The acetylcholinesterase gene and organophosphorus resistance in the Australian sheep blowfly, Lucilia cuprina

Jaburetox, a urease-derived peptide: Effects on enzymatic pathways of the cockroach Nauphoeta cinerea

Jaburetox is a recombinant peptide derived from one of the Canavalia ensiformis urease isoforms. This peptide induces several toxic effects on insects of different orders, including interference on muscle contractility in cockroaches, modulation of UDP-N-acetylglucosamine pyrophosphorylase (UAP) and nitric oxide synthase (NOS) activities in the central nervous system of triatomines, as well as activation of the immune system in Rhodnius prolixus. When injected, the peptide is lethal for R. prolixus and Triatoma infestans. Here, we evaluated Jaburetox toxicity to Nauphoeta cinerea cockroaches, exploring the effects on the central nervous system through the activities of UAP, NOS, acid phosphatases (ACP), and acetylcholinesterase (AChE). The results indicated that N. cinerea is not susceptible to the lethal effect of the peptide. Moreover, both in vivo and in vitro treatments with Jaburetox inhibited NOS activity, without modifying the protein levels. No alterations on ACP activity were observed. In addition, the enzyme activity of UAP only had its activity affected at 18 hr after injection. The peptide increased the AChE activity, suggesting a mechanism involved in overcoming the toxic effects. In conclusion, our findings indicate that Jaburetox affects the nitrinergic signaling as well as the AChE and UAP activities and establishes N. cinerea as a Jaburetox-resistant model for future comparative studies.

Propoxur resistance associated with insensitivity of acetylcholinesterase (AChE) in the housefly, Musca domestica (Diptera: Muscidae)

Two unique housefly strains, PSS and N-PRS (near-isogenic line with the PSS), were used to clarify the mechanisms associated with propoxur resistance in the housefly, Musca domestica. The propoxur-selected resistant (N-PRS) strain exhibited >1035-fold resistance to propoxur and 1.70-, 12.06-, 4.28-, 57.76-, and 57.54-fold cross-resistance to beta-cypermethrin, deltamethrin, bifenthrin, phoxim, and azamethiphos, respectively, compared to the susceptible (PSS) strain. We purified acetylcholinesterase (AChE) from the N-PRS and PSS strains using a procainamide affinity column and characterized the AChE. The sensitivity of AChE to propoxur based on the bimolecular rate constant (Ki) was approximately 100-fold higher in the PSS strain compared to the N-PRS strain. The cDNA encoding Mdace from both the N-PRS strain and the PSS strain were cloned and sequenced using RT-PCR. The cDNA was 2073 nucleotides long and encoded a protein of 691 amino acids. A total of four single nucleotide polymorphisms (SNPs), I162M, V260L, G342A, and F407Y, were present in the region of the active site of AChE from the N-PRS strain. The transcription level and DNA copy number of Mdace were significantly higher in the resistant strain than in the susceptible strain. These results indicated that mutations combined with the up-regulation of Mdace might be essential in the housefly resistance to propoxur.

Acute oral and contact toxicity of new ethyl-carbamates on the mortality and acetylcholinesterase activity of honey bee (Apis mellifera)

The effects produced by the ethyl-carbamates: ethyl-4-bromophenyl carbamate (LQM 919) and ethyl-4-chlorophenyl carbamate (LQM 996) on the mortality and behavior of Apis mellifera were evaluated by the acute oral toxicity test and the acute contact toxicity test. The oral lethal dose, 50% of the ethyl-carbamates was >145.24 μg per bee, and the oral lethal dose, 50% of propoxur was 0.072 μg per bee. Therefore, according to the OECD criteria, the ethyl-carbamates were classified as relatively nontoxic orally; meanwhile, propoxur was classified as highly toxic orally. In the contact test, lethal concentrations 50% of the ethyl-carbamates were 4.83 and 2.23 μg/cm² for LQM 919 and LQM 996, respectively; therefore, they were at least 10-fold less lethal (p < 0.05) than propoxur (0.22 μg/cm²). The ethyl-carbamates reduced the activity of A. mellifera acetylcholinesterase by up to 30%. The k_i and k_d values of both ethyl-carbamates were lower (p < 0.05) than those of propoxur and indicated that they are weak inhibitors and with low affinity to A. mellifera acetylcholinesterase, which along with the absence of behavioral alterations suggests that the mortality caused by ethyl carbamates is not related to damage to the nervous system. According to these results, the evaluated ethyl-carbamates can be considered a low ecotoxic risk for A. mellifera.

Overcoming insecticide resistance through computational inhibitor design

Insecticides allow control of agricultural pests and disease vectors and are vital for global food security and health. The evolution of resistance to insecticides, such as organophosphates (OPs), is a serious and growing concern. OP resistance often involves sequestration or hydrolysis of OPs by carboxylesterases. Inhibiting carboxylesterases could, therefore, restore the effectiveness of OPs for which resistance has evolved. Here, we use covalent virtual screening to produce nano-/picomolar boronic acid inhibitors of the carboxylesterase αE7 from the agricultural pest Lucilia cuprina as well as a common Gly137Asp αE7 mutant that confers OP resistance. These inhibitors, with high selectivity against human acetylcholinesterase and low to no toxicity in human cells and in mice, act synergistically with the OPs diazinon and malathion to reduce the amount of OP required to kill L. cuprina by up to 16-fold and abolish resistance. The compounds exhibit broad utility in significantly potentiating another OP, chlorpyrifos, against the common pest, the peach-potato aphid (Myzus persicae). These compounds represent a solution to OP resistance as well as to environmental concerns regarding overuse of OPs, allowing significant reduction of use without compromising efficacy.

Toxic effects of new ethyl-carbamates on the morphology, mortality and acetylcholinesterase activity of Eisenia foetida

The toxicity of the ixodicidal carbamates ethyl-4-bromophenyl carbamate (LQM 919), ethyl-4-chlorophenyl carbamate (LQM 996) and propoxur on Eisenia foetida adults was evaluated to estimate their ecotoxic potential. The earthworm mortality and weight loss produced by the three evaluated carbamates showed a concentration-dependent effect (p < 0.0001) in the contact filter paper test (CFPT). In the artificial soil test (AST), mortality increased in relation to the exposure time (p < 0.0001) and the concentration (p < 0.01) of the carbamates. Only the earthworms exposed in the CFPT showed morphological alterations. According to the LC₅₀ obtained in the CFPT, the three carbamates were classified as very toxic and, according to the LC₅₀ obtained in the AST, the three carbamates were classified as highly toxic for E. foetida. The values of k_i and k_d indicated that LQM 919 and LQM 996 are weak inhibitors with lower affinity for the acetylcholinesterase of E. foetida than that of propoxur. The concentrations in the CFPT and AST at which 100% mortality was observed in E. foetida were 64- and 4-fold higher, respectively, than the egg hatching inhibitory concentration 99% reported for ticks.

