Résumé
We investigated the genetic determinism of high chlorpyrifos resistance
(HCR), a phenotype first described in 1999 in Culex pipiens mosquitoes
surviving chlorpyrifos doses greater than or equal to1 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-1R 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-1R 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-1V or the duplicated
ace-1D 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.