Development, application and evaluation of three novel TaqMan qPCR assays for phosphine resistance monitoring in major stored product pests Tribolium castaneum and Rhyzopertha dominica

BACKROUND

Stored product protection from insect pests relies heavily on the use of phosphine. The most serious drawback of phosphine is the development of resistance in major stored product insects worldwide, including the red flour beetle, Tribolium castaneum (Herbst) and the lesser grain borer, Rhyzopertha dominica (F.). Two genetic loci are responsible for phosphine resistance: the rph1 (S349G mutation in the cyt-b5-r homolog) in T. castaneum and the rph2 (P45/49S mutation in the dihydrolipoamide dehydrogenase (dld) gene) in T. castaneum and R. dominica.

RESULTS

In this study, we have developed and applied high-throughput, practical and specific molecular diagnostics (TaqMan qPCR) for monitoring mutations S349G, P45S and P49S. In our pilot monitoring application, we have included phosphine-resistant and susceptible populations from different parts of the world (USA, Australia, Brazil) and European strains from Greece and Serbia. Our results for the resistant T. castaneum showed a P45S mutant allele frequency (MAF) of 100% and 75.0% in the populations from Serbia and Brazil, respectively. Regarding the susceptible T. castaneum, P45S was detected in Greece (MAF = 62.5%) and was absent in Australia (MAF = 0.0%). Additionally, the S349G mutation was found to be fixed in all resistant populations, while it was also detected in susceptible ones (frequencies: 65.0% and 100.0%). The only case where both mutations were fixed (100%) was a resistant population from Serbia. In R. dominica, the P49S mutation was found only in the two resistant R. dominica populations from Serbia and Greece (50.0% and 100%) and was absent from the susceptible one from Greece; thus, P49S seems to be a satisfactory indicator for monitoring phosphine resistance.

CONCLUSIONS

Our P49S detection assay in R. dominica seems to be a viable option in this direction, yet its utilization needs additional large-scale confirmatory work. The identification of additional resistance markers also should be prioritized. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Mitochondrial coding genes mediate insecticide tolerance in the oriental fruit fly, Bactrocera dorsalis (Hendel)

The oriental fruit fly, Bactrocera dorsalis (Hendel), an invasive insect pest infesting fruits and vegetables, possesses a remarkable capacity for environmental adaptation. The investigation of behind mechanisms of the stress adaptability in B. dorsalis holds significantly practical relevance. Previous studies on the molecular mechanism underlying stress resistance in B. dorsalis have predominantly focused on nuclear-coding genes, with limited exploration on organelle-coding genes. In this study, we assessed alterations in the mitochondrial physiological parameters of B. dorsalis under exposure to malathion, avermectin, and beta-cypermethrin at LD50 dosages. The results showed that all three insecticides were capable of reducing mitochondrial complex IV activity and ATP content. Expression patterns of mitochondrial coding genes across different developmental stages, tissues and insecticide exposures were analyzed by RT-qPCR. The results revealed that these mitochondrial coding genes were expressed in various tissues and at different developmental stages. Particularly noteworthy, atp6, cox2, and cytb exhibited substantial up-regulation in response to malathion and avermectin treatment. Furthermore, RNAi-mediated knockdown of atp6 and cox2 resulted in the increased toxicity of malathion and avermectin against B. dorsalis, and cox2 silencing was also associated with the decreased complex IV activity. These findings suggest that atp6 and cox2 most likely play pivotal roles in mediating tolerance or resistance to malathion and avermectin in B. dorsalis. Our results provide novel insights into the role of mitochondrial coding genes in conferring tolerance to insecticides in B. dorsalis, with practical implications for controlling this pest in the field.

Comparative Analysis of the Metabolic Profiles of Strains of Tribolium castaneum (Herbst) Adults with Different Levels of Phosphine Resistance Based on Direct Immersion Solid-Phase Microextraction and Gas Chromatography-Mass Spectrometry

The management of phosphine (PH3) resistance in stored grain pests is an essential component of implementing timely and effective pest control strategies. The prevailing standard method for PH3 resistance testing involves the exposure of adult insects to a specific concentration over a fixed period. Although it is widely adopted, this method necessitates an extensive period for assay preparation and diagnosis. To address this issue, this study employed Direct Immersion Solid-Phase Microextraction (DI-SPME) coupled with Gas Chromatography-Mass Spectrometry (GC-MS) to compare and analyze the metabolic profiles of PH3-sensitive (TC-S), PH3 weak-resistant (TC-W), and PH3 strong-resistant (TC-SR) Tribolium castaneum (Herbst) adults. A total of 36 metabolites were identified from 3 different PH3-resistant strains of T. castaneum; 29 metabolites were found to present significant differences (p < 0.05) across these groups, with hydrocarbon and aromatic compounds being particularly prevalent. Seven metabolites showed no significant variations among the strains, consisting of four hydrocarbon compounds, two iodo-hydrocarbon compounds, and one alcohol compound. Further multivariate statistical analysis revealed a total of three, two, and nine differentially regulated metabolites between the TC-S versus TC-W, TC-S versus TC-SR, and TC-W versus TC-SR groups, respectively. Primarily, these metabolites comprised hydrocarbons and iodo-hydrocarbons, with the majority being associated with insect cuticle metabolism. This study demonstrates that DI-SPME technology is an effective method for studying differentially expressed metabolites in T. castaneum with different levels of PH3 resistance. This approach may help to provide a better understanding of the development of insect PH3 resistance and act as a valuable reference for the establishment of rapid diagnostic techniques for insect PH3 resistance.

Transcriptome profiling and in silico docking analysis of phosphine resistance in rice weevil, Sitophilus oryzae (Coleoptera: Curculionidae)

The rice weevil, Sitophilus oryzae (Linnaeus, Coleoptera: Curculionidae), is a serious cosmopolitan pest that affects grain in storage and has developed high levels of resistance toward phosphine. In this study, RNA-seq data was used to study the phosphine resistance mechanisms in S. oryzae. Resistant and susceptible populations of S. oryzae were identified based on phosphine bioassays conducted in 32 populations collected across Tamil Nadu, India. Differential expression of mitochondrial (COX1, COX2, COX3, ND2, ND3, ATP6, and ATP8) and detoxification genes (Cyps, Gsts, and Cbe) were observed in the resistant and susceptible populations of S. oryzae. The previously characterized phosphine resistant gene, dld (dihydrolipoamide dehydrogenase) linked to the rph2 locus, was found to be up-regulated in resistant S. oryzae population (ISO-TNAU-RT) treated with phosphine. Also, the genes involved in Tricarboxylic acid (TCA) cycle were significantly down-regulated. In addition, a significant up-regulation in the expression of the antioxidant enzymes superoxide dismutase (2.5×) and catalase (2.1×) in ISO-TNAU-RT populations was recorded. Furthermore, a distinct amino acid substitution, Lysine > Glutamic acid (K141E) was identified in resistant phenotypes. In silico docking studies of both resistant and susceptible DLD protein with phosphine molecule revealed that the amino acid residues involved in the interaction were different. This suggested that the amino acid substitution might lead to structural modifications which reduces the affinity of the target (phosphine). This study provides insight on the various genes, pathways, and functional mechanisms having a significant role in phosphine resistance in S. oryzae.

MCSNet+: Enhanced Convolutional Neural Network for Detection and Classification of Tribolium and Sitophilus Sibling Species in Actual Wheat Storage Environments

Insect pests like Tribolium and Sitophilus siblings are major threats to grain storage and processing, causing quality and quantity losses that endanger food security. These closely related species, having very similar morphological and biological characteristics, often exhibit variations in biology and pesticide resistance, complicating control efforts. Accurate pest species identification is essential for effective control, but workplace safety in the grain bin associated with grain deterioration, clumping, fumigator hazards, and air quality create challenges. Therefore, there is a pressing need for an online automated detection system. In this work, we enriched the stored-grain pest sibling image dataset, which includes 25,032 annotated Tribolium samples of two species and five geographical strains from real warehouse and another 1774 from the lab. As previously demonstrated on the Sitophilus family, Convolutional Neural Networks demonstrate distinct advantages over other model architectures in detecting Tribolium. Our CNN model, MCSNet+, integrates Soft-NMS for better recall in dense object detection, a Position-Sensitive Prediction Model to handle translation issues, and anchor parameter fine-tuning for improved matching and speed. This approach significantly enhances mean Average Precision (mAP) for Sitophilus and Tribolium, reaching a minimum of 92.67 ± 1.74% and 94.27 ± 1.02%, respectively. Moreover, MCSNet+ exhibits significant improvements in prediction speed, advancing from 0.055 s/img to 0.133 s/img, and elevates the recognition rates of moving insect sibling species in real wheat storage and visible light, rising from 2.32% to 2.53%. The detection performance of the model on laboratory-captured images surpasses that of real storage facilities, with better results for Tribolium compared to Sitophilus. Although inter-strain variances are less pronounced, the model achieves acceptable detection results across different Tribolium geographical strains, with a minimum recognition rate of 82.64 ± 1.27%. In real-time monitoring videos of grain storage facilities with wheat backgrounds, the enhanced deep learning model based on Convolutional Neural Networks successfully detects and identifies closely related stored-grain pest images. This achievement provides a viable solution for establishing an online pest management system in real storage facilities.

