Amplification of a serine esterase gene is involved in insecticide resistance in Sri Lankan Culex tritaeniorhynchus

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

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

Carboxylesterase-mediated insecticide resistance: Quantitative increase induces broader metabolic resistance than qualitative change

Carboxylesterases are mainly involved in the mediation of metabolic resistance of many insects to organophosphate (OP) insecticides. Carboxylesterases underwent two divergent evolutionary events: (1) quantitative mechanism characterized by the overproduction of carboxylesterase protein; and (2) qualitative mechanism caused by changes in enzymatic properties because of mutation from glycine/alanine to aspartate at the 151 site (G/A151D) or from tryptophan to leucine at the 271 site (W271L), following the numbering of Drosophila melanogaster AChE. Qualitative mechanism has been observed in few species. However, whether this carboxylesterase mutation mechanism is prevalent in insects remains unclear. In this study, wild-type, G/A151D and W271L mutant carboxylesterases from Culex pipiens and Aphis gossypii were subjected to germline transformation and then transferred to D. melanogaster. These germlines were ubiquitously expressed as induced by tub-Gal4. In carboxylesterase activity assay, the introduced mutant carboxylesterase did not enhance the overall carboxylesterase activity of flies. This result indicated that G/A151D or W271L mutation disrupted the original activities of the enzyme. Less than 1.5-fold OP resistance was only observed in flies expressing A. gossypii mutant carboxylesterases compared with those expressing A. gossypii wild-type carboxylesterase. However, transgenic flies universally showed low resistance to OP insecticides compared with non-transgenic flies. The flies expressing A. gossypii W271L mutant esterase exhibited 1.5-fold resistance to deltamethrin, a pyrethroid insecticide compared with non-transgenic flies. The present transgenic Drosophila system potentially showed that a quantitative increase in carboxylesterases induced broader resistance of insects to insecticides than a qualitative change.

Gene amplification and insecticide resistance

Pesticide resistance in arthropods has been shown to evolve by two main mechanisms, the enhanced production of metabolic enzymes, which bind to and/or detoxify the pesticide, and mutation of the target protein, which makes it less sensitive to the pesticide. One route that leads to enhanced metabolism is the duplication or amplification of the structural gene(s) encoding the detoxifying enzyme, and this has now been described for the three main families (esterases, glutathione S-transferases and cytochrome P450 monooxygenases) implicated in resistance. More recently, a direct or indirect role for gene duplication or amplification has been described for target-site resistance in several arthropod species. This mini-review summarises the involvement of gene duplication/amplification in the insecticide/acaricide resistance of insect and mite pests and highlights recent developments in this area in relation to P450-mediated and target-site resistance.

Two single mutations commonly cause qualitative change of nonspecific carboxylesterases in insects

Carboxylesterases provide key mechanisms of resistance to insecticides, particularly organophosphates (OPs), in insects. One resistance mechanism is a qualitative change in the properties of a carboxylesterase. Two mutant forms, G151D and W271L, have been observed, mostly in dipteran species, to affect substrate specificity of enzymes. But whether these two single mutations can commonly change character of insect carboxylesterases is unknown. In our study carboxylesterase genes from seven insects distributed among four orders were cloned, mutated at position 151 or 271 and expressed in Escherichia coli. The kinetics of the purified recombinant proteins was examined towards an artificial carboxylester and two OP insecticides. The G/A151D and W271L mutation significantly reduced carboxylesterase activity in 87.5% and 100% cases, respectively, and at the same time conferred OP hydrolase activities in 62.5% and 87.5% cases, respectively. Thus, the change at position 271 is more effective to influence substrate specificity than that at position 151. These results may suggest that these two mutations have the potential to cause insecticide resistance broadly in insects.

