustainable management practices to avoid the development of resistance in field populations.

Nicotinic acetylcholine receptor α6 subunit mutation (G275V) found in a spinosad-resistant strain of the flower thrips, Frankliniella intonsa (Thysanoptera: Thripidae)

The flower thrips Frankliniella intonsa strain showing resistance to spinosad was established in the laboratory. The resistant strain showed an LC₅₀ value of 1398.7 mg/L in a leaf dipping/contact assay. The LC₅₀ value was ca. 280 times higher than that of the most susceptible strain. An insecticidal assay using synergists suggested no involvement of degradation enzymes, such as cytochrome P450, glutathione S-transferase, and carboxyl esterase, in the resistance. Glycine at amino acid position 275 of the nicotinic acetylcholine receptor (nAChR) α6 subunit was mutated to valine in the resistant strain. These results suggest that spinosad resistance in F. intonsa is conferred by the reduced sensitivity of nAChR.

Resistance development, stability, cross-resistance potential, biological fitness and biochemical mechanisms of spinetoram resistance in Thrips hawaiiensis (Thysanoptera: Thripidae)

BACKGROUND

Spinetoram, a new type of spinosyn with novel modes of action, has been used in effective thrips control programs, but resistance remains a threat. In the present study, a laboratory Thrips hawaiiensis population was subjected to spinetoram for resistance selection to investigate resistance development, stability, cross-resistance potential, biological fitness and underlying biochemical mechanisms.

RESULTS

Resistance to spinetoram in T. hawaiiensis rapidly increased 103.56-fold (for 20 generations of selection with spinetoram) compared with a laboratory susceptible population, and the average realized heritability (h² ) of resistance was calculated as 0.1317. Maintaining the resistant population for five generations without any further selection pressure resulted in a decline in the resistance ratio from 19.42- to 9.50-fold, suggesting that spinetoram resistance in T. hawaiiensis is unstable. Moreover, the spinetoram-resistant population exhibited a lack of cross-resistance to other classes of insecticides, and showed biological fitness costs. The results of synergism experiments using enzyme inhibitors and biochemical analyses revealed that metabolic mechanisms might not be responsible for the development of spinetoram resistance in T. hawaiiensis.

CONCLUSION

The current study expands understanding of spinosyn resistance in thrips species, providing a basis for proposing better integrated pest management strageties for thrips control programs and defining the most appropriate tools for such resistance management. © 2018 Society of Chemical Industry.

Inheritance and fitness costs of Spodoptera frugiperda (Lepidoptera: Noctuidae) resistance to spinosad in Brazil

BACKGROUND

Spodoptera frugiperda is a pest of economically important crops in South America. In Brazil, this species is considered the most destructive pest of maize. Use of spinosyn insecticides in insect resistance management (IRM) has been one strategy to control this pest. In this study, we selected a strain of S. frugiperda resistant to spinosad and evaluated the inheritance and fitness costs of the resistance.

RESULTS

Estimated LC₅₀ (concentration required to kill 50% of larvae) values were 0.011 and 9.80 µg cm^-2 for the spinosad-susceptible (Sus) and -resistant (Spin-res) strains, respectively. This represents an 890-fold resistance ratio. LC₅₀ values for reciprocal crosses were 0.18 and 0.14 µg cm^-2 , indicating that resistance to spinosad is an autosomal incompletely recessive trait. Backcrosses of the F₁ progeny from reciprocal crosses with the parental Spin-res strain showed a polygenic effect. The estimated minimum number of independent segregations was ∼ 2.45, indicating that resistance to spinosad is associated with multiple genes. In greenhouse assays, third-instar larvae from the Spin-res strain showed >92% survival on spinosad-treated maize. By contrast Sus and reciprocal crosses exhibited 0% and <5% survival, respectively, indicating that resistance is recessive. Life history studies to investigate the fitness cost of resistance revealed a 41% reduction in the rate of survival to adulthood and a 49% lower reproductive rate in the Spin-res strain compared with the Sus strain.

CONCLUSIONS

The autosomal, incompletely recessive and polygenic resistance to spinosad in S. frugiperda and the fitness costs associated with this resistance can be exploited in IRM strategies to preserve the lifetime of spinosad for control of S. frugiperda in Brazil. © 2017 Society of Chemical Industry.

