The evolution of a-esterase mediated organophosphate resistance in Musca domestica

Abstract

Ten carboxylesterase genes were cloned and sequenced from the house fly Musca domestica, and were found to be related to a-esterase genes in Drosophila melanogaster and Lucilia Cuprina. Gene sequence comparisons indicated that a series of gene duplication events occurred in the evolution of the a-esterase cluster in the Diptera that must have preceded the divergence of the Drosophilidae and Muscoidea, 80-100 Mya. An encompassing carboxylesterase phylogeny from bacteria to humans and a genome assessment of the Caenorhabditis elegans DNA sequencing project confirmed a recurring pattern of gene duplication in the evolution of the carboxylesterase superfamily. Gene sequence analysis of the MdaE7 carboxylesterase gene in house flies revealed an intriguing difference between the alleles in organophosphate (OP) -resistant and - susceptible flies. The MdaE7 enzyme of the Rutgers OP-resistant strain had a mutation that resulted in a Gly137->Asp change. This same mutation was previously observed in the orthologous gene (LcaE7) in sheep blowflies, and reported to convert the enzyme from a carboxylesterase to an OP hydrolase and confers insecticide resisitance on L. Cuprina. Bacterial expression of the Rutgers allele shows its product also has OP hydrolase activity. The data indicate identical catalytic mechanisms have evolved in orthologous MdaE7 and LcaE7 molecules to endow diazinon-type resistance on the two species of higher Diptera. Two MdaE7 malathion-resistant alleles were also identified and characterised. Structural mutations (Trp251->Ser and Trp251->Leu) in different alleles of MdaE7 were again equivalent to the LcaE7 (Trp251->Leu) mutation in the blowfly associated with increased malathion carboxylesterase activity. A novel transposable element from the house fly discovered in this study, dubbed "Theseus", was subsequently identified from the distantly related soil nematode Caenorhabditis elegans. Phylogenetic reconstructions show Theseus is derived from the basal node of the Tel-mariner family of transposable elements. Interestingly, a Theseus element is closely associated with the MdaE7 allele from the Rutgers strain, and this novel association of transposable element and OP-resistance gene may be implicated in trans-regulatory effects on other metabolic resistance genes. Population studies show a significant selective advantage associated with, and possible worldwide sweep by, the Rutgers diazinon resistance allele.