Characterization of biochemical based insecticide resistance mechanism by thermal bioassay and the variation of esterase activity in Culex quinquefasciatus

Biochemical mechanisms of insecticide resistance of thermal exposed and unexposed Culex quinquefasciatus strains are evaluated, which were not studied earlier. The activity of alpha- and beta-carboxylesterases and acetylcholinesterase of malathion susceptible and resistant strains were compared after thermal treatment. Three-day-old adult females were used for the malathion susceptibility test and biochemical assays, and males were used only for the susceptibility test. Thermal exposure brought about increase in resistance levels from 85% to 90% in males and 91% to 96.6% in females of resistant strain. The resistance status of the susceptibility strain was unchanged after thermal exposure. The activities of alpha- and beta-carboxylesterase of susceptible mosquitoes were within 800 and 700 U/mg protein, respectively. The alpha-carboxylesterase activity of the thermal exposed malathion-resistant population was significantly (t test, P < 0.05) higher than the unexposed resistant population, and the reverse was recorded in beta-carboxylesterase. The alpha-carboxylesterase activity of susceptible population was lower than the resistant population. The activity of alpha-carboxylesterase was higher than the beta-carboxylesterase in both the strains. Among the malathion resistant C. quinquefasciatus population, 2.3% population exhibited 30-40% inhibition which increased to 5.8% after the thermal exposure. Thermal exposure of mosquitoes increased the activity of both alpha-carboxylesterases and acetylcholinesterase but decreased the activity of beta-carboxylesterase.

Molecular mechanisms of metabolic resistance to synthetic and natural xenobiotics

Xenobiotic resistance in insects has evolved predominantly by increasing the metabolic capability of detoxificative systems and/or reducing xenobiotic target site sensitivity. In contrast to the limited range of nucleotide changes that lead to target site insensitivity, many molecular mechanisms lead to enhancements in xenobiotic metabolism. The genomic changes that lead to amplification, overexpression, and coding sequence variation in the three major groups of genes encoding metabolic enzymes, i.e., cytochrome P450 monooxygenases (P450s), esterases, and glutathione-S-transferases (GSTs), are the focus of this review. A substantial number of the adaptive genomic changes associated with insecticide resistance that have been characterized to date are transposon mediated. Several lines of evidence suggest that P450 genes involved in insecticide resistance, and perhaps insecticide detoxification genes in general, may share an evolutionary association with genes involved in allelochemical metabolism. Differences in the selective regime imposed by allelochemicals and insecticides may account for the relative importance of regulatory or structural mutations in conferring resistance.

Immunological assessment of an insecticide resistance-associated esterase in the Western corn rootworm

In previous investigations, we have determined that organophosphate resistance in the western corn rootworm, Diabrotica virgifera virgifera, is at least partially attributed to a group of non-specific carboxylesterases referred to as group II. Antiserum raised against a purified 66-kDa group II esterase is specific for the denatured enzyme. This antiserum reacts similarly with both beetle homogenates from resistant and susceptible populations, although there is much higher signal intensity in immunoblots of resistant relative to susceptible beetles. These results suggest that overproduction of group II esterases is the underlying basis of esterase-mediated resistance in D. v. virgifera by demonstrating that (1) group II esterases are immunologically indistinguishable between the resistant and susceptible populations, and (2) the intensity differences are due to increased group II esterase proteins in the resistant population. The diagnostic potential of immunological-based assays was tested with a traditional diagnostic concentration bioassay and a biochemical-based native PAGE assay. Significant correlations were observed among all three diagnostic assays (regression coefficients ranging from 0.95 to 0.96). These results demonstrate the importance of the 66-kDa protein as a resistance-associated biochemical marker, thus emphasizing the potential for 66-kDa protein-targeted immunoassays in resistance monitoring programs.

Mechanisms of resistance to DDT and pyrethroids in Patagonian populations of Simulium blackflies

Mixed populations of the pest blackflies Simulium bonaerense Coscarón & Wygodzinsky, S. wolffhuegeli (Enderlein) and S. nigristrigatum Wygodzinsky & Coscarón (Diptera: Simuliidae) are highly resistant to DDT and pyrethroids in the Neuquén Valley, a fruit-growing area of northern Patagonia, Argentina. As these insecticides have not been used for blackfly control, resistance is attributed to exposure to agricultural insecticides. Pre-treatment with the synergist piperonyl butoxide (PBO) reduced both DDT and fenvalerate resistance, indicating that resistance was partly due to monooxygenase inhibition. Pre-treatment with the synergist tribufos to inhibit esterases slightly increased fenvalerate toxicity in the resistant population. Even so, biochemical studies indicated almost three-fold higher esterase activity in the resistant population, compared to the susceptible. Starch gel electrophoresis confirmed higher frequency and staining intensity of esterase electromorphs in the resistant population. Incomplete synergism against metabolic resistance indicates additional involvement of a non-metabolic resistance mechanism, such as target site insensitivity, assumed to be kdr-like in this case. Glutathione S-transferase activities were low and inconsistent, indicating no role in Simulium resistance. Knowing these spectra of insecticide activity and resistance mechanisms facilitates the choice of more effective products for Simulium control and permits better coordination with agrochemical operations.