Nicotinic acetylcholine receptor subunit α6 associated with spinosad resistance in Rhyzopertha dominica (Coleoptera: Bostrichidae)

The lesser grain borer, Rhyzopertha dominica, which is a primary pest of stored products, breaks up whole grains and makes them susceptible to secondary infestation by other pests. Insecticide application is the main control measure against this borer. A resistant strain of R. dominica against the insecticide, spinosad, was selected in the laboratory. The full-length cDNA of the target site of spinosad, nicotinic acetylcholine receptor subunit α6, from R. dominica (Rdα6) was cloned and analyzed using reverse transcription PCR and rapid amplification of cDNA ends. The complete 2133-bp cDNA contains the open reading frame of 1497 bp encoding a 498-amino-acid protein. There are four predicted transmembrane (TM) regions, and six extracellular ligand-binding sites at the N-terminus, upstream from the first TM in Rdα6. Three mutations have been found in the resistant strain compared with the susceptible one: (1) a 181-bp fragment truncated at the N-terminus, resulting in the appearance of a premature stop codon, (2) one missing bp at the position 997, causing a frame-shift mutation, and (3) an 87-bp fragment truncated in the TM2 region. In addition, real-time quantitative PCR was applied to detect the transcriptional expression of Rdα6 in both the susceptible and resistant strains. The results indicated that the expression of Rdα6 was significantly lower in then resistant strain than in susceptible one. In conclusion, mutation of Rdα6 may cause R. dominica resistant to spinosad due to target site insensitivity.

Two distinctive β subunits are separately involved in two binding sites of imidacloprid with different affinities in Locusta migratoria manilensis

Due to great diversity of nicotinic acetylcholine receptor (nAChR) subtypes in insects, one β subunit may be contained in numerous nAChR subtypes. In the locust Locusta migratoria, a model insect species with agricultural importance, the third β subunits (Locβ3) was identified in this study, which reveals at least three β subunits in this insect species. Imidacloprid was found to bind nAChRs in L. migratoria central nervous system at two sites with different affinities, with K_d values of 0.16 and 10.31nM. The specific antisera (L1-1, L2-1 and L3-1) were raised against fusion proteins at the large cytoplasmic loop of Locβ1, Locβ2 and Locβ3 respectively. Specific immunodepletion of Locβ1 with antiserum L1-1 resulted in the selective loss of the low affinity binding site for imidacloprid, whereas the immunodepletion of Locβ3 with L3-1 caused the selective loss of the high affinity site. Dual immunodepletion with L1-1 and L3-1 could completely abolish imidacloprid binding. In contrast, the immunodepletion of Locβ2 had no significant effect on the specific [³H]imidacloprid binding. Taken together, these data indicated that Locβ1 and Locβ3 were respectively contained in the low- and high-affinity binding sites for imidacloprid in L. migratoria, which is different to the previous finding in Nilaparvata lugens that Nlβ1 was in two binding sites for imidacloprid. The involvement of two β subunits separately in two binding sites may decrease the risk of imidacloprid resistance due to putative point mutations in β subunits in L. migratoria.

Does resistance really carry a fitness cost?

Insecticide resistance mutations are widely assumed to carry fitness costs. However studies to measure such costs are rarely performed on genetically related strains and are often only done in the laboratory. Theory also suggests that once evolved the cost of resistance can be offset by the evolution of fitness modifiers. But for insecticide resistance only one such example is well documented. Here we critically examine the literature on fitness costs in the absence of pesticide and ask if our knowledge of molecular biology has helped us predict the costs associated with different resistance mechanisms. We find that resistance alleles can arise from pre-existing polymorphisms and resistance associated variation can also be maintained by sexual antagonism. We describe novel mechanisms whereby both resistant and susceptible alleles can be maintained in permanent heterozygosis and discuss the likely consequences for fitness both in the presence and absence of pesticide. Taken together these findings suggest that we cannot assume that resistance always appears de novo and that our assumptions about the associated fitness costs need to be informed by a deeper understanding of the underlying molecular biology.