Current status of phosphine resistance in Indian field populations of Tribolium castaneum and its influence on antioxidant enzyme activities

Resistance to phosphine is widely reported in several stored product insect pests globally. However, knowledge of its prevalence and the association of antioxidant enzymes with phosphine resistance is limited. Herein, we assessed the levels of phosphine susceptibility and estimated the antioxidant enzyme activities viz., superoxide dismutase (SOD), peroxidase (POX), and catalase (CAT) in selected Indian populations of red flour beetle Tribolium castaneum (Herbst). Dose-response probit assays revealed that the LC50 values ranged from 0.038 to 1.277 mg L-1 showing 2.11 to 70.94-fold resistance to phosphine compared to susceptible check. Activities of antioxidant enzymes varied significantly between the T. castaneum populations following phosphine exposure. The magnitude of SOD activity ranged from 8.77 to18.82 U mg-1 protein, while, the activities of POX and CAT varied between 52.42 and 408.32 and 61.11 to 247.49 µM H2O2 reduced min-1 mg-1 of protein, respectively. The correlation analysis revealed a significant positive association of SOD (r = 0.89) and POX (r = 0.98) with increased resistance ratio, while the CAT (r = - 0.98) is negatively linked with resistance to phosphine. A principal component analysis identified phosphine resistance was closely associated with POX and SOD activities but was unrelated to the CAT activity. Our results throw light on the varied association of antioxidant enzyme activities in response to phosphine fumigation in field populations of T. castaneum. Further studies on the biochemical and molecular basis of phosphine stress in insects may help to devise suitable strategies to safeguard storage commodities and ensure a sustainable environment.

Quick knockdown results in high mortality: is this theory correct? A case study with phosphine and the red flour beetle

BACKGROUND

The fumigant phosphine is used all over the world for disinfestation of stored grains and commodities. Adults of 23 different populations of Tribolium castaneum from 10 different countries were evaluated for phosphine resistance using a modification of the Detia Degesch Phosphine Tolerance Test Kit (DDPTTK). Adults were exposed to 3000 ppm and recorded for 5-270 min for their mobility.

RESULTS

Among the tested populations, high levels of phosphine resistance were recorded in populations from Brazil, Serbia, and Spain. No survivals were recorded after 7 days post exposure for eight of 23 in a tested population.

CONCLUSIONS

Our work revealed four scenarios: 1, quick knockdown-low (or no) recovery; 2, Slow knockdown-high recovery; 3, Quick knockdown-high recovery; and 4, Slow knockdown-low recovery. Our data indicate that post exposure period is critical for the evaluation and characterization of phosphine resistance. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Identification of cuticular protein genes and analysis of their roles in phosphine resistance of the rusty grain beetle Cryptolestes ferrugineus

The rusty grain beetle, Cryptolestes ferrugineus (Stephens) is one of the most economically important stored grain pests, and it has evolved the high resistance to phosphine. Cuticular proteins (CPs) are the major structural components of insect cuticle, and previous studies have confirmed that CPs were involved in insecticide resistance. However, the CPs of C. ferrugineus are still poorly characterized, and thus we conducted transcriptome-wide identification of CP genes and analyze their possible relationships with phosphine resistance in this pest. In this study, a total of 122 putative CPs were annotated in the C. ferrugineus transcriptome data by blasting with the known CPs of Tribolium castaneum. The analysis of conserved motifs revealed these CPs of C. ferrugineus belonging to 9 different families, including 87 CPR, 13 CPAP1, 7 CPAP3, 3 Tweedle, 1 CPLCA, 1 CPLCG, 5 CPLCP, 2 CPCFC, and 3 CPFL proteins. The further phylogenetic analysis showed the different evolutionary patterns of CPs. Namely, we found some CPs (CPR family) formed species-specific protein clusters, indicating these CPs might occur independently among insect taxa, and while some other CPs (CPAP1 and CPAP3 family) shared a closer correlation based on the architecture of protein domains. Subsequently, the previous RNA-seq data were applied to establish the expression profiles of CPs in a phosphine susceptible and resistant populations of C. ferrugineus, and a large amount of CP genes were found to be over-expressed in resistant insects. Lastly, an up-regulated CP gene (CPR family) was selected for the further functional analysis, and after this gene was silenced via RNA interference (RNAi), the sensitivity to phosphine was significantly enhanced in C. ferrugineus. In conclusion, the present results provided us an overview of C. ferrugineus CPs, and which suggested that the CPs might play the critical roles in phosphine resistance.

Insight into the molecular mechanism of phosphine toxicity provided by functional analysis of cytochrome b5 fatty acid desaturase and dihydrolipoamide dehydrogenase in the red flour beetle, Tribolium castaneum

Phosphine is the dominant chemical used in postharvest pest control. Widespread and highly frequent use of phosphine has been selected for pest insects, including Tribolium castaneum, which is highly resistant. Lipid peroxidation and reactive oxygen species (ROS) are two major factors determining phosphine toxicity; however, the mechanisms of production of these two factors in phosphine toxicity are still unknown. Here, we first determined the time course of phosphine-induced lipid peroxidation and ROS production in T. castaneum. Our results showed that lipid peroxidation occurs before ROS in the process of phosphine toxicity, and fumigated beetles with higher resistance levels were associated with weaker activity on lipid peroxidation and ROS. A significant decline in lipid peroxidation was observed in fumigated individuals after knockdown of cytochrome b5 fatty acid desaturase (Cyt-b5-r) via RNA interference (RNAi), indicating that Cyt-b5-r is critical for triggering phosphine-induced lipid peroxidation. Moreover, significant decreases in both ROS and mortality were detected in fumigated T. castaneum adults fed melatonin for 7 days, an inhibitor of lipid peroxidation. Cyt-b5-r RNAi also inhibited ROS production and mortality in phosphine-treated beetles. Meanwhile, a significant decrease in ROS production (68.4%) was detected in dihydrolipoamide dehydrogenase (DLD) knockdown individuals with phenotypes susceptible to phosphine, suggesting that lipid peroxidation initiates ROS with the expression of DLD. However, a significant increase in ROS (122.1%) was detected in the DLD knockdown beetles with strongly resistant phenotypes, indicating that the DLD-involved pathway may not be the only mechanism of ROS generation in phosphine toxicity and the existence of a moonlighting role in downregulating ROS in strongly resistant T. castaneum.

Comparison of phenotypic and genotypic frequency of phosphine resistance in select field populations of Tribolium castaneum from India

BACKGROUND

Tribolium castaneum causes substantial damage to stored grains, leading to economic losses. The present study evaluates phosphine resistance in adult and larval stages of T. castaneum from north and northeast India, where continuous and long-term phosphine use in large-scale storage conditions intensifies resistance, posing risks to grain quality, safety, and industry profitability.

METHODS AND RESULTS

This study utilized T. castaneum bioassays and CAPS markers restriction digestion methodology to assess resistance. The phenotypic results indicated a lower LC50 value in larvae compared to adults, while the resistance ratio remained consistent across both stages. Similarly, the genotypic analysis revealed comparable resistance levels regardless of the developmental stage. We categorized the freshly collected populations based on resistance ratios, with Shillong showing weak resistance, Delhi and Sonipat displaying moderate resistance, and Karnal, Hapur, Moga, and Patiala exhibiting strong resistance to phosphine. Further validation by accessing findings and exploring the relationship between phenotypic and genotypic variations using Principal Component Analysis (PCA). This comprehensive study enhances our understanding of T. castaneum resistance levels, providing valuable insights for the development of targeted pest management strategies.

CONCLUSION

This study provides insights into the current phenotypic and genotypic resistance levels of T. castaneum in North and North East India. Understanding this is crucial for developing effective pest management strategies and future research on biological and physiological aspects of phosphine resistance in insects, enabling the formulation of effective management practices. Addressing phosphine resistance is vital for sustainable pest management and the long-term viability of the agricultural and food industries.

