THE INDEPENDENT GENE AMPLIFICATION OF ELECTROPHORETICALLY INDISTINGUISHABLE B-ESTERASES FROM THE INSECTICIDE-RESISTANT MOSQUITO CULEX-QUINQUEFASCIATUS

Resistance to organophosphates in Culex mosquitoes is typically associated with increased activity of non-specific esterases. The commonest phenotype involves two elevated esterases, A(2) and B-2, while some strains have elevation of esterase B-1 alone. Overexpression of the two B esterase electromorphs is due to gene amplification. Full-length cDNAs coding for amplified esterase B genes from a resistant Cuban strain (MRES, with amplified B-1 esterase) and a Sri Lankan strain (PeIRR, with amplified B-2 esterase) of C. quinquefasciatus have been sequenced. In addition, a partial-length cDNA coding for a B esterase from an insecticide-susceptible Sri Lankan strain (PelSS) has been sequenced. All the nucleotide sequences and the inferred amino acid sequences show a high level of identity (> 95% at the nucleotide and amino acid level), confirming that they are an allelic series. The two B-1 esterase nucleotide sequences {MRES and the previously published TEM-R [Mouches, Pauplin, Agarwal, Lemieux, Herzog, Abadon, Beyssat-Arnaouty, Hyrien, De Saint Vincent, Georghiou and Pasteur (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 2574-2578]} showed the lowest identity, and restriction-fragment-length-polymorphism analysis of the two strains was different. On the basis of these data we suggest that the two electrophoretically identical B-1 esterase isoenzymes from California and Cuba have been amplified independently. Alternatively, if amplification has occurred only once, the original amplification has not occurred recently.