Ion channels as insecticide targets

Ion channels remain the primary target of most of the small molecule insecticides. This review examines how the subunit composition of heterologously expressed receptors determines their insecticide-specific pharmacology and how the pharmacology of expressed receptors differs from those found in the insect nervous system. We find that the insecticide-specific pharmacology of some receptors, like that containing subunits of the Rdl encoded GABA receptor, can be reconstituted with very few of the naturally occurring subunits expressed. In contrast, workers have struggled even to express functional insect nicotinic acetylcholine receptors (nAChRs), and work has therefore often relied upon the expression of vertebrate receptor subunits in their place. We also examine the extent to which insecticide-resistance-associated mutations, such as those in the para encoded voltage-gated sodium channel, can reveal details of insecticide-binding sites and mode of action. In particular, we examine whether mutations are present in the insecticide-binding site and/or at sites that allosterically affect the drug preferred conformation of the receptor. We also discuss the ryanodine receptor as a target for the recently developed diamides. Finally, we examine the lethality of the genes encoding these receptor subunits and discuss how this might determine the degree of conservation of the resistance-associated mutations found.

Insecticide resistance mediated by an exon skipping event

Many genes increase coding capacity by alternate exon usage. The gene encoding the insect nicotinic acetylcholine receptor (nAChR) α6 subunit, target of the bio‐insecticide spinosad, is one example of this and expands protein diversity via alternative splicing of mutually exclusive exons. Here, we show that spinosad resistance in the tomato leaf miner, Tuta absoluta is associated with aberrant regulation of splicing of Taα6 resulting in a novel form of insecticide resistance mediated by exon skipping. Sequencing of the α6 subunit cDNA from spinosad selected and unselected strains of T. absoluta revealed all Taα6 transcripts of the selected strain were devoid of exon 3, with comparison of genomic DNA and mRNA revealing this is a result of exon skipping. Exon skipping cosegregated with spinosad resistance in survival bioassays, and functional characterization of this alteration using modified human nAChR α7, a model of insect α6, demonstrated that exon 3 is essential for receptor function and hence spinosad sensitivity. DNA and RNA sequencing analyses suggested that exon skipping did not result from genetic alterations in intronic or exonic cis‐regulatory elements, but rather was associated with a single epigenetic modification downstream of exon 3a, and quantitative changes in the expression of trans‐acting proteins that have known roles in the regulation of alternative splicing. Our results demonstrate that the intrinsic capacity of the α6 gene to generate transcript diversity via alternative splicing can be readily exploited during the evolution of resistance and identifies exon skipping as a molecular alteration conferring insecticide resistance.

Mutation (G275E) of the nicotinic acetylcholine receptor α6 subunit is associated with high levels of resistance to spinosyns in Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

The tomato leafminer, Tuta absoluta, now a major pest of tomato crops worldwide, is primarily controlled using chemical insecticides. Recently, high levels of resistance to the insecticide spinosad have been described in T. absoluta populations in Brazil. Selection of a resistant field-collected strain led to very high levels of resistance to spinosad and cross-resistance to spinetoram, but not to other insecticides that target the nicotinic acetylcholine receptor (nAChR). In this study the mechanisms underlying resistance to spinosad were investigated using toxicological, biochemical and molecular approaches. Inhibition of metabolic enzymes using synergists and biochemical assessment of detoxification enzyme activity provided little evidence of metabolic resistance in the selected strain. Cloning and sequencing of the nAChR α6 subunit from T. absoluta, the spinosad target-site, from susceptible and spinosad-resistant strains were done to investigate the role of a target-site mechanism in resistance. A single nucleotide change was identified in exon 9 of the α6 subunit of the resistant strain, resulting in the replacement of the glycine (G) residue at position 275 observed in susceptible T. absoluta strains with a glutamic acid (E). A high-throughput DNA-based diagnostic assay was developed and used to assess the prevalence of the G275E mutation in 17 field populations collected from different geographical regions of Brazil. The resistant allele was found at low frequency, and in the heterozygous form, in seven of these populations but at much higher frequency and in the homozygous form in a population collected in the Iraquara municipality. The frequency of the mutation was significantly correlated with the mortality of these populations in discriminating dose bioassays. In summary our results provide evidence that the G275E mutation is an important mechanism of resistance to spinosyns in T. absoluta, and may be used as a marker for resistance monitoring in field populations.