Pesticidal Toxicity of Phosphine and Its Interaction with Other Pest Control Treatments

Phosphine is the most widely used fumigant for stored grains due to a lack of better alternatives, all of which have serious shortcomings that restrict their use. The extensive use of phosphine has led to the development of resistance among insect pests of grain, which threatens its status as a reliable fumigant. Understanding the mode of action of phosphine as well as its resistance mechanisms provides insight that may lead to improved phosphine efficacy and pest control strategies. The mechanisms of action in phosphine vary from disrupting metabolism and oxidative stress to neurotoxicity. Phosphine resistance is genetically inherited and is mediated by the mitochondrial dihydrolipoamide dehydrogenase complex. In this regard, laboratory studies have revealed treatments that synergistically enhance phosphine toxicity that may be used to suppress resistance development and enhance efficacy. Here, we discuss the reported phosphine modes of action, mechanisms of resistance and interactions with other treatments.

Phosphine gas in the dark induces severe phytotoxicity in Arabidopsis thaliana by increasing a hypoxia stress response and disrupting the energy metabolism: Transcriptomic approaches

Phosphine (PH₃) is an ideal fumigant alternative on methyl bromide (MB) as MB has been classified as an ozone-depleting substance. However, several challenges limit its efficient use in crop production, including the emergence of PH₃-resistant insect pests and the incidence of phytotoxic effects on nursery plants. Therefore, this study aims to elucidate the mechanism underlying PH₃ phytotoxicity in plants using transcriptomic techniques. Fumigation with 2 g/m³ PH₃ induced phytotoxic effects in A. thaliana, as evidenced by a decrease in growth and vegetation indices compared to the control group. Transcriptomic analysis revealed that PH₃ fumigation phytotoxicity responses in A. thaliana involve genes related to hypoxia stress and energy metabolism. Additionally, pretreatment with ethylene induced pre-adaptation to hypoxia under light conditions during fumigation effectively suppressed the phytotoxic effects of PH₃ in A. thaliana by increasing the expression of hypoxia-adaptive genes. Moreover, the phytotoxicity of PH₃ was also confirmed in pumpkin (Cucurbita moschata Duch.), and was dependent on light. Overall, our findings showed that fumigation under light conditions and ethylene pretreatment could be used to minimize PH₃-induced phytotoxic effects in plants.

Rapid adaptation in a fast-changing world: Emerging insights from insect genomics

Many researchers have questioned the ability of biota to adapt to rapid anthropogenic environmental shifts. Here, we synthesize emerging genomic evidence for rapid insect evolution in response to human pressure. These new data reveal diverse genomic mechanisms (single locus, polygenic, structural shifts; introgression) underpinning rapid adaptive responses to a variety of anthropogenic selective pressures. While the effects of some human impacts (e.g. pollution; pesticides) have been previously documented, here we highlight startling new evidence for rapid evolutionary responses to additional anthropogenic processes such as deforestation. These recent findings indicate that diverse insect assemblages can indeed respond dynamically to major anthropogenic evolutionary challenges. Our synthesis also emphasizes the critical roles of genomic architecture, standing variation and gene flow in maintaining future adaptive potential. Broadly, it is clear that genomic approaches are essential for predicting, monitoring and responding to ongoing anthropogenic biodiversity shifts in a fast-changing world.

Amplification refractory mutation system based real-time PCR (ARMS-qPCR) for rapid resistance characterization of Tribolium castaneum to phosphine

Resistance of Tribolium castaneum to phosphine is related to point mutations in DNA code corresponding to amino acid changes associated with a core metabolic enzyme dihydrolipoamide dehydrogenase (DLD), but the mutation patterns vary among different resistant populations. Thus, there is a great need to develop a cost-effective method to detect core mutations in T. castaneum, which would be the key factor to understand the molecular basis of phosphine resistance. Amplification refractory mutation system-based quantitative Real-Time PCR (ARMS-qPCR) is an ideal method that can rapidly detect point mutations. Here, the P45S and G131D mutations existed in the DLD of T. castaneum selected from strong Chinese resistance phenotypes, and the DLD P45S mutation, which represents a strong phosphine resistance allele, was confirmed as the most abundant mutation to determine strong resistance genotypes. Our study found that 85 out of 120 beetles carried the P45S resistance allele, including 51 homozygous and 34 heterozygous individuals. Moreover, there was a strong linear relationship (R² = 0.917) between the resistance ratio and the resistance allele frequency among the strongly resistant populations. Our data showed that the ARMS-qPCR method that we developed could rapidly determine strong resistance phenotypes of T. castaneum to phosphine by detecting the DLD P45S mutation. These results not only provide a detailed example for developing an ARMS-qPCR-based method to characterize pesticide resistance, but also support further elucidation of the molecular basis of phosphine resistance.

Role of Lipids in Phosphine Resistant Stored-Grain Insect Pests Tribolium castaneum and Rhyzopertha dominica

Insects rely on lipids as an energy source to perform various activities, such as growth, flight, diapause, and metamorphosis. This study evaluated the role of lipids in phosphine resistance by stored-grain insects. Phosphine resistant and susceptible strains of the two main stored-grain insects, Tribolium castaneum and Rhyzopertha dominica, were analyzed using liquid chromatography-mass spectroscopy (LC-MS) to determine their lipid contents. Phosphine resistant strains of both species had a higher amount of lipids than susceptible stains. Significant variance ratios between the resistant and susceptible strains of T. castaneum were observed for glycerolipids (1.13- to 53.10-fold) and phospholipids (1.05- to 20.00-fold). Significant variance ratios between the resistant and susceptible strains of R. dominica for glycerolipids were 1.04- to 31.50-fold and for phospholipids were 1.04- to 10.10-fold. Glycerolipids are reservoirs to face the long-term energy shortage. Phospholipids act as a barrier to isolate the cells from the surrounding environment and allow each cell to perform its specific function. Thus, lipids offer a consistent energy source for the resistant insect to survive under the stress of phosphine fumigation and provide a suitable environment to protect the mitochondria from phosphine. Hence, it was proposed through this study that the lipid content of phosphine-resistant and phosphine-susceptible strains of T. castaneum and R. dominica could play an important role in the resistance of phosphine.

The Genome of Rhyzopertha dominica (Fab.) (Coleoptera: Bostrichidae): Adaptation for Success

The lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae), is a major global pest of cereal grains. Infestations are difficult to control as larvae feed inside grain kernels, and many populations are resistant to both contact insecticides and fumigants. We sequenced the genome of R. dominica to identify genes responsible for important biological functions and develop more targeted and efficacious management strategies. The genome was assembled from long read sequencing and long-range scaffolding technologies. The genome assembly is 479.1 Mb, close to the predicted genome size of 480.4 Mb by flow cytometry. This assembly is among the most contiguous beetle assemblies published to date, with 139 scaffolds, an N₅₀ of 53.6 Mb, and L₅₀ of 4, indicating chromosome-scale scaffolds. Predicted genes from biologically relevant groups were manually annotated using transcriptome data from adults and different larval tissues to guide annotation. The expansion of carbohydrase and serine peptidase genes suggest that they combine to enable efficient digestion of cereal proteins. A reduction in the copy number of several detoxification gene families relative to other coleopterans may reflect the low selective pressure on these genes in an insect that spends most of its life feeding internally. Chemoreceptor genes contain elevated numbers of pseudogenes for odorant receptors that also may be related to the recent ontogenetic shift of R. dominica to a diet consisting primarily of stored grains. Analysis of repetitive sequences will further define the evolution of bostrichid beetles compared to other species. The data overall contribute significantly to coleopteran genetic research.

A Preliminary Analysis on the Insecticidal Effect of Cyantraniliprole against Stored-Product Pests

While existing insecticides are becoming obsolete, the need for research and application of alternative substances is growing. Cyantraniliprole is a second-generation ryanodine receptor with the ability to bind and activate the ryanodine receptors in insect striated muscle cells, causing continuous muscular contraction, paralysis, and death. Many studies indicate its effectiveness on a variety of insects, but its action on storage pests has not yet been reported. We successfully conducted experiments for the first time on adult Tenebrio molitor, Tribolium confusum, Alphitobius diaperinus, Rhyzopertha dominica, and Trogoderma granarium with application of cyantraniliprole, causing dose-dependent mortality. Bioassays were carried out in the laboratory, where experimental adults were sprayed with six concentrations of cyantraniliprole. Mortality was recorded at 7, 14, 21, and 28 days after exposure. Mean mortality, survival concentration, and survival time were estimated for each species. The concentrations with both the highest mortality and the lowest survival rate were 2500 and 3000 ppm. Our results indicate that the tested insecticide was effective against T. confusum adults and is a promising pesticide for use in storage facilities.

