Insecticides Development of Safer and More Effective Technologies
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These databases are invaluable resources, providing entomologists with a vast array of genomic data, both structural and functional, and thus assists with comparative studies on global genome architecture and evolution, in addition to the identification of genes associated with host preference, insecticide resistance, or immunity. The advent of the post-genomics era, therefore, has been contributing towards the improvement of global health by i aiding in the identification of new targets for novel insecticides and vaccines in the combat against vector-borne diseases, and ii improving the understanding of specific genes and mechanisms underlying pathogenicity of microbes and insecticide resistance.
Data mining and bioinformatics tools have become critical in the analysis and interpretation of the biological datasets attained from large-scale sequencing efforts. These tools have been widely utilised for targeted therapeutics in the human population, more specifically to identify i the genetic variation between populations and ii biomarkers for pathogenesis and progression of particular disease Mansmann While the myriad of advantages to this approach is recognised across applications, with insecticide development benefiting in a similar manner to human drug development, there is limited exploitation.
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Insect genomics, however, has enabled the discovery of insecticide targets via functional characterisation of proteins that are essential to survival or other biological functions. The introduction of draft insect genomes with the respective gene annotations has played a crucial role in the mining of protein targets for insecticides. Conventional neuroactive insecticides have targeted acetylcholinesterase, nicotinic acetylcholine receptors and voltage-gated ion channels, but G-protein coupled receptors GPCRs , kinases, ATPases, synthases and carboxylesterases have also demonstrated potential as novel insecticide targets.
Comparative genomics has aided in the detection of gene targets through homologous sequences, and as a model organism, knowledge surrounding Drosophila melanogaster is the most robust, providing a good source of comparison for gene prediction and annotation. Due to its notoriety as an agricultural pest, Wang et al. De novo genome assembly resulted in a 6. A total of genes were described as potential insecticide gene targets; were noted as either a GPCR or ion channel, while the other , including synthases, carboxylesterases, ATPases and kinases, were identified after in silico screening for orthologues of lethal genes in Caenorhabditis elegans and D.
Similarly, Karatolos et al. After classifying the function of the putative proteins, transcripts encoding traditional insecticide targets, including the acetylcholinesterase enzyme, nicotinic acetylcholine receptor subunits, acetyl-CoA carboxylase, voltage-gated sodium channel, g-aminobutyric acid receptor, glutamate-gated chloride channel, and ryanodine receptor, were identified Karatolos et al. Additionally, Nene shed light on the genome of Aedes aegypti by whole-genome shotgun sequencing purified larval DNA.
This played a key role in the functional classification of putative proteins through computational analysis of domains, secretion signaling sequence and transmembrane motifs, and more significantly, identification of transcripts encoding GPCRS that are potential insecticide targets Nene et al. Comparative genomics has also aided in the elucidation of the molecular mechanisms underlying insecticide resistance, including metabolic resistance and target-site resistance.
Due to their involvement in insecticide resistance, these proteins have been identified in many insect genomes. The An. The crop pests, T. The ABC transporter family has also been annotated in several arthropod genomes, including 73, 52, 49, and 41 genes being detected in T. The analysis of copy number variation and identification of polymorphisms has aided in the discovery of gene amplification and overtranscription of detoxification genes in resistant insect populations. In highly insecticide-resistant Culex pipiens quinquefasciatus , the copy number of carboxylesterase enzymes esta2 and estb2 is increased by up to 80 fold.
Elevated level and activity of cytochrome P is often found in insecticide-resistant mosquitoes as a result of constitutive overexpression regulated by cis- and trans-acting factors. The upregulation of genes within the Cyp6 P family, more specifically, has been implicated in the insecticide-resistant mosquitoes. Elevated transcripts of these genes have been observed in pyrethroid-resistant strains of An.
Similar gene amplification and upregulation in transcription activity is observed for GST enzymes; a specific GST involved in dehydrochlorination has been detected to function at a higher rate in DDT-resistant Ae. The Meligethes aeneus transcriptome, the European pollen beetle responsible for the decline of rapeseed crop, illustrates a comparable pattern. After mapping RNAseq reads and conducting a GO analysis of susceptible and pyrethroid-resistant beetle populations, Zimmer et al.
Insecticides - Development of Safer and More Effective Technologies
Target-site resistance, on the other hand, is characterised by polymorphisms within the target region, functionally changing the protein and its ability to induce a mode of action. Typically, organophosphates and methylcarbamate insecticides target the serine residue within the active site gorge of acetylcholinesterase, preventing acetylcholine hydrolysis Hemingway et al.
However, through comparative sequencing of susceptible and resistant mosquitoes, specific amino acid substitutions within the coding region of the AChE1 catalytic site have been described in insecticide-resistant mosquitoes. The mutations are found to constrict the opening of the active site, restricting access to the catalytic residues. More specifically, a glycine to serine substitution has been a consistent find in several mosquito species, including An.
The current acetylcholinesterase target is ubiquitous in all animals, causing high off-target toxicity. However, the availability of AChE sequences for multiple species including insects, birds, amphibians and mammals has enabled the identification of a potential target specific to insects. Sequence alignments of different species have revealed a cysteine residue conserved in 22 insects, primarily those that transmit human diseases such as mosquitoes Ae. Pyrethroids, a common insecticide class, act by binding to voltage-gated sodium channels and altering its gating kinetics.