A CRISPR/Cas9 mediated point mutation in the alpha 6 subunit of the nicotinic acetylcholine receptor confers resistance to spinosad in Drosophila melanogaster

Spinosad, a widely used and economically important insecticide, targets the nicotinic acetylcholine receptor (nAChRs) of the insect nervous system. Several studies have associated loss of function mutations in the insect nAChR α6 subunit with resistance to spinosad, and in the process identified this particular subunit as the specific target site. More recently a single non-synonymous point mutation, that does not result in loss of function, was identified in spinosad resistant strains of three insect species that results in an amino acid substitution (G275E) of the nAChR α6 subunit. The causal role of this mutation has been called into question as, to date, functional evidence proving its involvement in resistance has been limited to the study of vertebrate receptors. Here we use the CRISPR/Cas9 gene editing platform to introduce the G275E mutation into the nAChR α6 subunit of Drosophila melanogaster. Reverse transcriptase-PCR and sequencing confirmed the presence of the mutation in Dα6 transcripts of mutant flies and verified that it does not disrupt the normal splicing of the two exons in close vicinity to the mutation site. A marked decrease in sensitivity to spinosad (66-fold) was observed in flies with the mutation compared to flies of the same genetic background minus the mutation, clearly demonstrating the functional role of this amino acid substitution in resistance to spinosad. Although the resistance levels observed are 4.7-fold lower than exhibited by a fly strain with a null mutation of Dα6, they are nevertheless predicated to be sufficient to result in resistance to spinosad at recommended field rates. Reciprocal crossings with susceptible fly strains followed by spinosad bioassays revealed G275E is inherited as an incompletely recessive trait, thus resembling the mode of inheritance described for this mutation in the western flower thrips, Frankliniella occidentalis. This study both resolves a debate on the functional significance of a target-site mutation and provides an example of how recent advances in genome editing can be harnessed to study insecticide resistance.

A three amino acid deletion in the transmembrane domain of the nicotinic acetylcholine receptor α6 subunit confers high-level resistance to spinosad in Plutella xylostella

Spinosad is a macrocyclic lactone insecticide that acts primarily at the nicotinic acetylcholine receptors (nAChRs) of target insects. Here we describe evidence that high levels of resistance to spinosad in the diamondback moth (Plutella xylostella) are associated with a three amino acid (3-aa) deletion in the fourth transmembrane domain (TM4) of the nAChR α6 subunit (Pxα6). Following laboratory selection with spinosad, the SZ-SpinR strain of P. xylostella exhibited 940-fold resistance to spinosad. In addition, the selected insect population had 1060-fold cross-resistance to spinetoram but, in contrast, no cross-resistance to abamectin was observed. Genetic analysis indicates that spinosad resistance in SZ-SpinR is inherited as a recessive and autosomal trait, and that the 3-aa deletion (IIA) in TM4 of Pxα6 is tightly linked to spinosad resistance. Because of well-established difficulties in functional expression of cloned insect nAChRs, the analogous resistance-associated deletion mutation was introduced into a prototype nAChR (the cloned human α7 subunit). Two-electrode voltage-clamp recording with wild-type and mutated nAChRs expressed in Xenopus laevis oocytes indicated that the mutation causes a complete loss of agonist activation. In addition, radioligand binding studies indicated that the 3-aa deletion resulted in significantly lower-affinity binding of the extracellular neurotransmitter-binding site. These findings are consistent with the 3-amino acid (IIA) deletion within the transmembrane domain of Pxα6 being responsible for target-site resistance to spinosad in the SZ-SpinR strain of P. xylostella.

Putative nicotinic acetylcholine receptor subunits express differentially through the life cycle of codling moth, Cydia pomonella (Lepidoptera: Tortricidae)

Nicotinic acetylcholine receptors (nAChRs) are the targets of neonicotinoids and spinosads, two insecticides used in orchards to effectively control codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae). Orchardists in Washington State are concerned about the possibility of codling moth field populations developing resistance to these two insecticides. In an effort to help mitigate this issue, we initiated a project to identify and characterize codling moth nAChR subunits expressed in heads. This study had two main goals; (i) identify transcripts from a codling moth head transcriptome that encode for nAChR subunits, and (ii) determine nAChR subunit expression profiles in various life stages of codling moth. From a codling moth head transcriptome, 24 transcripts encoding for 12 putative nAChR subunit classes were identified and verified by PCR amplification, cloning, and sequence determination. Characterization of the deduced protein sequences encoded by putative nAChR transcripts revealed that they share the distinguishing features of the cys-loop ligand-gated ion channel superfamily with 9 α-type subunits and 3 β-type subunits identified. Phylogenetic analysis comparing these protein sequences to those of other insect nAChR subunits supports the identification of these proteins as nAChR subunits. Stage expression studies determined that there is clear differential expression of many of these subunits throughout the codling moth life cycle. The information from this study will be used in the future to monitor for potential target-site resistance mechanisms to neonicotinoids and spinosads in tolerant codling moth populations.