Variation in general esterase activity within a population of Haematobia irritans (Diptera: Muscidae)

Control of the horn fly, Hematobia irritans (L.), is generally dependent on chemical insecticides. However, the biology and behavior of the horn fly favors rapid development of insecticide resistance. To prolong the effectiveness of the insecticide option, information is required regarding the mechanisms of insecticide resistance. Metabolic hydrolysis of insecticides by esterases is a detoxification mechanism in many insect species. Measurement of general esterase activity within populations of horn flies may provide a diagnostic tool for resistance management. In this study we evaluated the amount of variation in general esterase activity within female and male horn fly samples from a population that had not been exposed to insecticides for 8 yr. We found considerable variation in general esterase activity within samples of each sex, with females demonstrating the greater variation. The observed variation is thought to be the result of age-structure dynamics within the population. The amount of inherent variation makes it difficult to detect small mean differences between populations, thus limiting the utility of general esterase assays. Thus, effective diagnosis of esterase-mediated resistance mechanisms can only be achieved by the identification of specific detoxification esterases and the design of assays, either biochemical or molecular, for their detection and measurement.

Comparative linkage map development and identification of an autosomal locus for insensitive acetylcholinesterase-mediated insecticide resistance in Culex tritaeniorhynchus

A comparative linkage map for Culex tritaeniorhynchus was constructed based on restriction fragment length polymorphism markers using cDNAs from Aedes aegypti. Linear orders of marker loci in Cx. tritaeniorhynchus were identical to Culex pipiens wherein chromosomes 2 and 3 reflect whole-arm rearrangements compared to A. aegypti. However, the sex determination locus in Cx. tritaeniorhynchus maps to chromosome 3, in contrast to Cx. pipiens and Ae. aegypti where it is located on chromosome 1. Our results indicate that insensitive acetylcholinesterase (AChE)-mediated organophosphate resistance is controlled by a single major gene (AChE) on chromosome 2, while the AChE structural gene (Ace) is located on chromosome 1. No evidence for a second Ace gene was observed, even under very low stringency hybridization conditions.

Insecticide resistance spectra and resistance mechanisms in populations of Japanese encephalitis vector mosquitoes, Culex tritaeniorhynchus and Cx. gelidus, in Sri Lanka

