The nicotinic acetylcholine receptor subunits Mdα5 and Mdβ3 on autosome 1 of Musca domestica are not involved in spinosad resistance

Spinosad is a relatively new insecticide that exerts its toxic action via nicotinic acetylcholine receptors (nAChRs). Spinosad resistance in house flies appears to be due to an altered target site. To evaluate the molecular basis of spinosad resistance, two nAChR subunit genes, Mdα5 and Mdβ3 were cloned and characterized from an insecticide-susceptible (aabys) and spinosad resistant (rspin) strain of the house fly, Musca domestica. The Mdα5 and Mdβ3 cDNAs encode proteins of 781 and 432 amino acid residues, respectively. Phylogenetic analysis with insect nAChR subunits suggested that Mdα5 and Mdβ3 are most closely related to Dα5 and Dβ3 of Drosophila melanogaster, respectively. Mdβ3 is intronless, which is unique among all previously described nAChR genes. A-to-I RNA editing was found at 13 sites in Mdα5, eleven of which resulted in amino acid substitutions. No evidence for A-to-I RNA editing was found in Mdβ3. Mdα5 expression, quantified by real time PCR, was 340and 23-fold higher in the head and thorax than in the abdomen. Mdβ3 expression was more uniform, being only 2.4-fold higher in the head and 1.4-fold lower in the thorax, compared to the abdomen. There was no difference in the expression of Mdα5 and Mdβ3 between the aabys and rspin strains. Although Mdα5 and Mdβ3 both map to the same chromosome as spinosad resistance, there were no unique features of either gene in rspin, relative to the aabys strain. This suggests neither Mdα5 nor Mdβ3 is responsible for spinosad resistance in house flies.