High chlorpyrifos resistance in Culex pipiens mosquitoes: strong synergy between resistance genes

We investigated the genetic determinism of high chlorpyrifos resistance (HCR), a phenotype first described in 1999 in Culex pipiens mosquitoes surviving chlorpyrifos doses ⩾1 mg l(-1) and more recently found in field samples from Tunisia, Israel or Indian Ocean islands. Through chlorpyrifos selection, we selected several HCR strains that displayed over 10 000-fold resistance. All strains were homozygous for resistant alleles at two main loci: the ace-1 gene, with the resistant ace-1(R) allele expressing the insensitive G119S acetylcholinesterase, and a resistant allele of an unknown gene (named T) linked to the sex and ace-2 genes. We constructed a strain carrying only the T-resistant allele and studied its resistance characteristics. By crossing this strain with strains harboring different alleles at the ace-1 locus, we showed that the resistant ace-1(R) and the T alleles act in strong synergy, as they elicited a resistance 100 times higher than expected from a simple multiplicative effect. This effect was specific to chlorpyrifos and parathion and was not affected by synergists. We also examined how HCR was expressed in strains carrying other ace-1-resistant alleles, such as ace-1(V) or the duplicated ace-1(D) allele, currently spreading worldwide. We identified two major parameters that influenced the level of resistance: the number and the nature of the ace-1-resistant alleles and the number of T alleles. Our data fit a model that predicts that the T allele acts by decreasing chlorpyrifos concentration in the compartment targeted in insects.

Acetylcholinesterase point mutations in European strains of Tetranychus urticae (Acari: Tetranychidae) resistant to organophosphates

BACKGROUND

In Tetranychus urticae Koch, acetylcholinesterase insensitivity is often involved in organophosphate (OP) and carbamate (CARB) resistance. By combining toxicological, biochemical and molecular data from three reference laboratory and three OP selected strains (OP strains), the AChE1 mutations associated with resistance in T. urticae were characterised.

RESULTS

The resistance ratios of the OP strains varied from 9 to 43 for pirimiphos-methyl, from 78 to 586 for chlorpyrifos, from 8 to 333 for methomyl and from 137 to 4164 for dimethoate. The insecticide concentration needed to inhibit 50% of the AChE1 activity was, in the OP strains, at least 2.7, 55, 58 and 31 times higher for the OP pirimiphos-methyl, chlorpyrifos oxon, paraoxon and omethoate respectively, and 87 times higher for the CARB carbaryl. By comparing the AChE1 sequence, four amino acid substitutions were detected in the OP strains: (1) F331W (Torpedo numbering) in all the three OP strains; (2) T280A found in the three OP strains but not in all clones; (3) G328A, found in two OP strains; (4) A201S found in only one OP strain.

CONCLUSIONS

Four AChE1 mutations were found in resistant strains of T. urticae, and three of them, F331W, G328A and A201S, are possibly involved in resistance to OP and CARB insecticides. Among them, F331W is probably the most important and the most common in T. urticae. It can be easily detected by the diagnostic PCR-RLFP assay developed in this study.

Multiple duplications of the rare ace-1 mutation F290V in Culex pipiens natural populations

Two amino acid substitutions in acetylcholinesterase 1 (AChE1), G119S and F290V, are responsible for resistance to organophosphate and carbamate insecticides in Culex pipiens mosquitoes. These mutations generate very different levels of insensitivity to insecticide inhibitors. We described here a biochemical method that rapidly identifies AChE1 variants (susceptible, G119S and F290V, named S, R and V, respectively) present in individual mosquitoes. We investigated the frequency of AChE1 phenotypes in 41 field samples collected around the Mediterranean Sea. F290V substitution was found only in 15 samples and at low frequency, whereas G119S was highly spread in all samples. However, seven V distinct alleles were identified whereas only one R allele was present. The [V] enzymatic phenotype was never observed alone, and the V allele was always found associated with the susceptible and/or G119S AChE1 ([VS], [VR] or [VRS] phenotypes). Furthermore, we showed the presence of duplicated alleles, associating a susceptible and a V copy of the ace-1 gene, in most individuals analyzed for its presence. Evolutionary forces driving the large number of F290V ace-1 alleles and their low frequency in Mediterranean countries are discussed.

Comparison of Anopheles gambiae and Culex pipiens acetycholinesterase 1 biochemical properties

Selection of insensitive acetycholinesterase 1 (AChE1) has occurred in several mosquito species controlled with carbamate (CX) and organophosphate (OP) insecticides. In case of pyrethroid resistance, these insecticides represent an alternative for disease vector control program. Their heavy use in agriculture has selected resistant populations of Anopheles gambiae in West Africa. The evolution of resistance has to be studied to prevent, or at least slow down, the spread of resistant mosquito in wild populations. An. gambiae shares the same resistance mechanism to CX and OP insecticides as Culex pipiens, which was attributed to the G119S substitution in the AChE1 enzyme. By comparing resistant AChE1 from both species, we show here that similar resistance levels are obtained toward 10 insecticides of both classes. Moreover, similar AChE1 activity levels are recorded between either susceptible or resistant mosquitoes of both species. Enzymes belonging to both species seem thus to share identical properties. Consequently, we hypothesize that fitness cost associated with AChE1 insensitivity in C. pipiens mosquitoes should be similar in An. gambiae and thus be used in strategies to control resistant populations where malaria is prevalent.