Tribolium castaneum: A Model Insect for Fundamental and Applied Research

Tribolium castaneum has a long history as a model species in many distinct subject areas, but improved connections among the genetics, genomics, behavioral, ecological, and pest management fields are needed to fully realize this species' potential as a model. Tribolium castaneum was the first beetle whose genome was sequenced, and a new genome assembly and enhanced annotation, combined with readily available genomic research tools, have facilitated its increased use in a wide range of functional genomics research. Research into T. castaneum's sensory systems, response to pheromones and kairomones, and patterns of movement and landscape utilization has improved our understanding of behavioral and ecological processes. Tribolium castaneum has also been a model in the development of pest monitoring and management tactics, including evaluation of insecticide resistance mechanisms. Application of functional genomics approaches to behavioral, ecological, and pest management research is in its infancy but offers a powerful tool that can link mechanism with function and facilitate exploitation of these relationships to better manage this important food pest.

Dihydrolipoamide dehydrogenase, pyruvate oxidation, and acetylation-dependent mechanisms intersecting drug iatrogenesis

In human metabolism, pyruvate dehydrogenase complex (PDC) is one of the most intricate and large multimeric protein systems representing a central hub for cellular homeostasis. The worldwide used antiepileptic drug valproic acid (VPA) may potentially induce teratogenicity or a mild to severe hepatic toxicity, where the underlying mechanisms are not completely understood. This work aims to clarify the mechanisms that intersect VPA-related iatrogenic effects to PDC-associated dihydrolipoamide dehydrogenase (DLD; E3) activity. DLD is also a key enzyme of α-ketoglutarate dehydrogenase, branched-chain α-keto acid dehydrogenase, α-ketoadipate dehydrogenase, and the glycine decarboxylase complexes. The molecular effects of VPA will be reviewed underlining the data that sustain a potential interaction with DLD. The drug-associated effects on lipoic acid-related complexes activity may induce alterations on the flux of metabolites through tricarboxylic acid cycle, branched-chain amino acid oxidation, glycine metabolism and other cellular acetyl-CoA-connected reactions. The biotransformation of VPA involves its complete β-oxidation in mitochondria causing an imbalance on energy homeostasis. The drug consequences as histone deacetylase inhibitor and thus gene expression modulator have also been recognized. The mitochondrial localization of PDC is unequivocal, but its presence and function in the nucleus were also demonstrated, generating acetyl-CoA, crucial for histone acetylation. Bridging metabolism and epigenetics, this review gathers the evidence of VPA-induced interference with DLD or PDC functions, mainly in animal and cellular models, and highlights the uncharted in human. The consequences of this interaction may have significant impact either in mitochondrial or in nuclear acetyl-CoA-dependent processes.

Suppression of a core metabolic enzyme dihydrolipoamide dehydrogenase (dld) protects against amyloid beta toxicity in C. elegans model of Alzheimer's disease

A decrease in energy metabolism is associated with Alzheimer's disease (AD), but it is not known whether the observed decrease exacerbates or protects against the disease. The importance of energy metabolism in AD is reinforced by the observation that variants of dihydrolipoamide dehydrogenase (DLD), is genetically linked to late-onset AD. To determine whether DLD is a suitable therapeutic target, we suppressed the dld-1 gene in Caenorhabditis elegans that express human Aβ peptide in either muscles or neurons. Suppression of the dld-1 gene resulted in significant restoration of vitality and function that had been degraded by Aβ pathology. This included protection of neurons and muscles cells. The observed decrease in proteotoxicity was associated with a decrease in the formation of toxic oligomers rather than a decrease in the abundance of the Aβ peptide. The mitochondrial uncoupler, carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP), which like dld-1 gene expression inhibits ATP synthesis, had no significant effect on Aβ toxicity. Proteomics data analysis revealed that beneficial effects after dld-1 suppression could be due to change in energy metabolism and activation of the pathways associated with proteasomal degradation, improved cell signaling and longevity. Thus, some features unique to dld-1 gene suppression are responsible for the therapeutic benefit. By direct genetic intervention, we have shown that acute inhibition of dld-1 gene function may be therapeutically beneficial. This result supports the hypothesis that lowering energy metabolism protects against Aβ pathogenicity and that DLD warrants further investigation as a therapeutic target.

The exposome paradigm to predict environmental health in terms of systemic homeostasis and resource balance based on NMR data science

The environment, from microbial ecosystems to recycled resources, fluctuates dynamically due to many physical, chemical and biological factors, the profile of which reflects changes in overall state, such as environmental illness caused by a collapse of homeostasis. To evaluate and predict environmental health in terms of systemic homeostasis and resource balance, a comprehensive understanding of these factors requires an approach based on the "exposome paradigm", namely the totality of exposure to all substances. Furthermore, in considering sustainable development to meet global population growth, it is important to gain an understanding of both the circulation of biological resources and waste recycling in human society. From this perspective, natural environment, agriculture, aquaculture, wastewater treatment in industry, biomass degradation and biodegradable materials design are at the forefront of current research. In this respect, nuclear magnetic resonance (NMR) offers tremendous advantages in the analysis of samples of molecular complexity, such as crude bio-extracts, intact cells and tissues, fibres, foods, feeds, fertilizers and environmental samples. Here we outline examples to promote an understanding of recent applications of solution-state, solid-state, time-domain NMR and magnetic resonance imaging (MRI) to the complex evaluation of organisms, materials and the environment. We also describe useful databases and informatics tools, as well as machine learning techniques for NMR analysis, demonstrating that NMR data science can be used to evaluate the exposome in both the natural environment and human society towards a sustainable future.

A co-fumigation strategy utilizing reduced rates of phosphine (PH3 ) and sulfuryl fluoride (SF) to control strongly resistant rusty grain beetle, Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae)

BACKGROUND

Managing resistance to phosphine (PH₃ ) in rusty grain beetle, Cryptolestes ferrugineus, is challenging, as strongly resistant insects of this species require very high concentrations over lengthy exposure periods (>10 days). Recently, approaches that enhance the efficacy of PH₃ have gained momentum to control this pest, especially co-fumigations. In this study, efficacy of co-fumigating PH₃ with another commercially available fumigant, sulfuryl fluoride (SF), has been evaluated against adults and eggs of two PH₃ -resistant strains of C. ferrugineus. Concentrations of the mixture, representing lower than current application rates of both fumigants, were tested towards its field use.

RESULTS

Co-fumigation of PH₃ with SF was achieved in two patterns: over a continuous exposure period of 168 h simultaneously and sequentially over two periods of 78 h, in which insects were exposed to SF first followed by PH₃ with 12 h aeration in-between. Results of simultaneous fumigations identified two effective co-fumigation rates, SF 185 + PH₃ 168 g hm^-3 and SF 370 + PH₃ 84 g hm^-3 that yielded complete control of adults and eggs. These two rates also were equally effective when they were applied sequentially and produced consistent results. Irrespective of application methods, concentrations of both PH₃ and SF failed individually in achieving complete mortality of either adults or eggs or both.

CONCLUSION

Our results confirmed that a co-fumigation strategy involving half the current standard rate of PH₃ (84 g hm^-3 ) with one-fourth of the current maximal registered rate of SF (370 g hm^-3 ) can provide effective control of strongly PH₃ -resistant C. ferrugineus.

Non-Rodent Genetic Animal Models for Studying Tauopathy: Review of Drosophila, Zebrafish, and C. elegans Models

Tauopathy refers to a group of progressive neurodegenerative diseases, including frontotemporal lobar degeneration and Alzheimer's disease, which correlate with the malfunction of microtubule-associated protein Tau (MAPT) due to abnormal hyperphosphorylation, leading to the formation of intracellular aggregates in the brain. Despite extensive efforts to understand tauopathy and develop an efficient therapy, our knowledge is still far from complete. To find a solution for this group of devastating diseases, several animal models that mimic diverse disease phenotypes of tauopathy have been developed. Rodents are the dominating tauopathy models because of their similarity to humans and established disease lines, as well as experimental approaches. However, powerful genetic animal models using Drosophila, zebrafish, and C. elegans have also been developed for modeling tauopathy and have contributed to understanding the pathophysiology of tauopathy. The success of these models stems from the short lifespans, versatile genetic tools, real-time in-vivo imaging, low maintenance costs, and the capability for high-throughput screening. In this review, we summarize the main findings on mechanisms of tauopathy and discuss the current tauopathy models of these non-rodent genetic animals, highlighting their key advantages and limitations in tauopathy research.