Metabolic mechanisms and acetylcholinesterase sensitivity involved in tolerance to chlorpyrifos-ethyl in the earwig Forficula auricularia

Apple orchards are highly treated crops, in which organophosphorus (OP) are among the most heavily sprayed insecticides. These pesticides are toxic to non-target arthropods and their repeated use increases the risk of resistance. We studied mechanisms involved in tolerance and resistance to OP insecticides in the earwig Forficula auricularia, an effective generalist predator in pomefruit orchards. Adult earwigs were sampled in three apple orchards managed under contrasting strategies: conventional, Integrated Pest Management, and organic. The threshold activities of enzyme families involved in pesticides tolerance: Glutathione-S-transferases (GSTs) and Carboxylesterases (CbEs) were measured in earwig extracts. Acetylcholinesterase (AChE) was monitored as a toxicological endpoint. Variations in these activities were assessed prior to and after exposure to chlorpyrifos-ethyl at the normal application rate. We observed that the mortality of earwigs exposed to chlorpyrifos-ethyl depended on the management strategy of orchards. Significantly lower mortality was seen in individuals sampled from conventional orchard. The basal activities of CbEs and GSTs of collected organisms were higher in conventional orchard. After in vivo exposure, AChE activity appeared to be inhibited in surviving males with no difference between orchards. However an in vitro inhibition trial with chlorpyrifos-oxon showed that AChE from earwigs collected in organic and IPM orchards were more sensitive than from conventional ones. These observations support the hypothesis of a molecular target modification in AChE and highlight the possible role of CbEs in effective protection of AChE. Our findings suggest that the earwigs with a high historic level of insecticide exposure could acquire resistance to chlorpyrifos-ethyl.

Molecular markers and their application in the monitoring of acaricide resistance in Rhipicephalus microplus

Monitoring acaricide resistance and understanding the underlying mechanisms are critically important in developing strategies for resistance management and tick control. Identification of single nucleotide polymorphisms in the acaricide-resistant associated gene of Rhipicephalus microplus has enabled the development of molecular markers for detection and monitoring of resistance against different types of acaricide. There are many molecular markers developed for resistance monitoring, including mutations on target genes such as sodium channel, acetylcholinesterase, carboxylesterase, β-adrenergic octopamine receptor, octopamine-tyramine etc. Molecular genotyping through molecular markers can detect the presence of resistance-associated genes in a tick population before it reaches high frequency. This review aims to provide an update on the various molecular markers discovered to date from different regions of the world.

Target-site mutations (AChE-G119S and kdr) in Guangxi Anopheles sinensis populations along the China-Vietnam border

Background

In South Asia, the epidemiology of malaria is complex, and transmission mainly occurs in remote areas near international borders. Vector control has been implemented as a key strategy in malaria prevention for decades. A rising threat to the efficacy of vector control efforts is the development of insecticide resistance, thus it is important to monitor the type and frequency of insecticide resistant alleles in the disease vectors such as An. sinensis along the China-Vietnam border. Such information is needed to synthesize effective malaria vector control strategies.

Methods

A total of 208 adults of An. sinensis, collected from seven sites in southwest Guangxi along the China-Vietnam border, were inspected for the resistance-conferring G119S mutation in acetylcholinesterase (AChE) by PCR-RFLP (polymerase chain reaction restriction fragment length polymorphism) and kdr mutations in the voltage-gated sodium channel (VGSC) by sequencing. In addition, the evolutionary origin of An. sinensis vgsc gene haplotypes was analyzed using Network 5.0.

Results

The frequencies of mutant 119S of AChE were between 0.61–0.85 in the seven An. sinensis populations. No susceptible homozygote (119GG) was detected in three of the seven sites (DXEC, LZSK and FCGDX). Very low frequencies of kdr (0.00–0.01) were detected in the seven populations, with most individuals being susceptible homozygote (1014LL). The 1014F mutation was detected only in the southeast part (FCGDX) at a low frequency of 0.03. The 1014S mutation was distributed in six of the seven populations with frequencies ranging from 0.04 to 0.08, but absent in JXXW. Diverse haplotypes of 1014L and 1014S were found in An. sinensis along the China-Vietnam border, while only one 1014F haplotype was detected in this study. Consistent with a previous report, resistant 1014S haplotypes did not have a single origin.

Conclusions

The G119S mutation of AChE was present at high frequencies (0.61–0.85) in the An. sinensis populations along the China-Vietnam border, suggesting that the vector control authorities should be cautious when considering carbamates and organophosphates as chemicals for vector control. The low frequencies (0.00–0.11) of kdr in these populations suggest that pyrethroids remain suitable for use against An. sinensis in these regions.

Identification and characterization of ace1-type acetylcholinesterase in insecticide-resistant and -susceptible Propylaea japonica (Thunberg)

Characterization and gene cloning of acetylecholinesterase (AChE) in the insecticide-resistant (R) and -susceptible (S) insects have been reported in the past. However, the studies focused mostly on herbivorous pests, rather than predacious species, such as ladybird beetles. Using R and S Propylaea japonica (thunberg), a full-length cDNA sequence (2928 bp) of the ace1-type AChE gene was determined for the first time. The ace1 encoding a protein of 645 amino acids contained typical conserved motifs, such as FGESAG domains, catalytic triad, acyl pocket, oxyanino hole, choline binding site, peripheral anionic site, omega loop and conserved aromatic residues. R P. japonica displayed 50-times greater resistance to chlorpyrifos or mathamidophos with a significantly lower AChE sensitivity to paraoxon, malaoxon, chlorpyrifos or methamidophos than its S counterpart. Five amino acids in the ace1 of R P. japonica differed from those found in S P. japonica. One of them, F358S, located in the acyl-binding pocket, might play a crucial role in the resistance of the insect to organophosphates (OPs). Whereas, K493E and I538V, which were close to some of the conserved aromatic amino acids (i.e., H509, Y511, and W499) in the gorge, and G571R and T576A near C593 that formed the disulfide bonds with C471, might also involve in the change of insecticide resistance in P. japonica. AChE insensitivity and amino acid replacements, particularly F358S, might be the determining factors in the alteration of OPs-resistance in P. japonica.

Two Bombyx mori acetylcholinesterase genes influence motor control and development in different ways

Among its other biological roles, acetylcholinesterase (AChE, EC 3.1.1.7), encoded by two ace in most insects, catalyses the breakdown of acetylcholine, thereby terminating synaptic transmission. ace1 encodes the synaptic enzyme and ace2 has other essential actions in many insect species, such as Chilo suppressalis and Plutella xylostella. The silkworm, Bombyx mori, has been domesticated for more than two thousand years and its aces have no history of pesticide exposure. Here, we investigated the functional differences between two ace genes, BmAce1 and BmAce2, in the silkworm. qPCR analysis indicated that BmAce1 is highly expressed in muscle and BmAce2 is more ubiquitously expressed among tissues and enriched in the head. Both genes were separately suppressed using chemically synthesized siRNAs. The mRNA abundance of the two ace genes was significantly reduced to about 13% - 75% of the control levels after siRNA injection. The AChE activities were decreased to 32% to 85% of control levels. Silencing BmAce2 resulted in about 26% mortality, faster and higher than the 20% in the siBmAce1-treated group. Silencing BmAce1 impacted motor control and development to a greater extent than silencing BmAce2, although both treatment groups suffered motor disability, slowed development and reduced cocoons. Both genes have essential, differing biological significance.