The subunit gene Ldα1 of nicotinic acetylcholine receptors plays important roles in the toxicity of imidacloprid and thiamethoxam against Leptinotarsa decemlineata

Nicotinic acetylcholine receptors (nAChRs) are pentameric ACh-gated ion channels. It is believed that nAChRs composed of different subunits may vary in their function and toxicological characteristics. Neonicotinoids are activators of nAChRs and important insecticides that are extensively used for crop protection and resistance has been developed by some pests. They are also major insecticides for the control of Leptinotarsa decemlineata, which is a destructive defoliator pest that invaded the Xinjiang region of China in the 1990s. However, little is known about the constitution or subunits of the target in this pest. In this study, the full-length cDNAs encoding four new nAChR subunits (named Ldα3, Ldα6, Ldα10, and Ldβ1) were cloned from L. decemlineata. These genes encode 822-, 753-, 672-, and 759-amino acid proteins, respectively, which share typical features of insect nAChRs subunits and closely resemble the corresponding subunits of the nAChRs from Tribolium castaneum. Temporal and spatial expression analyses showed that these genes, as well as the previously identified Ldα1, Ldα2, and Ldα8 genes, are widely expressed in all developmental stages, including eggs, larvae of various instars, pupae, and adults. All genes monitored were expressed at higher levels in the head than in the thorax and abdomen, except for Ldα10. Dietary ingestion of double-stranded RNA bacterially expressed for Ldα1 (dsLdα1) significantly reduced the mRNA level of Ldα1 in treated larvae and adults by 48.0% and 78.6%, respectively. Among the non-target genes, Ldα3, Ldα9, and Ldβ1 were significantly up-regulated in larvae. A toxicity bioassay showed that dsLdα1 treatment greatly decreased the sensitivity to imidacloprid and thiamethoxam in adults. The larval susceptibility to thiamethoxam but not to imidacloprid was also reduced because of the lower down-regulation of Ldα1. Thus, our results suggest that Ldα1 encodes a subunit of a functional nAChR that mediates the toxicity of imidacloprid and thiamethoxam against L. decemlineata and that the down-regulation of Ldα1 might be an important mechanism for resistance and/or tolerance of L. decemlineata to neonicotinoids.

In vivo functional analysis of the Drosophila melanogaster nicotinic acetylcholine receptor Dα6 using the insecticide spinosad

The vinegar fly, Drosophila melanogaster, has been used to identify and manipulate insecticide resistance genes. The advancement of genome engineering technology and the increasing availability of pest genome sequences has increased the predictive and diagnostic capacity of the Drosophila model. The Drosophila model can be extended to investigate the basic biology of the interaction between insecticides and the proteins they target. Recently we have developed an in vivo system that permits the expression and study of key insecticide targets, the nicotinic acetylcholine receptors (nAChRs), in controlled genetic backgrounds. Here this system is used to study the interaction between the insecticide spinosad and a nAChR subunit, Dα6. Reciprocal chimeric subunits were created from Dα6 and Dα7, a subunit that does not respond to spinosad. Using the in vivo system, the Dα6/Dα7 chimeric subunits were tested for their capacity to respond to spinosad. Only the subunits containing the C-terminal region of Dα6 were able to respond to spinosad, thus confirming the importance this region for spinosad binding. A new incompletely dominant, spinosad resistance mechanism that may evolve in pest species is also examined. First generated using chemical mutagenesis, the Dα6(P146S) mutation was recreated using the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 system, the first use of this technology to introduce a resistant mutation into a controlled genetic background. Both alleles present with the same incompletely dominant, spinosad resistance phenotype, proving the P146S replacement to be the causal mutation. The proximity of the P146S mutation to the conserved Cys-loop indicates that it may impair the gating of the receptor. The results of this study enhance the understanding of nAChR structure:function relationships.