Phenotypic and molecular analyses in rice weevil, Sitophilus oryzae (Linneaus) (Coleoptera: Curculionidae): identification of a super kdr mutation, T929I, conferring resistance to deltamethrin

BACKGROUND

The rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) is a cosmopolitan pest of stored cereal grains and other commodities globally. Infestations caused by S. oryzae makes grains unsuitable for consumption, processing, and export. Deltamethrin, a synthetic pyrethroid insecticide, is widely used in major grain storages in India as a prophylactic treatment to control this pest. However, recurrent use of this insecticide had led to genetic resistance in S. oryzae, questioning its ongoing use at the current recommended concentration.

RESULTS

Dose response analysis of resistant (Delta-R) and susceptible (Lab-S) strains of S. oryzae collected from grain storages across southern India, revealed that Delta-R was 134-fold more resistant than the Lab-S at median lethal concentration (LC₅₀ ). A concentration of 180 ppm over 48 h effectively discriminated 16 resistant field populations from Lab-S with per cent resistance ranging from 8.72% to 75.86%. Exposing all the resistant populations to 1000 ppm over 48 h identified 12 populations with strongly resistant individuals and confirmed the existence of two distinct resistance phenotypes, 'weak' and 'strong' in S. oryzae. Furthermore, sequence analysis of the voltage-gated sodium channel (vgsc) gene in Delta-R identified a single target site mutation, T929I conferring resistance in S. oryzae. CAPS (Cleaved Amplified Polymorphic Sequence) marker analysis of this allele confirmed that frequency of resistance is high (up to 0.96) supporting the results of phenotypic analysis.

CONCLUSION

Both phenotype and molecular marker analyses clearly demonstrated that deltamethrin at 180 and 1000 ppm can be used to discriminate weakly and strongly resistant populations in S. oryzae, respectively. Resistance diagnostics based on the mutation, T929I, supports our phenotypic data and indicates that resistance to deltamethrin in S. oryzae is prevalent in southern parts of India, stressing the need to identify a synergist or suitable alternatives. © 2021 Society of Chemical Industry.

Reinventing the wheel? Reassessing the roles of gene flow, sorting and convergence in repeated evolution

Biologists have long been intrigued by apparently predictable and repetitive evolutionary trajectories inferred across a variety of lineages and systems. In recent years, high-throughput sequencing analyses have started to transform our understanding of such repetitive shifts. While researchers have traditionally categorized such shifts as either "convergent" or "parallel," based on relatedness of the lineages involved, emerging genomic insights provide an opportunity to better describe the actual evolutionary mechanisms at play. A synthesis of recent genomic analyses confirms that convergence is the predominant driver of repetitive evolution among species, whereas repeated sorting of standing variation is the major driver of repeated shifts within species. However, emerging data reveal numerous notable exceptions to these expectations, with recent examples of de novo mutations underpinning convergent shifts among even very closely related lineages, while repetitive sorting processes have occurred among even deeply divergent taxa, sometimes via introgression. A number of very recent analyses have found evidence for both processes occurring on different scales within taxa. We suggest that the relative importance of convergent versus sorting processes depends on the interplay between gene flow among populations, and phylogenetic relatedness of the lineages involved.

Quo Vadis Caenorhabditis elegans Metabolomics-A Review of Current Methods and Applications to Explore Metabolism in the Nematode

Metabolomics and lipidomics recently gained interest in the model organism Caenorhabditis elegans (C. elegans). The fast development, easy cultivation and existing forward and reverse genetic tools make the small nematode an ideal organism for metabolic investigations in development, aging, different disease models, infection, or toxicology research. The conducted type of analysis is strongly depending on the biological question and requires different analytical approaches. Metabolomic analyses in C. elegans have been performed using nuclear magnetic resonance (NMR) spectroscopy, direct infusion mass spectrometry (DI-MS), gas-chromatography mass spectrometry (GC-MS) and liquid chromatography mass spectrometry (LC-MS) or combinations of them. In this review we provide general information on the employed techniques and their advantages and disadvantages in regard to C. elegans metabolomics. Additionally, we reviewed different fields of application, e.g., longevity, starvation, aging, development or metabolism of secondary metabolites such as ascarosides or maradolipids. We also summarised applied bioinformatic tools that recently have been used for the evaluation of metabolomics or lipidomics data from C. elegans. Lastly, we curated metabolites and lipids from the reviewed literature, enabling a prototypic collection which serves as basis for a future C. elegans specific metabolome database.

Variants in the Mitochondrial Genome Sequence of Rhyzopertha dominica (Fabricius) (Coleoptera: Bostrycidae)

Simple Summary

The lesser grain borer damages grains in storage worldwide. The major control method for this beetle is phosphine fumigation, but the increase in resistant populations has led to a loss in phosphine efficacy. Insect mitochondria are the major source of energy, and some phosphine-resistant insects have reduced energy production. Therefore, we want to understand whether changes in the mitochondrial genome may promote phosphine resistance in insects, but we need an accurate mitogenome sequence and annotation. We extracted and sequenced genomic DNA from a laboratory colony of the lesser grain borer and assembled and annotated the mitochondrial genome. The mitochondrial genome sequence was similar in structure to other insect mitochondria and encoded typical mitochondrial genes. We compared our predicted mitochondrial genome sequence to that of another lesser grain borer strain from Jingziguan (China). While there was mostly agreement among the two sequences, the data will be used to determine if key differences may suggest mutations in the two populations related to phosphine control pressure. However, differences also could be the result in different genome sequences and interpretations. The data will be useful as a research tool to examine the expression of mitochondrial genes in phosphine susceptible and -resistant insect populations.

Abstract

The lesser grain borer, Rhyzopertha dominica, is a coleopteran pest of stored grains and is mainly controlled by phosphine fumigation, but the increase in phosphine-resistant populations threatens efficacy. Some phosphine-resistant insects have reduced respiration, and thus studying the mitochondrial genome may provide additional information regarding resistance. Genomic DNA from an inbred laboratory strain of R. dominica was extracted and sequenced with both short (Illumina) and long (Pacific Biosciences) read technologies for whole genome sequence assembly and annotation. Short read sequences were assembled and annotated by open software to identify mitochondrial sequences, and the assembled sequence was manually annotated and verified by long read sequences. The mitochondrial genome sequence for R. dominica had a total length of 15,724 bp and encoded 22 trna genes, 2 rRNA genes, 13 protein coding genes (7 nad subunits, 3 cox, 2 atp, and 1 cytB), flanked by a long control region. We compared our predicted mitochondrial genome to that of another from a R. dominica strain from Jingziguan (China). While there was mostly agreement between the two assemblies, key differences will be further examined to determine if mutations in populations are related to insecticide control pressure, mainly that of phosphine. Differences in sequence data, assembly, and annotation also may result in different genome interpretations.

Detection of Phosphine Resistance in Field Populations of Four Key Stored-Grain Insect Pests in Pakistan

Simple Summary

The resistance of coleopteran stored-product pests to phosphine fumigation is becoming a global phenomenon. However, there is limited literature available and a lack of knowledge on this issue in Pakistan. Thus, in the current study, we estimated the status of phosphine resistance among ten populations of Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae), Sitophilus granarius (L.) (Coleoptera: Curculionidae), Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and Trogoderma granarium Everts (Coleoptera: Dermestidae), originated from different regions of Pakistan. Laboratory-susceptible populations of all insect species were also considered in the tests. Concentration–response bioassays were conducted for each insect species. All of the tested populations of each species were found to be resistant to phosphine. However, they varied with regard to their levels of resistance. Overall, R. dominica exhibited the highest resistance level, followed by T. castaneum, T. granarium and S. granarius. Although phosphine is effective against several stored-product pests, the development of resistance may lead to failures in its application in Pakistan.

