Estimation of allele-specific Ace-1 duplication in insecticide-resistant Anopheles mosquitoes from West Africa.

Luc S Djogbénou,Benoît Assogba,Edi A V Constant,David Weetman,Martin Akogbéto,Michel Makoutodé,Martin J Donnelly,John Essandoh

doi:10.1186/s12936-015-1026-3

Luc S Djogbénou, Benoît Assogba + Show 6 more

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https://doi.org/10.1186/s12936-015-1026-3

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Abstract

Background Identification of variation in Ace-1 copy number and G119S mutation genotype from samples of Anopheles gambiae and Anopheles coluzzii across West Africa are important diagnostics of carbamate and organophosphate resistance at population and individual levels. The most widespread and economical method, PCR–RFLP, suffers from an inability to discriminate true heterozygotes from heterozygotes with duplication. MethodsIn addition to PCR–RFLP, in this study three different molecular techniques were applied on the same mosquito specimens: TaqMan qPCR, qRTPCR and ddPCR. To group heterozygous individuals recorded from the PCR–RFLP analysis into different assumptive genotypes K-means clustering was applied on the Z-scores of data obtained from both the TaqMan and ddPCR methods. The qRTPCR analysis was used for absolute quantification of copy number variation.ResultsThe results indicate that most heterozygotes are duplicated and that G119S mutation must now be regarded as a complex genotype ranging from primarily single-copy susceptible Glycine homozygotes to balanced and imbalanced heterozygotes, and multiply-amplified resistant Serine allele homozygotes. Whilst qRTPCR-based gene copy analysis suffers from some imprecision, it clearly illustrates differences in copy number among genotype groups identified by TaqMan or ddPCR. Based on TaqMan method properties, and by coupling TaqMan and ddPCR methods simultaneously on the same type of mosquito specimens, it demonstrated that the TaqMan genotype assays associated with the K-means clustering algorithm could provide a useful semi-quantitative estimate method to investigate the level of allele-specific duplication in mosquito populations.ConclusionsAce-1 gene duplication is evidently far more complex in An. gambiae and An. coluzzii than the better-studied mosquito Culex quinquefasciatus, which consequently can no longer be considered an appropriate model for prediction of phenotypic consequences. These require urgent further evaluation in Anopheles. To maintain the sustained effectiveness carbamates and organophosphates as alternative products to pyrethroids for malaria vector control, monitoring of duplicated resistant alleles in natural populations is essential to guide the rational use of these insecticides.Electronic supplementary materialThe online version of this article (doi:10.1186/s12936-015-1026-3) contains supplementary material, which is available to authorized users.

Highlights

Identification of variation in Ace-1 copy number and G119S mutation genotype from samples of Anopheles gambiae and Anopheles coluzzii across West Africa are important diagnostics of carbamate and organophosphate resistance at population and individual levels
Mutations in the Ace-1 gene, which codes for AChE in insects such as the primary African malaria vector Anopheles gambiae species pair (Anopheles gambiae s.s. and Anopheles coluzzii), can confer resistance to carbamate and organophosphate compounds [9]
polymerase chain reaction (PCR)–RFLP Ace-1 genotypes were first determined by using the available polymerase-chain reaction-restriction fragment length polymorphism (PCR–RFLP) analysis

Summary

Introduction

Identification of variation in Ace-1 copy number and G119S mutation genotype from samples of Anopheles gambiae and Anopheles coluzzii across West Africa are important diagnostics of carbamate and organophosphate resistance at population and individual levels. Of the mutations in the coding sequence of the Ace-1 gene recorded to date, only one, G119S (a single amino acid substitution, from a glycine to a serine at the position 119 in the AChE catalytic site using Torpedo californica nomenclature), is found in Anopheles, and causes strong resistance to both organophosphates and carbamates [10, 11]. This substitution to the resistant allele (Ace-1R) has a high fitness costs in insecticide-free environments [12, 13]. Duplicated alleles have evolved on several occasions in Culex which link a resistant allele (Ace-1R) with a susceptible allele (Ace-1S) on the same chromosome in permanent ‘heterozygosity’ [14] alleviating significant costs in field populations [15]

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