Functional changes in the sodium channel have led to the development of knockdown resistance kdr in multiple insect species. First described in the model system of the house fly, Musca domestica L , the molecular mechanism underlying kdr has been well-investigated in disease vectors and agricultural pests. Analysis of sequence alignments of susceptible and kdr houseflies highlighted two specific missense point mutations, including a leucine to phenylalanine at residue of the sodium channel gene. Other species were examined for a similar gene mutation; the common kdr mutation has been identified in seven species, including West African pyrethroid-resistant An.
Characterisation of the sodium channel has led to the identification of over 20 unique polymorphisms of pyrethroid-resistant species. The leucine is substituted with a serine in the east African disease vector populations of An. Recently, investigations for new insecticide modes of action have surged, and RNA interference RNAi has facilitated in narrowing the search to specific protein targets by aiding in the functional and behavioural characterisation of genes.
RNAi mediated gene silencing is often completed via microinjection of double-stranded RNA or small interfering RNA, but uptake through topical application and ingestion have also demonstrated to be worthwhile. After targeting the Ae. Similar results were observed when the nanoparticles were used to target the transcription factor, singled-minded Zhang et al.
A RNAi screen was conducted for T. The iBeetle database was mined, and dsRNA described to elicit mortality post-injection was selected for further investigation. A titration curve of selected dsRNA was conducted to identify and validate the optimal response.
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Furthermore, similar experiments were implemented for the orthologues of RNAi target genes reported to cause high mortality in the western corn rootworm. Overall, the group identified 11 novel targets which displayed fatality. While premature mortality via RNAi targeting is a desired result, the molecular tool can be applied to disrupt other key functions. For example, the Ae. Two specific Ae. While RNAi has demonstrated its utility in identification of control targets, this technique has also been recently tested as an innovative biological control strategy.
In lieu of genetically modifying mosquitoes with the sterile insect technique, Whyard established an alternate method for mosquito sterilisation and sex sorting via RNAi. Ingestion of dsRNAs targeting testis and female sex determination genes by Ae. This RNAi methodology has also been effective in the agricultural realm. Baum identified 14 genes within Diabrotica virgifera that are essential for its survival, and developed a transgenic corn expressing dsRNA targeting one specific gene, the midgut enzyme vacuolar ATPase.
Results indicate that the dsRNA expression has a protective effect on the corn; a reduced infestation of the agricultural pest is the outcome of the transgenic corn line. Similarly, Mao fed H. Since this gene appears to play a role in resistance to gossypol, a natural insecticide produced by the cotton plant, gene silencing in H.
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While this RNAi strategy has not been introduced into field practice as a form of biological control, these experimental results demonstrate its potential. Since its introduction in mammalian cells, this system has been widely adopted and applied to many model and non-model organisms, including several insects, notably D. Guide RNA and the cas9 nuclease are delivered to induce double-stranded breaks at targeted sequence regions within the genome, perturbing the translation process and consequent protein. This relatively new methodology, therefore, facilitates the study of gene function both in terms of identification and validation, and can inform on suitable gene targets for novel insecticides.
For example, targeting the Nix gene of Ae. Genes responsible for insecticide resistance were further validated in C. It was found that for the species tested, the neem extract was as effective as a commercial product enriched with azadirachtin. This was attributed to the presence of additional terpenoids in the extract, which enhanced its effectiveness. The results demonstrated that the aqueous neem extracts reduced the viability of O.
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It was found that the extracts of T. One of the main causes of losses in cultivations is the action of insects and other pests, which can be controlled by products derived from natural sources such as botanical insecticides , as well as the use of nanotechnology to produce new formulations Forim et al. Nanobiotechnology offers great promise in this direction and O R O Nanotechnology and botanical insecticides P nanotechnological formulations can be used to improve both the stability and effectiveness of these natural products Ghormade et al.
They offer the ability to release the active compound to the target organism and then provide controlled release of the molecules at the site of action. Many different matrices can be used to produce nanostructured systems, including biodegradable polymers Fig.
The following section describes some of the nanostructured products that have been developed for use with active agents isolated from plants Table 2 , as well as essential oils Table 3. Despite these positive aspects, the use of nanotechnology to protect cultivations and combat agricultural pests requires careful evaluation Borm and Berube, Nanotechnology represents an interesting irony in terms of environmental safety, while its applications for environmental safety and remediation are growing. Moreover, reproducible studies are required to measure the toxicity and behavior of the systems in the environment, as well as to determine their bioavailability and durability Kah and Hofmann, ; Kah et al.
There is a growing body of evidence clearly indicating that nanotechnologies also have uncertainties and risk Bakshi et al. Previous studies proved that silica and titanium dioxide nanoparticles can cross the placenta barrier in pregnant mice Yamashta et al. Moreover, metallic nanoparticles can also reduce the seedling growth of agricultural crops Lee et al.
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Recently, Ma et al. Moreover, the uptake and accumulation of nanoparticles in crop plants and their translocation to edible plant parts should be studied in detail Fig. T toxic to the eggs of S. When the treatment was applied using the grains of T.