The Battle Against Flystrike - Past Research and New Prospects Through Genomics

Flystrike, or cutaneous myiasis, is caused by blow fly larvae of the genus Lucilia. This disease is a major problem in countries with large sheep populations. In Australia, Lucilia cuprina (Wiedemann, 1830) is the principal fly involved in flystrike. While much research has been conducted on L. cuprina, including physical, chemical, immunological, genetic and biological investigations, the molecular biology of this fly is still poorly understood. The recent sequencing, assembly and annotation of the draft genome and analyses of selected transcriptomes of L. cuprina have given a first global glimpse of its molecular biology and insights into host-fly interactions, insecticide resistance genes and intervention targets. The present article introduces L. cuprina, flystrike and associated issues, details past control efforts and research foci, reviews salient aspects of the L. cuprina genome project and discusses how the new genomic and transcriptomic resources for this fly might accelerate fundamental molecular research of L. cuprina towards developing new methods for the treatment and control of flystrike.

Functional and immunohistochemical characterization of CCEae3a, a carboxylesterase associated with temephos resistance in the major arbovirus vectors Aedes aegypti and Ae. albopictus

Temephos is a major organophosphate (OP) larvicide that has been used extensively for the control of Aedes albopictus and Aedes aegypti, the major vectors for viral diseases, such as dengue fever, zika and chikungunya. Resistance to temephos has been recently detected and associated with the upregulation of carboxylesterases (CCEs) through gene amplification, in both species. Here, we expressed the CCEae3a genes which showed the most striking up-regulation in resistant Aedes strains, using the baculovirus system. All CCEae3a variants encoded functional enzymes, with high activity and preference for p-nitrophenyl butyrate, a substrate that was shown capable to differentiate temephos resistant from susceptible Aedes larvae. Enzyme kinetic studies showed that CCEae3as from both Ae. aegypti and Ae. albopictus (CCEae3a_aeg and CCEae3a_alb, respectively) strongly interact with temephos oxon and slowly released the OP molecule, indicating a sequestration resistance mechanism. No difference was detected between resistant and susceptible CCEae3a_aeg variants (CCEae3a_aegR and CCEae3a_aegS, respectively), indicating that previously reported polymorphism is unlikely to play a role in temephos resistance. HPLC/MS showed that CCEae3as were able to metabolize temephos oxon to the temephos monoester [(4-hydroxyphenyl) sulfanyl] phenyl O,O-dimethylphosphorothioate. Western blot and immunolocalization studies, based on a specific antibody raised against the CCEae3a_alb showed that the enzyme is expressed at higher levels in resistant insects, primarily in malpighian tubules (MT) and nerve tissues.

Mapping the Accessible Conformational Landscape of an Insect Carboxylesterase Using Conformational Ensemble Analysis and Kinetic Crystallography

The proper function of enzymes often depends upon their efficient interconversion between particular conformational sub-states on a free-energy landscape. Experimentally characterizing these sub-states is challenging, which has limited our understanding of the role of protein dynamics in many enzymes. Here, we have used a combination of kinetic crystallography and detailed analysis of crystallographic protein ensembles to map the accessible conformational landscape of an insect carboxylesterase (LcαE7) as it traverses all steps in its catalytic cycle. LcαE7 is of special interest because of its evolving role in organophosphate insecticide resistance. Our results reveal that a dynamically coupled network of residues extends from the substrate-binding site to a surface loop. Interestingly, the coupling of this network that is apparent in the apoenzyme appears to be reduced in the phosphorylated enzyme intermediate. Altogether, the results of this work highlight the importance of protein dynamics to enzyme function and the evolution of new activity.

Pyrethroid Insecticide Resistance Mechanisms in the Adult Phlebotomus papatasi (Diptera: Psychodidae)

Phlebotomus papatasi is one of the most medically important sand fly species in the Old World, serving as a vector of Leishmania parasites and phleboviruses. Chemical control is still considered the most effective method for rapidly reducing populations of flying insects involved in vector-borne disease transmission, but is increasingly threatened by insecticide resistance in the target insect posing significant problems for entomologists responsible for control programs. This study was conducted to determine pyrethroid resistance mechanisms and the biological, physiological, and molecular impacts of resistance in Ph. papatasi, and to compare their resistance mechanisms against those reported for mosquitoes and other intensely studied dipterans. Field-collected Ph. papatasi from Aswan, Egypt, were subjected to sublethal doses of permethrin and reared as a resistant strain under laboratory conditions through 16 generations. Biological parameter observations of resistant Ph. papatasi revealed an association of resistance with productivity cost. Physiological analysis revealed that concentrations of oxidase and esterase enzymes increased in early generations of the resistant colony, and then subsided through the F16 generation to levels similar to those in a susceptible colony. The activity levels of acetylcholinesterase were higher in field-collected Ph. papatasi than in susceptible colony flies, but decreased significantly despite subsequent exposure to permethrin. The molecular search for gene mutations in the resistant strain of Ph. papatasi failed to identify any mutations common in pyrethroid-resistant mosquitoes. Our study revealed that the mechanism of pyrethroid resistance in sand flies is different than that in mosquitoes, at least at the genetic level.

Validating the importance of two acetylcholinesterases in insecticide sensitivities by RNAi in Pardosa pseudoannulata, an important predatory enemy against several insect pests

The pond wolf spider (Pardosa pseudoannulata) is an important predatory enemy against several insect pests and showed relative different sensitivities to organophosphate and carbamate insecticides compared to insect pests. In our previous studies, two acetylcholinesterases were identified in P. pseudoannulata and played important roles in insecticide sensitivities. In order to understand the contributions of the two acetylcholinesterases to insecticide sensitivities, we firstly employed the RNAi technology in the spider. For a suitable microinjection RNAi method, the injection site, injection volume and interference time were optimized, which then demonstrated that the injection RNAi method was applicable in this spider. With the new RNAi method, it was revealed that both Pp-AChE1 and Pp-AChE2, encoded by genes Ppace1 and Ppace2, were the targets of organophosphate insecticides, but Pp-AChE1 would be more important. In contrast, the carbamate acted selectively on Pp-AChE1. The results showed that Pp-AChE1 was the major catalytic enzyme in P. pseudoannulata and the major target of organophosphate and carbamate insecticides. In a word, an RNAi method was established in the pond wolf spider, which further validated the importance of two acetylcholinesterases in insecticide sensitivities in this spider.

Survey of acaricides resistance status of Rhipiciphalus (Boophilus) microplus collected from selected places of Bihar, an eastern state of India

Monitoring acaricide resistance in field ticks and use of suitable managemental practices are essential for controlling tick populations infesting animals. In the present study, the acaricide resistance status in Rhipicephalus (Boophilus) microplus ticks infesting cattle and buffaloes of five districts located in the eastern Indian state, Bihar were characterized using three data sets (AIT, Biochemical assays and gene sequences). Adult immersion test (AIT) was adopted using seven field isolates and their resistance factor (RF) was determined. Six isolates (DNP, MUZ, BEG, VSH, DRB and SUL) were found resistant to both deltamethrin and diazinon and except VSH all were resistant to cypermethrin. One isolate (PTN) was susceptible with a RF below 1.5. To understand the possible mode of resistance development, targeted enzymes and gene sequences of the para sodium channel and achetylcholinesterase 2 (AChE2) were analyzed. The esterase, monooxygenase and glutathione-S-transferase (GST) activity of reference susceptible IVRI-I line was determined as 2.47 ± 0.007 nmol/min/mg protein, 0.089 ± 0.0016 nmol/mg of protein and 0.0439 ± 0.0003 nmol/mg/min respectively, which increased significantly in the resistant field isolates. However, except esterases, the fold increase of monooxygenase (1.14-2.27 times) and GST (0.82-1.53 times) activities were not very high. A cytosine (C) to adenine (A) nucleotide substitution (CTC to ATC) at position 190 in domain II S4-5 linker region was detected only in one isolate (SUL) having RF of 34.9 and in the reference deltamethrin resistant line (IVRI-IV). However, the T2134A mutation was not detected in domain IIIS6 transmembrane segment of resistant isolates and also in reference IVRI-IV line despite of varying degree of resistance. The flumethrin specific G215T and the recently identified T170C mutations were also absent in domain II sequences under study. Four novel amino acid substitutions in AChE2 gene of field isolates and in organophosphate (OP) resistant reference IVRI-III line were identified which can possibly have a role in resistance development.