Abstract

In Pakistan, the control of stored-product insect pests mainly relies on the use of phosphine gas along with other control tactics. The aim of this study was to determine the level of phosphine resistance among ten differently located populations of the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae), the granary weevil, Sitophilus granarius (L.) (Coleoptera: Curculionidae), the red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and the khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae). Laboratory-susceptible populations of all insect species were also considered in the experiments. Concentration–response bioassays were conducted for each species. All of the tested populations (10 out of 10) of each species were found to be resistant to phosphine, but varied in their level of resistance. Probit analysis estimated LC₅₀ at 2.85, 1.90, 2.54 and 2.01 ppm for laboratory-susceptible populations of R. dominica, S. granarius, T. castaneum and T. granarium, respectively. Against R. dominica, the highest and lowest resistance levels were observed in the Rahim Yar Khan (LC₅₀ at 360.90 ppm) and Rawalpindi (LC₅₀ at 210.98 ppm) populations, respectively. These resistant populations were 126.67- and 74.02-fold more resistant than the laboratory population. The Multan and Lahore populations of S. granarius exhibited the maximum (LC₅₀ at 122.81 ppm) and minimum (LC₅₀ at 45.96 ppm) resistance levels, respectively, i.e., they were 64.63- and 24.18-fold more resistant than the laboratory population. The Layyah population of T. castaneum showed the maximum resistance level (LC₅₀ at 305.89 ppm) while the lowest was observed in the Lahore population (LC₅₀ at 186.52 ppm), corresponding to 120.42- and 73.43-fold more resistant than the laboratory population, respectively. Regarding T. granarium, the Layyah population showed the maximum resistance level (LC₅₀ at 169.99 ppm) while the Lahore population showed the minimum resistance (LC₅₀ at 74.50 ppm), i.e., they were 84.57- and 37.06-fold more resistant than the laboratory population, respectively. Overall, R. dominica presented the highest resistance level, followed by T. castaneum, T. granarium and S. granarius. The current study suggests that the application of phosphine may not be an adequate control strategy for the management of the above tested insect pests in Pakistan.

RNA-seq Analysis Reveals Mitochondrial and Cuticular Protein Genes Are Associated with Phosphine Resistance in the Rusty Grain Beetle (Coleoptera:Laemophloeidae)

The rusty grain beetle, Cryptolestes ferrugineus (Stephens), is a serious pest of stored grain, which has developed high levels of resistance to phosphine. In this study, five geographically distant populations of C. ferrugineus had been collected in China, specifically in granaries where phosphine fumigant is used for pest control, and they showed a high resistance ratio up to 1,907 (LC50 = 21.0 mg/liter). Then, a reference transcriptome was constructed to use as a basis for investigating the molecular mechanisms of phosphine resistance in this species, which consisted of 47,006 unigenes with a mean length of 1,090. Subsequently, the RNA-Seq analysis of individuals from the most susceptible and resistant populations led to the identification of 54 genes that are differentially expressed. GO and KEGG analysis demonstrated that genes associated with mitochondrial and respiration functions were significantly enriched. Also, the 'structural constituent of cuticle' term was annotated in the GO enrichment analysis and further qRT-PCR confirmed that the expression levels of nine cuticular protein genes were significantly increased in the resistant population. In conclusion, we present here a transcriptome-wide overview of gene expression changes between resistant and susceptible populations of C. ferrugineus, and this in turn documents that mitochondria and cuticular protein genes may play together a crucial role in phosphine resistance. Further gene function analysis should enable the provision of advice to expedite resistance management decisions.

Unique genetic variants in dihydrolipoamide dehydrogenase (dld) gene confer strong resistance to phosphine in the rusty grain beetle, Cryptolestes ferrugineus (Stephens)

The rusty grain beetle, Cryptolestes ferrugineus, a major pest of stored commodities, has developed very high levels (>1000×) of resistance to the fumigant phosphine. Resistance in this species is remarkably stronger than reported in any other stored product pests demanding the need to understand the molecular basis of this trait. Previous genetic studies in other grain insect pests identified specific variants in two major genes, rph1 and rph2 in conferring the strong resistance trait. However, in C. ferrugineus, although the gene, rph1 was identified as cytochrome-b5-fatty acid desaturase, the rph2 gene has not been reported so far. We tested the candidate gene for rph2, dihydrolipoamide dehydrogenase (dld) using the recently published transcriptome of C. ferrugineus and identified three variants, L73N and A355G + D360H, a haplotype, conferring resistance in this species. Our sequence analysis in resistant strain and phosphine selected resistant survivors indicates that these variants occur either alone as a homozygote or a mixture of heterozygotes (i.e complex heterozygotes) both conferring strong resistance. We also found that one of the three variants, possibly L73N expressing "dominant" trait at low frequency in resistant insects. Comparison of dld sequences between Australian and Chinese resistant strain of this species confirmed that the identified variants are highly conserved. Our fitness analysis indicated that resistant insects may not incur significant biological costs in the absence of phosphine selection for 19 generations. Thus, we propose that the observed high levels of resistance in C. ferrugineus could be primarily due to the characteristics of three unique variants, L73N and A355G + D360H within dld.

Identification and functional analysis of cytochrome P450 CYP346 family genes associated with phosphine resistance in Tribolium castaneum

Resistance to phosphine fumigation has been frequently reported in insect pests of stored products and remains one of the obstacles in controlling these pests, including Tribolium castaneum. In this study, six field populations of T. castaneum were collected from different localities in China. Bioassay data showed that SZ population was strongly resistant to phosphine, followed by moderate-resistance populations WL and SF and three susceptible populations JX, YN, and ML. In addition, synergism assays showed that piperonyl butoxide significantly increased the toxicity of phosphine in resistant population SZ. Furthermore, CYP346B subfamily genes, CYP346B1, CYP346B2, and CYP346B3, were significantly overexpressed in resistant populations. Expression of CYP346B1, CYP346B2, and CYP346B3 were significantly upregulated following exposure to phosphine. RNAi assays showed that depletions on the expression levels of CYP346B1, CYP346B2, and CYP346B3 resulted in an increase of susceptibility to phosphine in T. castaneum, respectively. Our data demonstrated that CYP346B subfamily genes in T. castaneum were associated with the resistance of phosphine. Moreover, the study also increased our understanding of the molecular basis of phosphine resistance in stored pest insects.

The Gene Introgression Approach and the Potential Cost of Genes that Confer Strong Phosphine Resistance in Red Flour Beetle (Coleoptera: Tenebrionidae)

Resistance in pest insects to the grain fumigant phosphine (PH3) poses a threat to trade and food security. The possible pleiotropic effects of PH3 resistance on development and reproduction were investigated in the red flour beetle, Tribolium castaneum (Herbst), by introgressing two genes known to be major contributors to strong resistance (tc_rph1 and tc_rph2) into a susceptible background. The tc_rph2 allele was the G135S variant, whereas the identity of tc_rph1 allele was unknown but could have been one of the three known variants (L119W, V123F, or S349G). The introgressed resistant strain was 288× more resistant than the susceptible strain, based on mortality after a 20 h fumigation with PH3. Molecular screening confirmed that the introgressed strain was homozygous for the resistance genes, but was otherwise indistinguishable from the susceptible strain based on screening with 12 neutral DNA markers. We found no differences of consequence in developmental time between the susceptible and introgressed resistant strains. Similarly, the number of F1 adults produced by these strains was more or less equal, as was the weight of individual F1 adults. The conclusions remained the same regardless of whether the experiments were conducted on a flour-based medium or wheat. Thus, we found no evidence that being fully strongly PH3 resistant (i.e., homozygous for tc_rph1 and tc_rph2) has major consequences in terms of development or reproduction in T. castaneum.

Tribolium castaneum: A Model for Investigating the Mode of Action of Insecticides and Mechanisms of Resistance

The red flour beetle, Tribolium castaneum, is a worldwide insect pest of stored products, particularly food grains, and a powerful model organism for developmental, physiological and applied entomological research on coleopteran species. Among coleopterans, T. castaneum has the most fully sequenced and annotated genome and consequently provides the most advanced genetic model of a coleopteran pest. The beetle is also easy to culture and has a short generation time. Research on this beetle is further assisted by the availability of expressed sequence tags and transcriptomic data. Most importantly, it exhibits a very robust response to systemic RNA interference (RNAi), and a database of RNAi phenotypes (iBeetle) is available. Finally, classical transposonbased techniques together with CRISPR/Cas-mediated gene knockout and genome editing allow the creation of transgenic lines. As T. castaneum develops resistance rapidly to many classes of insecticides including organophosphates, methyl carbamates, pyrethroids, neonicotinoids and insect growth regulators such as chitin synthesis inhibitors, it is further a suitable test system for studying resistance mechanisms. In this review, we will summarize recent advances in research focusing on the mode of action of insecticides and mechanisms of resistance identified using T. castaneum as a pest model.