Genotype to phenotype, the molecular and physiological dimensions of resistance in arthropods

The recent accumulation of molecular studies on mutations in insects, ticks and mites conferring resistance to insecticides, acaricides and biopesticides is reviewed. Resistance is traditionally classified by physiological and biochemical criteria, such as target-site insensitivity and metabolic resistance. However, mutations are discrete molecular changes that differ in their intrinsic frequency, effects on gene dosage and fitness consequences. These attributes in turn impact the population genetics of resistance and resistance management strategies, thus calling for a molecular genetic classification. Mutations in structural genes remain the most abundantly described, mostly in genes coding for target proteins. These provide the most compelling examples of parallel mutations in response to selection. Mutations causing upregulation and downregulation of genes, both in cis (in the gene itself) and in trans (in regulatory processes) remain difficult to characterize precisely. Gene duplications and gene disruption are increasingly reported. Gene disruption appears prevalent in the case of multiple, hetero-oligomeric or redundant targets.

Resistance to bio-insecticides or how to enhance their sustainability: a review

After more than 70 years of chemical pesticide use, modern agriculture is increasingly using biological control products. Resistances to conventional insecticides are wide spread, while those to bio-insecticides have raised less attention, and resistance management is frequently neglected. However, a good knowledge of the limitations of a new technique often provides greater sustainability. In this review, we compile cases of resistance to widely used bio-insecticides and describe the associated resistance mechanisms. This overview shows that all widely used bio-insecticides ultimately select resistant individuals. For example, at least 27 species of insects have been described as resistant to Bacillus thuringiensis toxins. The resistance mechanisms are at least as diverse as those that are involved in resistance to chemical insecticides, some of them being common to bio-insecticides and chemical insecticides. This analysis highlights the specific properties of bio-insecticides that the scientific community should use to provide a better sustainability of these products.

Large-scale detection and analysis of adenosine-to-inosine RNA editing during development in Plutella xylostella

Adenosine-to-inosine (A-to-I) RNA editing is site-specific modification of RNAs that increases the diversity of the transcriptome and proteome. Most insects undergo complete metamorphosis, including four life cycle stages: egg, larva, pupa and adult. Many previous studies have confirmed that RNA-editing events occur in a development-specific manner; in other words, RNA-editing levels change during metamorphosis. Here, we describe an effort to identify the developmental specificity of RNA-editing events using a large-scale computational analysis of RNA-seq data derived from four developmental stages of the diamondback moth, Plutella xylostella. One thousand one hundred and eighty-seven A-to-I RNA-editing sites were predicted to be developmental stage specific (false-discovery rate <0.01) and 1,094 of these sites were located in protein-coding regions. Editing of 152 sites resulted in an altered amino acid residue. A putative adult-specific A-to-I RNA-editing site was verified by comparing cDNA sequences with its corresponding genomic locus at different stages of the P. xylostella life cycle. Our findings will help elucidate the role of A-to-I RNA editing in the regulation of metamorphosis. Further studies detailing changes in the extent of editing are needed to establish how as yet unknown regulatory factors are involved in the editing mechanism and what biological functions' editing serves.

Olive fly transcriptomics analysis implicates energy metabolism genes in spinosad resistance

Background

The olive fly, Bactrocera oleae, is the most devastating pest of cultivated olives. Its control has been traditionally based on insecticides, mainly organophosphates and pyrethroids. In recent years, the naturalyte spinosad is used against the olive fly. As with other insecticides, spinosad is subject to selection pressures that have led to resistance development. Mutations in the α6 subunit of the nicotinic acetylcholine receptor (nAChR) have been implicated in spinosad resistance in several species (e.g., Drosophila melanogaster) but excluded in others (e.g., Musca domestica). Yet, additional mechanisms involving enhanced metabolism of detoxification enzymes (such as P450 monooxygenases or mixed function oxidases) have also been reported. In order to clarify the spinosad resistance mechanisms in the olive fly, we searched for mutations in the α6-subunit of the nAChR and for up-regulated genes in the entire transcriptome of spinosad resistant olive flies.