Identification and Molecular Characterization of Two Acetylcholinesterases from the Salmon Louse, Lepeophtheirus salmonis

Acetylcholinesterase (AChE) is an important enzyme in cholinergic synapses. Most arthropods have two genes (ace1 and ace2), but only one encodes the predominant synaptic AChE, the main target for organophosphates. Resistance towards organophosphates is widespread in the marine arthropod Lepeophtheirus salmonis. To understand this trait, it is essential to characterize the gene(s) coding for AChE(s). The full length cDNA sequences encoding two AChEs in L. salmonis were molecularly characterized in this study. The two ace genes were highly similar (83.5% similarity at protein level). Alignment to the L. salmonis genome revealed that both genes were located close to each other (separated by just 26.4 kbp on the L. salmonis genome), resulting from a recent gene duplication. Both proteins had all the typical features of functional AChE and clustered together with AChE-type 1 proteins in other species, an observation that has not been described in other arthropods. We therefore concluded the presence of two versions of ace1 gene in L. salmonis, named ace1a and ace1b. Ace1a was predominantly expressed in different developmental stages compared to ace1b and was possibly active in the cephalothorax, indicating that ace1a is more likely to play the major role in cholinergic synaptic transmission. The study is essential to understand the role of AChEs in resistance against organophosphates in L. salmonis.

Amino acid substitutions of acetylcholinesterase associated with carbofuran resistance in Chilo suppressalis

BACKGROUND

Over 1000-fold carbofuran resistance has been observed in Chilo suppressalis (Walker) collected from the Changhua (CH) and Chiayi (CY) prefectures of Taiwan. An understanding of the pertinent mechanisms will benefit effective insecticide resistance management of C. suppressalis.

RESULTS

Among the five amino acid substitutions of acetylcholinesterase (AChE) identified in C. suppressalis, A314S and H668P had been reported and E101D, F402V and R667Q were novel. Substitution frequencies in AChE of CH and CY populations were much higher than in the susceptible Hsinchu (HC) population. Significantly negative correlations were observed between the frequencies of E101D, A314S and R667Q and the kinetic parameters of AChEs in these populations. AChE from the resistant CH population was less susceptible to the inhibition of carbofuran, with an I50 that was 3.6-fold higher than that of the susceptible HC population. Although Km and Vmax of AChE from the CH and CY populations were reduced to 72-87% of those from the HC population, the overall catalytic efficiency (Vmax /Km ) remained constant for all three populations.

CONCLUSION

Amino acid substitutions identified in the AChE of C. suppressalis are associated with changes in AChE kinetics and its insensitivity to carbofuran. These observations are helpful for rapid monitoring, prediction and management of OP and CB resistance in the field.

Control of the sheep blowfly in Australia and New Zealand--are we there yet?

The last 50 years of research into infections in Australia and New Zealand caused by larvae of the sheep blowfly, Lucilia cuprina, have significantly advanced our understanding of this blowfly and its primary host, the sheep. However, apart from some highly effective drugs it could be argued that no new control methodologies have resulted. This review addresses the major areas of sheep blowfly research over this period describing the significant outcomes and analyses, and what is still required to produce new commercial control technologies. The use of drugs against this fly species has been very successful but resistance has developed to almost all current compounds. Integrated pest management is becoming basic to control, especially in the absence of mulesing, and has clearly benefited from computer-aided technologies. Biological control has more challenges but natural and perhaps transformed biopesticides offer possibilities for the future. Experimental vaccines have been developed but require further analysis of antigens and formulations to boost protection. Genetic technologies may provide potential for long-term control through more rapid indirect selection of sheep less prone to flystrike. Finally in the future, genetic analysis of the fly may allow suppression and perhaps eradication of blowfly populations or identification of new and more viable targets for drug and vaccine intervention. Clearly all these areas of research offer potential new controls but commercial development is perhaps inhibited by the success of current chemical insecticides and certainly requires a significant additional injection of resources.

Identification of two acetylcholinesterases in Pardosa pseudoannulata and the sensitivity to insecticides

Pardosa pseudoannulata is an important predatory enemy against insect pests, such as rice planthoppers and leafhoppers. In order to understand the insecticide selectivity between P. pseudoannulata and insect pests, two acetylcholinesterase genes, Pp-ace1 and Pp-ace2, were cloned from this natural enemy. The putative proteins encoded by Pp-ace1 and Pp-ace2 showed high similarities to insect AChE1 (63% to Liposcelis entomophila AChE1) and AChE2 (36% to Culex quinquefasciatus AChE2) with specific functional motifs, which indicated that two genes might encode AChE1 and AChE2 proteins respectively. The recombinant proteins by expressing Pp-ace1 and Pp-ace2 genes in insect sf9 cells showed high AChE activities. The kinetic parameters, Vmax and Km, of two recombinant AChE proteins were significantly different. The sensitivities to six insecticides were determined in two recombinant AChEs. Pp-AChE1 was more sensitive to all tested insecticides than Pp-AChE2, such as fenobucarb (54 times in Ki ratios), isoprocarb (31 times), carbaryl (13 times) and omethoate (6 times). These results indicated that Pp-AChE1 might be the major synaptic enzyme in the spider. By sequence comparison of P. pseudoannulata and insect AChEs, the key amino acid differences at or close to the functional sites were found. The locations of some key amino acid differences were consistent with the point mutation sites in insect AChEs that were associated with insecticide resistance, such as Phe331 in Pp-AChE2 corresponding to Ser331Phe mutation in Myzus persicae and Aphis gossypii AChE2, which might play important roles in insecticide selectivity between P. pseudoannulata and insect pests. Of course, the direct evidences are needed through further studies.

