Multiple resistance to pirimiphos-methyl and bifenthrin in Tribolium castaneum involves the activity of lipases, esterases, and laccase2

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.

Involvement of Laccase2 in Cuticle Sclerotization of the Whitefly, Bemisia tabaci Middle East–Asia Minor 1

Cuticle sclerotization is critical for insect survival. Laccase2 (Lac2) is a phenol oxidase that plays a key role in cuticle formation and pigmentation in a variety of insects. However, the function of Lac2 in whitefly, Bemisia tabaci, remains unclear. In this study, we identified a BtLac2 gene in B. tabaci MEAM1 and found that BtLac2 was expressed in all stages. It was highly expressed in the egg stage, followed by nymph and adult. Moreover, the expression of BtLac2 was higher in the cuticle than in other tissues. Knockdown of BtLac2 in nymphs produced thinner and fragile cuticles, which significantly increased the mortality rate, extended the development duration of nymphs, and decreased the emergence rate of adults. This result demonstrates that BtLac2 plays an important role in the cuticle hardening of B. tabaci and suggests a potential management strategy using RNAi to knock down BtLac2 expression.

RNAi-Mediated Suppression of Laccase2 Impairs Cuticle Tanning and Molting in the Cotton Boll Weevil (Anthonomus grandis)

The cotton boll weevil, Anthonomus grandis, is the most economically important pest of cotton in Brazil. Pest management programs focused on A. grandis are based mostly on the use of chemical insecticides, which may cause serious ecological impacts. Furthermore, A. grandis has developed resistance to some insecticides after their long-term use. Therefore, alternative control approaches that are more sustainable and have reduced environmental impacts are highly desirable to protect cotton crops from this destructive pest. RNA interference (RNAi) is a valuable reverse genetics tool for the investigation of gene function and has been explored for the development of strategies to control agricultural insect pests. This study aimed to evaluate the biological role of the Laccase2 (AgraLac2) gene in A. grandis and its potential as an RNAi target for the control of this insect pest. We found that AgraLac2 is expressed throughout the development of A. grandis with significantly higher expression in pupal and adult developmental stages. In addition, the immunolocalization of the AgraLac2 protein in third-instar larvae using specific antibodies revealed that AgraLac2 is distributed throughout the epithelial tissue, the cuticle and the tracheal system. We also verified that the knockdown of AgraLac2 in A. grandis resulted in an altered cuticle tanning process, molting defects and arrested development. Remarkably, insects injected with dsAgraLac2 exhibited defects in cuticle hardening and pigmentation. As a consequence, the development of dsAgraLac2-treated insects was compromised, and in cases of severe phenotypic defects, the insects subsequently died. On the contrary, insects subjected to control treatments did not show any visible phenotypic defects in cuticle formation and successfully molted to the pupal and adult stages. Taken together, our data indicate that AgraLac2 is involved in the cuticle tanning process in A. grandis and may be a promising target for the development of RNAi-based technologies.

Developmental changes in larvae of the oriental latrine fly, Chrysomya megacephala, exposed to deltamethrin

The Chrysomya megacephala (Fabricius, 1794) (Diptera: Calliphoridae) has a worldwide distribution and is considered of medical and veterinary importance. Its synanthropic behavior combined with the visitation of decaying organic matter allows it to mechanically transmit pathogens to human and animals. Pyrethroids are widely used pesticides, effective in low doses against a broad spectrum of insects. Our objective was to evaluate the action of K-Othrine® on the post-embryonic development of C. megacephala. First instar larvae were exposed to different concentrations of K-Othrine® (3.75 × 10^-5 g a.i/mL (low), 7.5 × 10^-5 g a.i/mL (intermediate low), 1.125 × 10^-4 g a.i/mL (intermediate), 1.5 × 10^-4 g a.i/mL (intermediate high), and 2.5 × 10^-4 g a.i/mL (high)) mixed into artificial diets and a control with no pesticide. We measured weight, time of development, sex ratio, larval and pupal mortality, and esterase patterns. The mean weight values at intermediate low and intermediate concentrations were significantly lower compared with the control; additionally, the development time was delayed compared with the control. The larval and pupal mortality differed among the treatments. The larvae had six esterase regions, which altered its relative activity at 48 h and 72 h after insecticide ingestion. Further studies must investigate the effects of different insecticides on the post-embryonic development of C. megacephala.

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.

Vgsc-interacting proteins are genetically associated with pyrethroid resistance in Aedes aegypti

Association mapping of factors that condition pyrethroid resistance in Aedes aegypti has consistently identified genes in multiple functional groups. Toward better understanding of the mechanisms involved, we examined high throughput sequencing data (HTS) from two Aedes aegypti aegypti collections from Merida, Yucatan, Mexico treated with either permethrin or deltamethrin. Exome capture enrichment for coding regions and the AaegL5 annotation were used to identify genes statistically associated with resistance. The frequencies of single nucleotide polymorphisms (SNPs) were compared between resistant and susceptible mosquito pools using a contingency χ² analysis. The -log₁₀(χ²p value) was calculated at each SNP site, with a weighted average determined from all sites in each gene. Genes with -log₁₀(χ²p value) ≥ 4.0 and present among all 3 treatment groups were subjected to gene set enrichment analysis (GSEA). We found that several functional groups were enriched compared to all coding genes. These categories were transport, signal transduction and metabolism, in order from highest to lowest statistical significance. Strikingly, 21 genes with demonstrated association to synaptic function were identified. In the high association group (n = 1,053 genes), several genes were identified that also genetically or physically interact with the voltage-gated sodium channel (VGSC). These genes were eg., CHARLATAN (CHL), a transcriptional regulator, several ankyrin-domain proteins, PUMILIO (PUM), a translational repressor, and NEDD4 (E3 ubiquitin-protein ligase). There were 13 genes that ranked among the top 10%: these included VGSC; CINGULIN, a predicted neuronal gap junction protein, and the aedine ortholog of NERVY (NVY), a transcriptional regulator. Silencing of CHL and NVY followed by standard permethrin bottle bioassays validated their association with permethrin resistance. Importantly, VGSC levels were also reduced about 50% in chl- or nvy-dsRNA treated mosquitoes. These results are consistent with the contribution of a variety of neuronal pathways to pyrethroid resistance in Ae. aegypti.

Cap n collar transcription factor regulates multiple genes coding for proteins involved in insecticide detoxification in the red flour beetle, Tribolium castaneum

In invertebrates, a heterodimer of xenobiotic transcription factors, cap n collar C isoform (CncC) and muscle aponeurosis fibromatosis (Maf) mediate cellular defense. In insects, these proteins regulate expression of genes involved in insecticide detoxification. In the current study, we performed sequencing of cDNA copied from RNA isolated from Tribolium castaneum pyrethroid resistant strain (QTC279) beetles injected with CncC or green fluorescence protein (GFP, control) dsRNA. Differential expression analysis of sequences identified 662 genes that showed a decrease and 91 genes that showed an increase in expression (p value ≤ 0.01 and log2 fold change of ≥ 1.5) in CncC knockdown insects when compared to their expression in control insects. We selected a subset of 27 downregulated genes and verified their differential expression using qRT-PCR. This subset of 27 genes included 21 genes with a predicted function in xenobiotic detoxification. RNAi and insecticide bioassays were employed to study the function of six of these genes coding for CYP4G7, CYP4G14, GST-1 and four ABC transporters, ABCA-UB, ABCA-A1 and ABCA-A1L and ABCA-9B involved in all three phases of insecticide detoxification. These data suggest that CncC regulates genes coding for proteins involved in detoxification of insecticides.