Results

The olive fly α6-subunit of the nAChR was cloned from the laboratory sensitive strain and a spinosad selected resistant line. The differences reflected silent nucleotide substitutions or conserved amino acid changes. Additionally, whole transcriptome analysis was performed in the two strains in order to reveal any underlying resistance mechanisms. Comparison of over 13,000 genes showed that in spinosad resistant flies nine genes were significantly over-expressed, whereas ~40 were under-expressed. Further functional analyses of the nine over-expressed and eleven under-expressed loci were performed. Four of these loci (Yolk protein 2, ATP Synthase FO subunit 6, Low affinity cationic amino acid transporter 2 and Serine protease 6) showed consistently higher expression both in the spinosad resistant strain and in wild flies from a resistant California population. On the other side, two storage protein genes (HexL1 and Lsp1) and two heat-shock protein genes (Hsp70 and Hsp23) were unfailingly under-expressed in resistant flies.

Conclusion

The observed nucleotide differences in the nAChR-α6 subunit between the sensitive and spinosad resistant olive fly strains did not advocate for the involvement of receptor mutations in spinosad resistance. Instead, the transcriptome comparison between the two strains indicated that several immune system loci as well as elevated energy requirements of the resistant flies might be necessary to lever the detoxification process.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-714) contains supplementary material, which is available to authorized users.

Spinosad resistance of melon thrips, Thrips palmi, is conferred by G275E mutation in α6 subunit of nicotinic acetylcholine receptor and cytochrome P450 detoxification

To examine the resistance mechanisms of Thrips palmi against spinosad, we cloned partial nucleotide sequences of the nicotinic acetylcholine receptor α6 subunit (TPα6) gene from susceptible (OK) and resistant (TS1 and TS5) strains and compared the deduced amino acid sequences among the three strains. The OK, TS1, and TS5 strains respectively showed LC50 values of 3.4mg/L, 2838.5mg/L, and 6655.5mg/L. The deduced amino acid sequence of TPα6 gene showed 96% identity with that of Frankliniella occidentalis. Comparison of the deduced amino acid sequences of TPα6 gene among the three strains showed that the TS1 and TS5 strains had a resistant amino acid, Glu, at amino acid position 275. On the other hand, a susceptible amino acid, Gly, was encoded at the corresponding amino acid position for the OK strain. The synergist, piperonyl butoxide, respectively caused 1.1-fold , 5.8-fold , and 9.0-fold decreases in the resistance ratios of the OK, TS1, and TS5 strains. These results suggest that spinosad resistance of T. palmi is conferred by reduced sensitivity of TPα6 and cytochrome P450-mediated detoxification.

Alternative splicing and highly variable cadherin transcripts associated with field-evolved resistance of pink bollworm to bt cotton in India

Evolution of resistance by insect pests can reduce the benefits of insecticidal proteins from Bacillus thuringiensis (Bt) that are used extensively in sprays and transgenic crops. Despite considerable knowledge of the genes conferring insect resistance to Bt toxins in laboratory-selected strains and in field populations exposed to Bt sprays, understanding of the genetic basis of field-evolved resistance to Bt crops remains limited. In particular, previous work has not identified the genes conferring resistance in any cases where field-evolved resistance has reduced the efficacy of a Bt crop. Here we report that mutations in a gene encoding a cadherin protein that binds Bt toxin Cry1Ac are associated with field-evolved resistance of pink bollworm (Pectinophora gossypiella) in India to Cry1Ac produced by transgenic cotton. We conducted laboratory bioassays that confirmed previously reported resistance to Cry1Ac in pink bollworm from the state of Gujarat, where Bt cotton producing Cry1Ac has been grown extensively. Analysis of DNA from 436 pink bollworm from seven populations in India detected none of the four cadherin resistance alleles previously reported to be linked with resistance to Cry1Ac in laboratory-selected strains of pink bollworm from Arizona. However, DNA sequencing of pink bollworm derived from resistant and susceptible field populations in India revealed eight novel, severely disrupted cadherin alleles associated with resistance to Cry1Ac. For these eight alleles, analysis of complementary DNA (cDNA) revealed a total of 19 transcript isoforms, each containing a premature stop codon, a deletion of at least 99 base pairs, or both. Seven of the eight disrupted alleles each produced two or more different transcript isoforms, which implicates alternative splicing of messenger RNA (mRNA). This represents the first example of alternative splicing associated with field-evolved resistance that reduced the efficacy of a Bt crop.