Functional study on the mutations in the silkworm (Bombyx mori) acetylcholinesterase type 1 gene (ace1) and its recombinant proteins

The acetylcholinesterase of Lepidoptera insects is encoded by two genes, ace1 and ace2. The expression of the ace1 gene is significantly higher than that of the ace2 gene, and mutations in ace1 are one of the major reasons for pesticide resistance in insects. In order to investigate the effects of the mutations in ace1’s characteristic sites on pesticide resistance, we generated mutations for three amino acids using site-directed mutagenesis, which were Ala(GCG)303Ser(TCG), Gly(GGA)329Ala(GCA) and Leu (TCT)554Ser(TTC). The Baculovirus expression system was used for the eukaryotic expression of the wild type ace1 (wace1) and the mutant ace1 (mace1). SDS-PAGE and Western blotting were used to detect the targeting proteins with expected sizeof about 76 kDa. The expression products were purified for the determination of AChE activity and the inhibitory effects of physostigmine and phoxim. We observed no significant differences in the overall activity of the wild type and mutant AChEs. However, with 10 min of physostigmine (10 μM) inhibition, the remaining activity of the wild type AChE was significantly lower than that of the mutant AChE. Ten min inhibition with 33.4 μM phoxim also resulted in significantly lower remaining activity of the wild type AChE than that of the mutant AChE. These results indicated that mutations for the three amino acids reduced the sensitivity of AChE to physostigmine and phoxim, which laid the foundation for future in vivo studies on AChE’s roles in pesticide resistance.

Structure and function of an insect α-carboxylesterase (αEsterase7) associated with insecticide resistance

Insect carboxylesterases from the αEsterase gene cluster, such as αE7 (also known as E3) from the Australian sheep blowfly Lucilia cuprina (LcαE7), play an important physiological role in lipid metabolism and are implicated in the detoxification of organophosphate (OP) insecticides. Despite the importance of OPs to agriculture and the spread of insect-borne diseases, the molecular basis for the ability of α-carboxylesterases to confer OP resistance to insects is poorly understood. In this work, we used laboratory evolution to increase the thermal stability of LcαE7, allowing its overexpression in Escherichia coli and structure determination. The crystal structure reveals a canonical α/β-hydrolase fold that is very similar to the primary target of OPs (acetylcholinesterase) and a unique N-terminal α-helix that serves as a membrane anchor. Soaking of LcαE7 crystals in OPs led to the capture of a crystallographic snapshot of LcαE7 in its phosphorylated state, which allowed comparison with acetylcholinesterase and rationalization of its ability to protect insects against the effects of OPs. Finally, inspection of the active site of LcαE7 reveals an asymmetric and hydrophobic substrate binding cavity that is well-suited to fatty acid methyl esters, which are hydrolyzed by the enzyme with specificity constants (∼10(6) M(-1) s(-1)) indicative of a natural substrate.

Testing insecticidal activity of novel chemically synthesized siRNA against Plutella xylostella under laboratory and field conditions

BACKGROUND

Over the last 60 years, synthetic chemical pesticides have served as a main tactic in the field of crop protection, but their availability is now declining as a result of the development of insect resistance. Therefore, alternative pest management agents are needed. However, the demonstration of RNAi gene silencing in insects and its successful usage in disrupting the expression of vital genes opened a door to the development of a variety of novel, environmentally sound approaches for insect pest management.

METHODOLOGY/PRINCIPAL FINDINGS

Six small interfering RNAs (siRNAs) were chemically synthesized and modified according to the cDNA sequence of P. xylostella acetylcholine esterase genes AChE1 and AChE2. All of them were formulated and used in insecticide activity screening against P. xylostella. Bioassay data suggested that Si-ace1_003 and Si-ace2_001 at a concentration of 3 µg cm(-2) displayed the best insecticidal activity with 73.7% and 89.0%, mortality, respectively. Additional bioassays were used to obtain the acute lethal concentrations of LC50 and LC90 for Si-ace2_001, which were 53.66 µg/ml and 759.71 µg/ml, respectively. Quantitative Real-time PCR was used to confirm silencing and detected that the transcript levels of P. xylostella AChE2 (PxAChE2) were reduced by 5.7-fold compared to the control group. Consequently, AChE activity was also reduced by 1.7-fold. Finally, effects of the siRNAs on treated plants of Brassica oleracea and Brassica alboglabra were investigated with different siRNA doses. Our results showed that Si-ace2_001 had no negative effects on plant morphology, color and growth of vein under our experimental conditions.

CONCLUSIONS

The most important finding of this study is the discovery that chemically synthesized and modified siRNA corresponding to P. xylostella AChE genes cause significant mortality of the insect both under laboratory and field conditions, which provides a novel strategy to control P. xylostella and to develop bio-pesticides based on the RNA interference technology.

Genomic structure, organization and localization of the acetylcholinesterase locus of the olive fruit fly, Bactrocera oleae

Acetylcholinesterase (AChE), encoded by the ace gene, is a key enzyme of cholinergic neurotransmission. Insensitive acetylcholinesterase (AChE) has been shown to be responsible for resistance to OPs and CBs in a number of arthropod species, including the most important pest of olives trees, the olive fruit fly Bactrocera oleae. In this paper, the organization of the B. oleae ace locus, as well as the structural and functional features of the enzyme, are determined. The organization of the gene was deduced by comparison to the ace cDNA sequence of B. oleae and the organization of the locus in Drosophila melanogaster. A similar structure between insect ace gene has been found, with conserved exon-intron positions and junction sequences. The B. oleae ace locus extends for at least 75 kb, consists of ten exons with nine introns and is mapped to division 34 of the chromosome arm IIL. Moreover, according to bioinformatic analysis, the Bo AChE exhibits all the common features of the insect AChE. Such structural and functional similarity among closely related AChE enzymes may implicate similarities in insecticide resistance mechanisms.

Biochemical identification and crystal structure of kynurenine formamidase from Drosophila melanogaster

KFase (kynurenine formamidase), also known as arylformamidase and formylkynurenine formamidase, efficiently catalyses the hydrolysis of NFK (N-formyl-L-kynurenine) to kynurenine. KFase is the second enzyme in the kynurenine pathway of tryptophan metabolism. A number of intermediates formed in the kynurenine pathway are biologically active and implicated in an assortment of medical conditions, including cancer, schizophrenia and neurodegenerative diseases. Consequently, enzymes involved in the kynurenine pathway have been considered potential regulatory targets. In the present study, we report, for the first time, the biochemical characterization and crystal structures of Drosophila melanogaster KFase conjugated with an inhibitor, PMSF. The protein architecture of KFase reveals that it belongs to the α/β hydrolase fold family. The PMSF-binding information of the solved conjugated crystal structure was used to obtain a KFase and NFK complex using molecular docking. The complex is useful for understanding the catalytic mechanism of KFase. The present study provides a molecular basis for future efforts in maintaining or regulating kynurenine metabolism through the molecular and biochemical regulation of KFase.

Novel and viable acetylcholinesterase target site for developing effective and environmentally safe insecticides

Insect pests are responsible for human suffering and financial losses worldwide. New and environmentally safe insecticides are urgently needed to cope with these serious problems. Resistance to current insecticides has resulted in a resurgence of insect pests, and growing concerns about insecticide toxicity to humans discourage the use of insecticides for pest control. The small market for insecticides has hampered insecticide development; however, advances in genomics and structural genomics offer new opportunities to develop insecticides that are less dependent on the insecticide market. This review summarizes the literature data that support the hypothesis that an insect-specific cysteine residue located at the opening of the acetylcholinesterase active site is a promising target site for developing new insecticides with reduced off-target toxicity and low propensity for insect resistance. These data are used to discuss the differences between targeting the insect-specific cysteine residue and targeting the ubiquitous catalytic serine residue of acetylcholinesterase from the perspective of reducing off-target toxicity and insect resistance. Also discussed is the prospect of developing cysteine-targeting anticholinesterases as effective and environmentally safe insecticides for control of disease vectors, crop damage, and residential insect pests within the financial confines of the present insecticide market.

