Chlorpyrifos resistance is associated with mutation and amplification of the acetylcholinesterase-1 gene in the tomato red spider mite, Tetranychus evansi

The tomato red spider mite, Tetranychus evansi is an invasive pest of many African countries where it causes significant damage to a range of solanaceous crops. In Malawi the control of T. evansi relies heavily on the use of chemical pesticides and this species has evolved resistance to members of the pyrethroid and organophosphate (OP) classes. In this study the molecular mechanisms underlying resistance to the organophosphate chlorpyrifos were investigated in two resistant strains of T. evansi from Malawi and France. Cloning and sequencing of the gene encoding the OP target (ace-1) revealed an amino acid substitution at just one of the positions (331) previously implicated in OP resistance across a range of different insect and mite species. The amino acid residue usually found at this position in susceptible insects and mites is a phenylalanine (F) but was a tyrosine (Y) in all sequenced clones of the France strain and a tyrosine or tryptophan (W) in sequenced clones of the Malawi strain. Additionally we found evidence that the ace-1 locus is amplified in the resistant strains, with direct measurement of gene copy number by quantitative PCR showing there are around 8–10 copies of the gene in both the France and the Malawi strain. Sequencing of clones of ace-1 from the Malawi strain indicated that individual mites have fewer copies of the W331 allele than the Y331 allele. The enhanced copy number of the ace-1 gene in T. evansi and copy number variation of the two alleles seen in the Malawi strain may be a mechanism to compensate for fitness costs associated with the mutant alleles as has been proposed for T. urticae.

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► Two Tetranychus evansi strains showed resistance to chlorpyrifos.
► Resistance was associated with mutation (Y331 and/or W331) of the ace-1 gene.
► Both strains also carry multiple copies of the ace-1 gene.
► Individual mites of the Malawi strain have fewer copies of W331 than Y331.
► This may be a mechanism to compensate for fitness costs associated with W331.