The α6 nicotinic acetylcholine receptor subunit of Frankliniella occidentalis is not involved in resistance to spinosad

Insects evolve resistance which constrains the sustainable use of insecticides. Spinosyns, a class of environmentally-friendly macrolide insecticides, is not an exception. The mode of inheritance and the mechanisms of resistance to spinosad (the most common spinosyn insecticide) in Frankliniella occidentalis (Western flower thrips, WFT) were investigated in this study. Resistance (170,000-fold) was autosomal and completely recessive. Recent studies showed that deletion of the nicotinic acetylcholine receptor α6 subunit gene resulted in strains of Drosophila melanogaster, Plutella xylostella and Bactrocera dorsalis that are resistant to spinosad, indicating that nAChRα6 subunit maybe important for the toxic action of this insecticide. Conversely, a G275E mutation of this subunit in F. occidentalis was recently proposed as the mechanism of resistance to spinosad. We cloned and characterized nAChRα6 from three susceptible and two spinosad resistant strains from China and the USA. The Foα6 cDNA is 1873bp and the open reading frame is 1458bp which encodes 485 amino acid residues with a predicted molecular weight of 53.5-kDa, the 5' and 3' UTRs are 121 and 294bp, respectively. There was no difference in the cDNA sequence between the resistant and susceptible thrips, suggesting the G275E mutation does not confer resistance in these populations. Ten isoforms of Foα6, arising from alternative splicing, were isolated and did not differ between the spinosad-susceptible and resistant strains. Quantitative real time PCR analysis showed Foα6 was highly expressed in the first instar larva, pupa and adult, and the expression levels were 3.67, 2.47, 1.38 times that of the second instar larva. The expression level was not significantly different between the susceptible and resistant strains. These results indicate that Foα6 is not involved in resistance to spinosad in F. occidentalis from China and the USA.

KONAGAbase: a genomic and transcriptomic database for the diamondback moth, Plutella xylostella

Background

The diamondback moth (DBM), Plutella xylostella, is one of the most harmful insect pests for crucifer crops worldwide. DBM has rapidly evolved high resistance to most conventional insecticides such as pyrethroids, organophosphates, fipronil, spinosad, Bacillus thuringiensis, and diamides. Therefore, it is important to develop genomic and transcriptomic DBM resources for analysis of genes related to insecticide resistance, both to clarify the mechanism of resistance of DBM and to facilitate the development of insecticides with a novel mode of action for more effective and environmentally less harmful insecticide rotation. To contribute to this goal, we developed KONAGAbase, a genomic and transcriptomic database for DBM (KONAGA is the Japanese word for DBM).

Description

KONAGAbase provides (1) transcriptomic sequences of 37,340 ESTs/mRNAs and 147,370 RNA-seq contigs which were clustered and assembled into 84,570 unigenes (30,695 contigs, 50,548 pseudo singletons, and 3,327 singletons); and (2) genomic sequences of 88,530 WGS contigs with 246,244 degenerate contigs and 106,455 singletons from which 6,310 de novo identified repeat sequences and 34,890 predicted gene-coding sequences were extracted. The unigenes and predicted gene-coding sequences were clustered and 32,800 representative sequences were extracted as a comprehensive putative gene set. These sequences were annotated with BLAST descriptions, Gene Ontology (GO) terms, and Pfam descriptions, respectively. KONAGAbase contains rich graphical user interface (GUI)-based web interfaces for easy and efficient searching, browsing, and downloading sequences and annotation data. Five useful search interfaces consisting of BLAST search, keyword search, BLAST result-based search, GO tree-based search, and genome browser are provided. KONAGAbase is publicly available from our website (http://dbm.dna.affrc.go.jp/px/) through standard web browsers.

Conclusions

KONAGAbase provides DBM comprehensive transcriptomic and draft genomic sequences with useful annotation information with easy-to-use web interfaces, which helps researchers to efficiently search for target sequences such as insect resistance-related genes. KONAGAbase will be continuously updated and additional genomic/transcriptomic resources and analysis tools will be provided for further efficient analysis of the mechanism of insecticide resistance and the development of effective insecticides with a novel mode of action for DBM.