Identification and characterization of three cholinesterases from the common bed bug, Cimex lectularius

We identified and characterized the full-length cDNA sequences encoding two acetylcholinesterases (ClAChE1 and ClAChE2) and a salivary gland-specific cholinesterase-like protein (ClSChE) from the common bed bug, Cimex lectularius. All three cholinesterase genes (Clac1, Clace2 and Clsce) have conserved motifs, including a catalytic triad, a choline-binding site and an acyl pocket. Phylogenetic analysis showed that ClAChE1 belongs to the insect AChE1 clade, whereas ClAChE2 belongs to the insect AChE2 clade. ClSChE was grouped into the clade containing all AChE1s, suggesting a paralogous relationship to ClAChE1. Transcription levels of Clace1 were higher than those of Clace2 in all tissues examined, including the central nervous system (CNS). In contrast, the Clsce transcript was not detected in the CNS but specifically found in the salivary gland at much higher levels (>3000-fold) than those of Clace1 and Clace2. Western blot analysis using anti-ClAChE antibodies, in conjunction with activity staining, revealed that ClAChE1 is more active than ClAChE2, whereas ClSChE has little enzyme activity. Three-dimensional structure modelling suggested that ClAChEs and ClSChE shared structural similarities, but had some differences in the residues forming the acyl pocket and oxyanion hole. The current findings should provide valuable insights into the evolution and functional diversification of insect cholinesterase.

RNA interference of two acetylcholinesterase genes in Plutella xylostella reveals their different functions

Acetylcholinesterase (AChE, EC 3.1.1.7) is an important enzyme with a typical function of degrading the neurotransmitter acetylcholine. Although two ace genes were reported in Plutella xylostella, their function differences remain largely unknown. The chemically synthesized siRNAs (si-Pxace1 and si-Pxace2) were injected into the second instar larvae to knock down Pxace1 and Pxace2, either respectively or simultaneously. The mRNA abundance of Pxace1 and Pxace2 was significantly reduced to 7-33.5% of the control levels at 72 h after siRNA injection. The AChE activities were significantly decreased at 96 h after treatment. Silencing of Pxace1 or Pxace2 resulted in mortality of 33.9 and 22.9%, respectively. The survivors in siRNA-treated groups had apparent growth inhibition such as reduction in larvae weights and lengths, malformation and motor retardation. Knockdown of Pxace1 apparently affected more on larvae growth than that of Pxace2, suggesting that Pxace1 had more important roles than Pxace2. Both Pxace1 and Pxace2 genes might have atypical functions in regulating larvae growth and motor ability.

Polymorphism in the acetylcholinesterase gene of Musca domestica L. field populations in Turkey

Acetylcholinesterase (AChE), encoded by the Ace gene, is the primary target of organophosphates (OPs) and carbamates (CBs) in insects. Ace mutations have been identified in OP and CB resistant strains of Musca domestica. In this study, the Ace gene was partially amplified and sequenced at amino acid positions 260, 342, and 407 to determine the frequencies of these mutations in housefly samples collected from farms and garbage disposal sites of 16 provinces in the Aegean and Mediterranean regions of Turkey. In addition, the percent remaining AChE activities in these samples were assayed by using three OPs (malaoxon, paraoxon, and dichlorvos) and one CB (carbaryl) compound as inhibitors. In all the analyzed samples, 13 different combinations at the three amino acid positions were identified and the L/V260-A/G342-F/Y407 combination was found in the highest frequency. No susceptible individual was detected. The highest mean percent remaining AChE activities were detected in the individuals having the L260-A/G342-F/Y407 genotype when malaoxon and paraoxon were used as inhibitors and in the individuals with the L260-A342-F/Y407 combination when dichlorvos and carbaryl were used as inhibitors. The obtained data were heterogeneous and there was no exact correlation between the molecular genetic background and the resistance phenotypes of the flies. The findings of this study at the molecular and biochemical levels indicate the presence of significant control problems in the field.

Acetylcholinesterase of Stomoxys calcitrans (L.) (Diptera: Muscidae): cDNA sequence, baculovirus expression, and biochemical properties

A 2193-nucleotide cDNA encoding acetylcholinesterase (AChE) of the stable fly, Stomoxys calcitrans (L.) was sequenced and expressed in the baculovirus system. The open reading frame encoded a 91 amino acid secretion signal peptide and a 613 amino acid mature protein with 96% and 94% identity to the AChEs of Haematobia irritans (L.) and Musca domestica (L.), respectively. Structural characteristics of M. domestica, H. irritans, and Drosophila melanogaster AChEs were conserved in the S. calcitrans AChE. The recombinant enzyme was inhibited by eserine, coroxon, and paraoxon and exhibited K(m) values of 63.9μM for acetylthiocholine and 96.7μM for butyrylthiocholine, confirming its biochemical identity as an acetylcholinesterase (EC 3.1.1.7). These data will enable rapid identification and assay for mutations that reduce AChE sensitivity to organophosphate (OP) pesticides, potentially aiding resistance management efforts to prevent fixation of the mutations in pest populations.

RNA interference of ace1 and ace2 in Chilo suppressalis reveals their different contributions to motor ability and larval growth

Acetylcholinesterase (AChE, EC 3.1.1.7) is a key enzyme in terminating synaptic transmission. We knocked down the expression of Csace1 or Csace2 using chemically synthesized small interfering RNAs (siRNAs) designed from divergent regions. The mRNA abundance of the two ace genes was reduced to 50-70% of control levels. The enzyme activities were decreased to 40-70%. Silencing of Csace1 or Csace2 resulted in a ~25% mortality rate. Knockdown of Csace1 had major effects on larval growth inhibition and resulted in reduced larval weight and length, malformation and motor disability, whereas silencing of Csace2 had only minor effects. These results suggested that both AChE-1 and AChE-2 have important roles in maintaining life in this insect and indicated that AChE-1 might have nontypical functions in regulating larval growth and motor ability.

The characterization of Lucilia cuprina acetylcholinesterase as a drug target, and the identification of novel inhibitors by high throughput screening

Acetylcholinesterase (AChE, EC3.1.1.7.) is the molecular target for the carbamate and organophosphate pesticides that are used to combat parasitic arthropods. In this paper we report the functional heterologous expression of AChE from Lucilia cuprina (the sheep blowfly) in HEK293 cells. We show that the expressed enzyme is cell-surface-exposed and possesses a glycosyl-phosphatidylinositol membrane anchor. The substrates acetyl-, propionyl- and butyrylthiocholine (AcTC, PropTC, ButTC), and also 11 further thiocholine and homo-thiocholine derivatives were chemically synthesized to evaluate and compare their substrate properties in L. cuprina AChE and recombinant human AChE. The Michaelis-Menten constants K(M) for AcTC, PropTC and ButTC were found to be 3-7-fold lower for the L. cuprina AChE than for the human AChE. Additionally, 2-methoxyacetyl-thiocholine and isobutyryl-thiocholine were better substrates for the insect enzyme than for the human AChE. The AcTC, PropTC and ButTC specificities and the Michaelis-Menten constants for recombinant L. cuprina AChE were similar to those determined for AChE extracted from L. cuprina heads, which are a particularly rich source of this enzyme. The median inhibition concentrations (IC(50) values) were determined for 21 organophosphates, 23 carbamates and also 9 known non-covalent AChE inhibitors. Interestingly, 11 compounds were 100- to >4000-fold more active on the insect enzyme than on the human enzyme. The substrate and inhibitor selectivity data collectively indicate that there are structural differences between L. cuprina and human AChE in or near the active sites, suggesting that it may be possible to identify novel, specific L. cuprina AChE inhibitors. To this end, a high throughput screen with 107,893 compounds was performed on the L. cuprina head AChE. This led to the identification of 195 non-carbamate, non-organophosphate inhibitors with IC(50) values below 10μM. Analysis of the most potent hit compounds identified 19 previously unknown inhibitors with IC(50) values below 200nM, which were up to 335-fold more potent on the L. cuprina enzyme than on the human AChE. Some of these compounds may serve as leads for lead optimization programs to generate fly-specific pesticides.