Preliminary Study on the Differences in Hydrocarbons Between Phosphine-Susceptible and -Resistant Strains of Rhyzopertha dominica (Fabricius) and Tribolium castaneum (Herbst) Using Direct Immersion Solid-Phase Microextraction Coupled with GC-MS

Phosphine resistance is a worldwide issue threatening the grain industry. The cuticles of insects are covered with a layer of lipids, which protect insect bodies from the harmful effects of pesticides. The main components of the cuticular lipids are hydrocarbon compounds. In this research, phosphine-resistant and -susceptible strains of two main stored-grain insects, T. castaneum and R. dominica, were tested to determine the possible role of their cuticular hydrocarbons in phosphine resistance. Direct immersion solid-phase microextraction followed by gas chromatography-mass spectrometry (GC-MS) was applied to extract and analyze the cuticular hydrocarbons. The results showed significant differences between the resistant and susceptible strains regarding the cuticular hydrocarbons that were investigated. The resistant insects of both species contained higher amounts than the susceptible insects for the majority of the hydrocarbons, sixteen from cuticular extraction and nineteen from the homogenized body extraction for T. castaneum and eighteen from cuticular extraction and twenty-one from the homogenized body extraction for R. dominica. 3-methylnonacosane and 2-methylheptacosane had the highest significant difference between the susceptible and resistant strains of T. castaneum from the cuticle and the homogenized body, respectively. Unknown5 from the cuticle and 3-methylhentriacontane from the homogenized body recorded the highest significant differences in R. dominica. The higher hydrocarbon content is a key factor in eliminating phosphine from entering resistant insect bodies, acting as a barrier between insects and the surrounding phosphine environment.

Oxygen and Arsenite Synergize Phosphine Toxicity by Distinct Mechanisms

Phosphine is the only fumigant approved globally for general use to control insect pests in stored grain. Due to the emergence of resistance among insect pests and the lack of suitable alternative fumigants, we are investigating ways to synergistically enhance phosphine toxicity, by studying the mechanism of action of known synergists, such as oxygen, temperature, and arsenite. Under normoxia, exposure of the model organism Caenorhabditis elegans for 24 h at 20°C to 70 ppm phosphine resulted in 10% mortality, but nearly 100% mortality if the oxygen concentration was increased to 80%. In wild-type C. elegans, toxicity of phosphine was negatively affected by a decrease in temperature to 15°C and positively affected by an increase in temperature to 25°C. The dld-1(wr4) strain of C. elegans is resistant to phosphine due to a mutation in the dihydrolipoamide dehydrogenase gene. It also exhibits increased mortality that is dependent on hyperoxia, when exposed to 70 ppm phosphine at 20°C. As with the wild-type strain, mortality decreased when exposure was carried out at 15°C. At 25°C, however, the strain was completely resistant to the phosphine exposure at all oxygen concentrations. Arsenite is also a synergist of phosphine toxicity, but only in the dld-1(wr4)-mutant strain. Thus, exposure to 4 mM arsenite resulted in 50% mortality, which increased to 89% mortality when 70 ppm phosphine and 4 mM arsenite were combined. In stark contrast, 70 ppm phosphine rendered 4 mM arsenite nontoxic to wild-type C. elegans. These results reveal two synergists with distinct modes of action, one of which targets individuals that carry a phosphine resistance allele in the dihydrolipoamide dehydrogenase gene.

Resistance to the Fumigant Phosphine and Its Management in Insect Pests of Stored Products: A Global Perspective

Development of resistance in major grain insect pest species to the key fumigant phosphine (hydrogen phosphide) across the globe has put the viability and sustainability of phosphine in jeopardy. The resistance problem has been aggravated over the past two decades, due mostly to the lack of suitable alternatives matching the major attributes of phosphine, including its low price, ease of application, proven effectiveness against a broad pest spectrum, compatibility with most storage conditions, and international acceptance as a residue-free treatment. In this review, we critically analyze the published literature in the area of phosphine resistance with special emphasis on the methods available for detection of resistance, the genetic basis of resistance development, key management strategies, and research gaps that need to be addressed.

Minimization of energy transduction confers resistance to phosphine in the rice weevil, Sitophilus oryzae

Infestation of phosphine (PH₃) resistant insects threatens global grain reserves. PH₃ fumigation controls rice weevil (Sitophilus oryzae) but not highly resistant insect pests. Here, we investigated naturally occurring strains of S. oryzae that were moderately resistant (MR), strongly resistant (SR), or susceptible (wild-type; WT) to PH₃ using global proteome analysis and mitochondrial DNA sequencing. Both PH₃ resistant (PH₃-R) strains exhibited higher susceptibility to ethyl formate-mediated inhibition of cytochrome c oxidase than the WT strain, whereas the disinfectant PH₃ concentration time of the SR strain was much longer than that of the MR strain. Unlike the MR strain, which showed altered expression levels of genes encoding metabolic enzymes involved in catabolic pathways that minimize metabolic burden, the SR strain showed changes in the mitochondrial respiratory chain. Our results suggest that the acquisition of strong PH₃ resistance necessitates the avoidance of oxidative phosphorylation through the accumulation of a few non-synonymous mutations in mitochondrial genes encoding complex I subunits as well as nuclear genes encoding dihydrolipoamide dehydrogenase, concomitant with metabolic reprogramming, a recognized hallmark of cancer metabolism. Taken together, our data suggest that reprogrammed metabolism represents a survival strategy of SR insect pests for the compensation of minimized energy transduction under anoxic conditions. Therefore, understanding the resistance mechanism of PH₃-R strains will support the development of new strategies to control insect pests.

Evaluation of Knockdown Bioassay Methods to Assess Phosphine Resistance in the Red Flour Beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

Resistance to the fumigant phosphine in Tribolium castaneum occurs worldwide. This study evaluated tests based on adult knockdown time, the time for a beetle to become immobile, when exposed to a high concentration of phosphine. We recorded knockdown times of beetles that remained completely still for 30 s when exposed to 3000 ppm of phosphine in a large, gas-tight glass tube. Beetles were used from 12 populations, of which six were 'susceptible' to phosphine, three were 'weakly resistant', and three were 'strongly resistant'. Knockdown times were determined for single beetles, as well as for groups of ten beetles for which the time to knockdown for either five beetles (KT50) or ten beetles (KT100) were recorded. Similar knockdown times occurred across susceptible and resistant populations. However, the KT100 tests generated conservative times for diagnosing strong vs. weak resistance. The strong resistant populations were all over 100 min with KT100, compared to 60 min or less for susceptible and weak resistant populations. Special tests on single beetles revealed higher knockdown times in insects that were deliberately disturbed compared to those without any disturbances. Work reported here suggests a knockdown test conducted on beetles in a matter of minutes or hours could help classify phosphine resistance status prior to decisions on phosphine fumigation.

Attenuation of radiation toxicity by the phosphine resistance factor dihydrolipoamide dehydrogenase (DLD)

Phosphine gas is an excellent fumigant for disinfesting stored grain of insect pests, but heavy reliance on phosphine has led to resistance in grain pests that threatens its efficacy. Phosphine-resistance was previously reported to be mediated by the enzyme DLD. Here we explore the relationship between phosphine toxicity and genotoxic treatments with the goal of understanding how phosphine works. Specifically, we utilized mutant lines either sensitive or resistant to phosphine, gamma irradiation or UV exposure. The phosphine-resistance mutation in the enzyme of energy metabolism, dihydrolipoamide dehydrogenase exhibited cross-resistance to UV and ionizing radiation. Two radiation-sensitive mutants that are defective in DNA repair as well as a mutant that is defective in the activation of the DAF-16 stress response transcription factor all exhibit sensitivity to phosphine that exceeds the sensitivity of the wild type control. A radiation resistance mutation in cep-1, the p53 orthologue, that is deficient in double strand break repair of DNA and is also deficient in apoptosis causes radiation-resistance results but sensitivity toward phosphine.

A high-throughput system used to determine frequency and distribution of phosphine resistance across large geographical regions

BACKGROUND

Next-generation sequencing can enable genetic surveys of large numbers of individuals. We developed a genotyping-by-sequencing assay for detecting strong phosphine resistance alleles in the dihydrolipoamide dehydrogenase (dld) gene of Rhyzopertha dominica populations. The assay can estimate the distribution and frequency of resistance variants in thousands of individual insects in a single run.

RESULTS

We analysed 1435 individual insects collected over a 1-year period from 59 grain-storage sites including farms (n = 29) and central storages (n = 30) across eastern Australia. Resistance alleles were detected in 49% of samples, 38% of farms and 60% of central storages. Although multiple alleles were detected, only two resistance variants (P49S and K142E) were widespread and each appeared to have a distinct but overlapping geographical distribution.