Culex tritaeniorhynchus Giles and Cx. gelidus Theobald (Diptera: Culicidae), both vectors of Japanese encephalitis, were collected in 1984 and 1998 from two disease endemic localities in Sri Lanka: Anaradhapura and Kandy. Using wild-caught adult mosquitoes from light traps, log dosage-probit mortality curves for insecticide bioassays were obtained for three insecticides: malathion (organophosphate), propoxur (carbamate) and permethrin (pyrethroid). LD50 values showed that, in 1998, Cx. tritaeniorhynchus was -100-fold more resistant to malathion and 10-fold more resistant to propoxur than was Cx. gelidus. This difference was attributed to Cx. tritaeniorhynchus breeding mostly in irrigated rice paddy fields, where it would have been exposed to pesticide selection pressure, whereas Cx. gelidus breeds in other types of aquatic habitats less prone to pesticide applications. Resistance in Cx. tritaeniorhynchus increased between 1984 and 1998, whereas Cx. gelidus remained predominantly susceptible. Propoxur inhibition of acetylcholinesterase (AChE) activity (the target site of organophosphates and carbamates) indicated that in 1998, frequencies of insensitive AChE-based resistance were 9% in Cx. gelidus and 2-23% in Cx. tritaeniorhynchus, whereas in 1984 this resistance mechanism was detected only in 2% of the latter species from Anaradhapura. The AChE inhibition coefficient (ki) with propoxur was 1.86+/-0.24 x 10(5) M(-)1 min(-1) for Cx. tritaeniorhynchus from Anaradhapura in 1998. Both species were tested for activity levels of detoxifying glutathione S-transferases (GSTs) and malathion-specific as well as general carboxylesterases. High activities of GSTs and carboxylesterases were detected in Cx. tritaeniorhynchus but not Cx. gelidus. Malathion-specific carboxylesterase was absent from both species. Native polyacrylamide gel electrophoresis resolved two elevated general carboxylesterases, CtrEstbeta1 and CtrEstalpha1, from Cx. tritaeniorhynchus and none from Cx. gelidus. CtrEstbeta1 was the most intensely staining band. Gel inhibition experiments showed that both elevated esterases were inhibited by organophosphates and carbamates but not by pyrethroids. The major elevated esterase CtrEstbeta1 was partially purified (15-fold) by sequential Q-Sepharose and phenyl Sepharose column chromatography. The bimolecular rate constant (ka) and the deacylation rate constant (k3) for the malaoxon/enzyme interaction were 9.9+/-1.1 x 10(3) M(-1) min(-1) and 3.5+/-0.05 x 10(-4) M(-1) min(-1), respectively, demonstrating that the role of this enzyme in organophosphorus insecticide resistance is sequestration.

The molecular basis of two contrasting metabolic mechanisms of insecticide resistance

The esterase-based insecticide resistance mechanisms characterised to date predominantly involve elevation of activity through gene amplification allowing increased levels of insecticide sequestration, or point mutations within the esterase structural genes which change their substrate specificity. The amplified esterases are subject to various types of gene regulation in different insect species. In contrast, elevation of glutathione S-transferase activity involves upregulation of multiple enzymes belonging to one or more glutathione S-transferase classes or more rarely upregulation of a single enzyme. There is no evidence of insecticide resistance associated with gene amplification in this enzyme class. The biochemical and molecular basis of these two metabolically-based insecticide resistance mechanisms is reviewed.

Insecticide resistance in insect vectors of human disease

Insecticide resistance is an increasing problem in many insect vectors of disease. Our knowledge of the basic mechanisms underlying resistance to commonly used insecticides is well established. Molecular techniques have recently allowed us to start and dissect most of these mechanisms at the DNA level. The next major challenge will be to use this molecular understanding of resistance to develop novel strategies with which we can truly manage resistance. State-of-the-art information on resistance in insect vectors of disease is reviewed in this context.

The role of gene splicing, gene amplification and regulation in mosquito insecticide resistance

The primary routes of insecticide resistance in all insects are alterations in the insecticide target sites or changes in the rate at which the insecticide is detoxified. Three enzyme systems, glutathione S-transferases, esterases and monooxygenases, are involved in the detoxification of the four major insecticide classes. These enzymes act by rapidly metabolizing the insecticide to non-toxic products, or by rapidly binding and very slowly turning over the insecticide (sequestration). In Culex mosquitoes, the most common organophosphate insecticide resistance mechanism is caused by co-amplification of two esterases. The amplified esterases are differentially regulated, with three times more Est beta 2(1) being produced than Est alpha 2(1). Cis-acting regulatory sequences associated with these esterases are under investigation. All the amplified esterases in different Culex species act through sequestration. The rates at which they bind with insecticides are more rapid than those for their non-amplified counterparts in the insecticide-susceptible insects. In contrast, esterase-based organophosphate resistance in Anopheles is invariably based on changes in substrate specificities and increased turnover rates of a small subset of insecticides. The up-regulation of both glutathione S-transferases and monooxygenases in resistant mosquitoes is due to the effects of a single major gene in each case. The products of these major genes up-regulate a broad range of enzymes. The diversity of glutathione S-transferases produced by Anopheles mosquitoes is increased by the splicing of different 5' ends of genes, with a single 3' end, within one class of this enzyme family. The trans-acting regulatory factors responsible for the up-regulation of both the monooxygenase and glutathione S-transferases still need to be identified, but the recent development of molecular tools for positional cloning in Anopheles gambiae now makes this possible.