Altered GPI modification of insect AChE improves tolerance to organophosphate insecticides

The olive fruit fly Bactrocera oleae is the most destructive and intractable pest of olives. The management of B. oleae has been based on the use of organophosphate (OP) insecticides, a practice that induced resistance. OP-resistance in the olive fly was previously shown to be associated with two mutations in the acetylcholinesterase (AChE) enzyme that, apparently, hinder the entrance of the OP into the active site. The search for additional mutations in the ace gene that encodes AChE revealed a short deletion of three glutamines (Δ3Q) from a stretch of five glutamines, in the C-terminal peptide that is normally cleaved and substituted by a GPI anchor. We verified that AChEs from B. oleae and other Dipterans are actually GPI-anchored, although this is not predicted by the "big-PI" algorithm. The Δ3Q mutation shortens the unusually long hydrophilic spacer that follows the predicted GPI attachment site and may thus improve the efficiency of GPI anchor addition. We expressed the wild type B. oleae AChE, the natural mutant Δ3Q and a constructed mutant lacking all 5 consecutive glutamines (Δ5Q) in COS cells and compared their kinetic properties. All constructs presented identical K(m) and k(cat) values, in agreement with the fact that the mutations did not affect the catalytic domain of the enzyme. In contrast, the mutants produced higher AChE activity, suggesting that a higher proportion of the precursor protein becomes GPI-anchored. An increase in the number of GPI-anchored molecules in the synaptic cleft may reduce the sensitivity to insecticides.

Frequencies of organophosphate resistance-associated mutations in the acetylcholinesterase gene of field collected olive fly (Bactrocera oleae) populations under different insecticide regimes

In the present study, the frequencies of three organophosphate (OP) resistance-associated mutations in acetylcholinesterase gene of Bactrocera oleae (BoAce) populations collected from 8 different important olivegrowing areas in the west part of Turkey were determined. Populations were sampled from the areas that have been treated with only the pyrethroid α-cypermethrin; pyrethroids plus OPs; deltamethrin with pheromone eco-traps, and no insecticide treatment applied areas for many years. For Ile214Val and Gly488Ser point mutations PCR-RFLP and for Δ3Q deletion mutation PCR diagnostic tests were carried out. Seventy-two percent of the total individuals analyzed in the study were exhibited heterozygous genotype (RS) for both Ile214Val and Gly488Ser point and homozygous susceptible genotype (SS) for Δ3Q deletion mutations. This RS/RS/SS combination together with RS/RR/SS with the frequency of 13% were the most common two combinations observed in all of the populations under different insecticide regimes, even in the populations under no insecticide pressure for many years. Independent evaluation of the three mutations resulted in 0.450, 0.534 and 0.037 frequency values for the resistant alleles of 214Val, 488Ser and Δ3Q mutations, respectively. Among the studied populations, the frequencies of resistant alleles for the positions of 214 and 488 were not differed from each other. However, in 3 of the populations the frequency of the R allele of Δ3Q was zero and it changed between 0.025 and 0.100 in the remaining five populations. Results of this study contributed to the distribution pattern of the two point mutations in Europe and a pattern for Δ3Q mutation was determined for the first time in the field collected olive fly samples.

Selective sweeps at the organophosphorus insecticide resistance locus, Rop-1, have affected variation across and beyond the α-esterase gene cluster in the Australian sheep blowfly, Lucilia cuprina

A major theoretical consequence of selection at a locus is the genetic hitchhiking of linked sites (selective sweep). The extent of hitchhiking around a gene is related to the strength of selection and the rate of recombination, with its impact diminishing with distance from the selected site. At the Rop-1 locus of the sheep blowfly, Lucilia cuprina, polymorphisms at two different sites within the LcαE7 gene encode forms of the protein that confer organophosphorus insecticide resistance. To assess the impact of selection at these two sites on variation around LcαE7, we sequenced regions within six other genes along chromosome IV across isogenic (IV) strains of L. cuprina. High levels of linkage disequilibrium, characterized by low haplotype number (K) and diversity (H), and significant R(2) values were observed for two genes, LcαE1 and LcαE10, both members of the same α-esterase gene cluster as LcαE7. A significant R(2) value was also observed for a gene predicted to be the next closest to LcαE7, AL03, but not for any of the other genes, LcRpL13a, Lcdsx, or LcAce. Skews in the site frequency spectra toward high-frequency variants were significant for LcαE1 (Fay and Wu's H = -2.91), LcαE10 (H = -1.85), and Lcdsx (H = -2.00). Since the selective sweeps, two forms of likely returning variation were observed, including variation in microsatellites in an intron of LcαE10 and a recombination event between LcαE7 and LcαE10. These data suggest that two incomplete soft sweeps have occurred at LcαE7 that have significantly affected variation across, and beyond, the α-esterase gene cluster of L. cuprina. The speed and impact of these selective sweeps on surrounding genomic variation and the ability of L. cuprina to respond to future environmental challenges are discussed.

Deep sequencing of New World screw-worm transcripts to discover genes involved in insecticide resistance

BACKGROUND

The New World screw-worm (NWS), Cochliomyia hominivorax, is one of the most important myiasis-causing flies, causing severe losses to the livestock industry. In its current geographical distribution, this species has been controlled by the application of insecticides, mainly organophosphate (OP) compounds, but a number of lineages have been identified that are resistant to such chemicals. Despite its economic importance, only limited genetic information is available for the NWS. Here, as a part of an effort to characterize the C. hominivorax genome and identify putative genes involved in insecticide resistance, we sampled its transcriptome by deep sequencing of polyadenylated transcripts using the 454 sequencing technology.

RESULTS

Deep sequencing on the 454 platform of three normalized libraries (larval, adult male and adult female) generated a total of 548,940 reads. Eighteen candidate genes coding for three metabolic detoxification enzyme families, cytochrome P450 monooxygenases, glutathione S-transferases and carboxyl/cholinesterases were selected and gene expression levels were measured using quantitative real-time polymerase chain reaction (qRT-PCR). Of the investigated candidates, only one gene was expressed differently between control and resistant larvae with, at least, a 10-fold down-regulation in the resistant larvae. The presence of mutations in the acetylcholinesterase (target site) and carboxylesterase E3 genes was investigated and all of the resistant flies presented E3 mutations previously associated with insecticide resistance.