CONCLUSION

The type of structure in which the grain is stored had a strong effect on resistance allele frequency. We observed higher frequencies of resistance alleles in bunker storages at central sites compared with other storage types. This contributed to the higher frequencies of resistance alleles in bulk-handling facilities relative to farms. The discovery of a storage structure that predisposes insects to resistance highlights the utility of our high-throughput assay system for improvement of phosphine resistance management practices. © 2018 Society of Chemical Industry.

Potential of Co-Fumigation with Phosphine (PH3) and Sulfuryl Fluoride (SO2F2) for the Management of Strongly Phosphine-Resistant Insect Pests of Stored Grain

Resistance to phosphine (PH3) in key insect pests of stored grain is increasing, with a requirement for maintaining a dose as high as 1 mg l-1 for 14 d for effective fumigation, which is difficult to achieve under most commercial storage conditions. There is no suitable replacement for PH3, as most of the available alternatives suffer from specific weaknesses, creating an urgent need to increase the efficacy of this fumigant. One such possibility is co-fumigation of PH3 with another fumigant, sulfuryl fluoride (SO2F2-SF), with the goal of decreasing the time required for a successful fumigation. In this study, adult of two PH3-resistant strains in each of four key grain insect pests, Rhyzopertha dominica, (F.) (Coleoptera: Bostrichidae) Tribolium castaneum Herbst (Coleoptera: Tenebrionidae), Sitophilus oryzae (L.) (Coleoptera: Curculionidae), and Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae) were fumigated with PH3 and SF individually as well as in combinations at 25°C over 48 h. Mortality responses in each species were subjected to probit analysis to determine the LC50 and LC99.9 of PH3, SF, and PH3 + SF. Co-fumigation of PH3 with SF resulted in a 50% reduction of the PH3 concentration required to achieve 99.9% mortality in two pest species. For example, the PH3 + SF mixture, reduced the amount of PH3 required from 14.2-14.5 to 5.6-6.36 mg l-1 and from 2.71-5.03 to 0.93-1.2 mg l-1, respectively, for C. ferrugineus and S. oryzae. The overall mortality response to the PH3 + SF mixture followed an "additive model" suggesting that mutual enhancement in toxicity can be achieved with this mixture specifically to control PH3-resistant insects.

Enantioselective Coupling of Dienes and Phosphine Oxides

We report a Pd-catalyzed intermolecular hydrophosphinylation of 1,3-dienes to afford chiral allylic phosphine oxides. Commodity dienes and air stable phosphine oxides couple to generate organophosphorus building blocks with high enantio- and regiocontrol. This method constitutes the first asymmetric hydrophosphinylation of dienes.

Identification and expression profiles of twenty-six glutathione S-transferase genes from rice weevil, Sitophilus oryzae (Coleoptera: Curculionidae)

The rice weevil, Sitophilus oryzae, is one of the most destructive pests in stored cereals products. In this study, 26 cDNAs encoding glutathione S-transferases (GSTs) were sequenced and characterized in S. oryzae. Phylogenetic analysis displayed the categorization of 26 GSTs into six different cytosolic classes, including two in the delta, twelve in epsilon, three in omega, six in sigma, two in theta, and one in zeta class. RT-qPCR assay illustrated that the relative expression of ten GST genes was significantly higher in adult stages than in larval and pupal developmental stages. Tissue-specific expression analysis revealed that the SoGSTe5, SoGSTe7, SoGSTe12, and SoGSTz1 were up-regulated in the midgut, SoGSTe2, SoGSTe6, and SoGSTs2 were up-regulated in the fat body, and three GSTs (SoGSTd1, SoGSTd2 and SoGSTe4) were up-regulated in Malpighian tubules. RT-qPCR indicated that five GST genes were over expressed after exposure to phosphine at various times and concentrations. The increase in GST gene expressions after phosphine exposure in S. oryzae may lead to an improved tolerance for fumigations and xenobiotics.

Toxicity of Chlorine Dioxide Gas to Phosphine-Susceptible and -Resistant Adults of Five Stored-Product Insect Species: Influence of Temperature and Food During Gas Exposure

Adults of phosphine-susceptible and -resistant strains of the red flour beetle, Tribolium castaneum (Herbst); lesser grain borer, Rhyzopertha dominica (F.); sawtoothed grain beetle, Oryzaephilus surinamensis (L.); maize weevil, Sitophilus zeamais Motschulsky; and rice weevil, Sitophilus oryzae (L.), were exposed for 2-12 h to a chlorine dioxide gas concentration of 1.40 g/m3 (520 ppm) in an outdoor trailer during July and October of 2015. The mean ± SE temperatures in July and October were 32.8 ± 0.5°C and 24.8 ± 0.6°C, respectively. In July, complete mortality after 5 d was achieved for all species and strains in vials with wheat after a 4- or 8-h exposure; in October, a longer exposure time was needed for complete mortality of insects in vials with wheat. Chlorine dioxide was more toxic to all insect species and strains at warmer than cooler temperature and in vials without wheat than those with wheat. Both phosphine-resistant and -susceptible strains were equally susceptible to chlorine dioxide. The presence of wheat resulted in delayed mortality of insects because of reaction of chlorine dioxide with active sites on kernels. Progeny production 8 wk after chlorine dioxide exposure showed a significant reduction (72-100%), compared with that in control vials for strains of R. dominica, S. zeamais, and S. oryzae. There was no progeny production in control and treatment vials for T. castaneum and O. surinamensis, as these species require dockage. Chlorine dioxide is a potential fumigant to control phosphine-resistant strains of the five stored-product insect species.

Dihydrolipoamide dehydrogenase suppression induces human tau phosphorylation by increasing whole body glucose levels in a C. elegans model of Alzheimer's Disease

The microtubule associated tau protein becomes hyperphosphorylated in Alzheimer's disease (AD). While hyperphosphorylation promotes neurodegeneration, the cause and consequences of this abnormal modification are poorly understood. As impaired energy metabolism is an important hallmark of AD progression, we tested whether it could trigger phosphorylation of human tau protein in a transgenic Caenorhabditis elegans model of AD. We found that inhibition of a mitochondrial enzyme of energy metabolism, dihydrolipoamide dehydrogenase (DLD) results in elevated whole-body glucose levels as well as increased phosphorylation of tau. Hyperglycemia and tau phosphorylation were induced by either RNAi suppression of the dld-1 gene or by inhibition of the DLD enzyme by the inhibitor, 2-methoxyindole-2-carboxylic acid (MICA). Although the calcium ionophore A23187 could reduce tau phosphorylation induced by either chemical or genetic suppression of DLD, it was unable to reduce tau phosphorylation induced by hyperglycemia. While inhibition of the dld-1 gene or treatment with MICA partially reversed the inhibition of acetylcholine neurotransmission by tau, neither treatment affected tau inhibited mobility. Conclusively, any abnormalities in energy metabolism were found to significantly affect the AD disease pathology.

Variant Linkage Analysis Using de Novo Transcriptome Sequencing Identifies a Conserved Phosphine Resistance Gene in Insects

Next-generation sequencing methods enable identification of the genetic basis of traits in species that have no prior genomic information available. The combination of next-generation sequencing, variant analysis, and linkage is a powerful way of identifying candidate genes for a trait of interest. Here, we used a comparative transcriptomics [RNA sequencing (RNAseq)] and genetic linkage analysis approach to identify the rph1 gene. rph1 variants are responsible for resistance to the fumigant phosphine (PH₃) that is used to control insect pests of stored grain. In each of the four major species of pest insect of grain we have investigated, there are two major resistance genes, rph1 and rph2, which interact synergistically to produce strongly phosphine-resistant insects. Using RNAseq and genetic linkage analyses, we identified candidate resistance (rph1) genes in phosphine-resistant strains of three species: Rhyzopertha dominica (129 candidates), Sitophilus oryzae (206 candidates), and Cryptolestes ferrugineus (645 candidates). We then compared these candidate genes to 17 candidate resistance genes previously mapped in Tribolium castaneum and found only one orthologous gene, a cytochrome b5 fatty acid desaturase (Cyt-b5-r), to be associated with the rph1 locus in all four species. This gene had either missense amino acid substitutions and/or insertion/deletions/frameshift variants in each of 18 phosphine-resistant strains that were not observed in the susceptible strains of the four species. We propose a model of phosphine action and resistance in which phosphine induces lipid peroxidation through reactive oxygen species generated by dihydrolipoamide dehydrogenase, whereas disruption of Cyt-b5-r in resistant insects decreases the polyunsaturated fatty acid content of membranes, thereby limiting the potential for lipid peroxidation.