CONCLUSIONS

Here, we provided the largest database of NWS expressed sequence tags that is an important resource, not only for further studies on the molecular basis of the OP resistance in NWS fly, but also for functional and comparative studies among Calliphoridae flies. Among our candidates, only one gene was found differentially expressed in resistant individuals, and its role on insecticide resistance should be further investigated. Furthermore, the absence of mutations in the OP target site and the high frequency of mutant carboxylesterase E3 indicate that metabolic resistance mechanisms have evolved predominantly in this species.

Acetylcholinesterase cDNA sequencing and identification of mutations associated with organophosphate resistance in Cochliomyia hominivorax (Diptera: Calliphoridae)

Altered acetylcholinesterase (AChE) has been identified in numerous arthropod species resistant to organophosphate (OP) and carbamate insecticides. The New World screwworm (NWS) Cochliomyia hominivorax (Coquerel), one of the most important myiasis-causing flies in the Neotropics, has been controlled mainly by the application of OP insecticides in its current geographical distribution. However, few studies have investigated insecticide resistance in this species. Based on previous studies about mutations conferring OP resistance in related dipteran species, AChE cDNA was sequenced allowing a survey for mutations (I298V, G401A, F466Y) in NWS populations. In addition, the G137D mutation in the carboxylesterase E3 gene, also associated with OP resistance, was analyzed in the same NWS populations. Only 2/135 individuals presented an altered AChE gene (F466Y). In contrast, a high frequency of the G137D mutation in the E3 gene was found in some localities of Brazil and Uruguay, while the mutant allele was not found in Cuba, Venezuela or Colombia. These findings suggest that the alteration in the carboxylesterase E3 gene may be one of the main resistance mechanisms selected in this ectoparasite. The knowledge of the frequency of these resistance-associated mutations in the NWS natural populations may contribute to the selection of appropriate chemicals for control as part of pest management strategies.

Comparative studies of acetylcholinesterase purified from three field populations of Liposcelis entomophila (enderlein) (psocoptera: liposcelididae)

Acetylcholinesterace (AChE) is known to be the major target for organophophate and carbamate insecticides and biomolecular changes to AChE have been demonstrated to be an important mechanism for insecticide resistance in many insect species. In this study, AChE from three field populations of Liposcelis entomophila (Enderlein) (Psocoptera: Liposcelididae) was purified by affinity chromatography and subsequently characterized by its Michaelis-Menten kinetics to determine if detectable changes to AChE have occurred. Bioassays revealed that the potential resistance threat of psocids in Sichuan Province (GH) was greater than either Hubei Province (WH) or Chongqing Municipality (BB). Compared to the other two populations, the WH population possessed the highest specific activity of purified AChE. Kinetic analyses indicated that the purified AChE from GH population expressed a significantly lower affinity to the substrate and a higher catalytic activity toward acetylthiocholine iodide (ATChI) (i.e., higher K(m) and V(max) values) than BB and WH populations. In vitro studies of AChE suggest that five inhibitors (aldicarb, eserine, BW284C51, omethoate, and propoxur) all possess strong inhibitory effects with eserine having the strongest inhibitory effect against purified AChE. According to bimolecular rate constants (k(i)), the purified AChE from GH population was least sensitive to all inhibitors except for omethoate. The differences in AChE among the three populations may be partially attributed to the differences in pesticide application and control practices for psocids among the three locations.

Molecular characterization and inhibition analysis of the acetylcholinesterase gene from the silkworm maggot, Exorista sorbillans

Several organophosphorus (OP) insecticides can selectively kill the silkworm maggot, Exorista sorbillans (Es) (Diptera: Tachinidae), while not obviously affecting the host (Bombyx mori) larvae, but the mechanism is not yet clear. In this study, the cDNA encoding an acetylcholinesterase (AChE) from the field Es was isolated. One point mutation (Gly353Ala) was identified. The Es-353G AChE and Es-353A AChE were expressed in baculovirus- insect cell system, respectively. The inhibition results showed that for eserine and Chlorpyrifos, Es-353A AChE was significantly less sensitive than Es-353G AChE. Meanwhile, comparison of the I(50) values of eserine, dichlorvos, Chlorpyrifos and omethoate of recombinant Es AChEs with its host (Bombyx mori) AChEs indicated that, both Es AChEs are more sensitive than B. mori AChEs. The results give an insight of the mechanism that some OP insecticides can selectively kills Es while without distinct effect on its host, B. mori.

Genotyping mutations in BmAChE3: A survey of organophosphate-resistant and -susceptible strains of Rhipicephalus (Boophilus) microplus

Mutations I48L, I54V, R86Q, V137I, I492M, and T548A were identified previously in BmAChE3, a gene encoding acetylcholinesterase, from the organophosphate (OP) acaricide-resistant San Rommn strain of Rhipicephalus (Boophilus) microplus. Recombinant BmAChE3 acetylcholinesterase containing the R86Q mutation was shown to exhibit nearly 20-fold reduction in the rate of phosphorylation by paraoxon relative to the wild-type sequence. In addition, the R86Q mutation was present in resistant laboratory strains at elevated frequency compared with OP-susceptible strains but was insufficient to alone generate the OP-resistant phenotype (J. Med. Entomol. 44: 1013-1018). Here, we developed assays to genotype the remaining five mutations and evaluated frequency of all six BmAChE3 mutations in individual R. microplus ticks from laboratory and Mexican field-collected strains. We found a substantial number of individuals in known OP-susceptible strains that seemed to be homozygous for each of the mutations surveyed, the exception being I48L, which was infrequent in all strains, leading us to conclude that none of the mutations alone were responsible for generation of phenotypic resistance to OP acaricide.

Recombinant expression and biochemical characterization of the catalytic domain of acetylcholinesterase-1 from the African malaria mosquito, Anopheles gambiae

Acetylcholinesterases (AChEs) and their genes from susceptible and resistant insects have been extensively studied to understand the molecular basis of target site insensitivity. Due to the existence of other resistance mechanisms, however, it can be problematic to correlate directly a mutation with the resistant phenotype. An alternative approach involves recombinant expression and characterization of highly purified wild-type and mutant AChEs, which serves as a reliable platform for studying structure-function relationships. We expressed the catalytic domain of Anopheles gambiae AChE1 (r-AgAChE1) using the baculovirus system and purified it 2,500-fold from the conditioned medium to near homogeneity. While K(M)'s of r-AgAChE1 were comparable for ATC, AbetaMTC, PTC, and BTC, V(max)'s were substantially different. The IC(50)'s for eserine, carbaryl, paraoxon, BW284C51, malaoxon, and ethopropazine were 8.3, 72.5, 83.6, 199, 328, and 6.59 x 10(4) nM, respectively. We determined kinetic constants for inhibition of r-AgAChE1 by four of these compounds. The enzyme bound eserine or paraoxon stronger than carbaryl or malaoxon. Because the covalent modification of r-AgAChE1 by eserine occurred faster than that by the other compounds, eserine is more potent than paraoxon, carbaryl, and malaoxon. Furthermore, we found that choline inhibited r-AgAChE1, a phenomenon related to the enzyme activity decrease at high concentrations of acetylcholine.