Molecular and kinetic evidence for allelic variants of esterase Estβ1 in the mosquito Culex quinquefasciatus

Elevated esterase Estβ1 was purified from larvae of newly isolated strains of the mosquito Culex quinquefasciatus from Colombia (COL) and Trinidad (TRI) with resistance to organophosphate (OP) insecticides. Insecticide interactions were compared with those of elevated Estβ12 from the OP-resistant Habana strain and the non-elevated Estβ13 from the susceptible Pe1SS strain. On the basis of insecticide binding efficiency, all elevated Estβ1 esterases were readily distinguishable. Differences between the EcoRI restriction fragment patterns of the amplified estβ1 gene in COL and TRI strains compared with each other, and between amplified estβ11, estβ12 and the non-amplified estβ13, suggest differences in their nucleotide sequence. Considering their variable insecticide binding efficiencies, these genetic differences would imply that, in contrast to estα2 and estβ2, amplification of estβ1 has occurred several times independently. Generally, the elevated Estβ1s were more reactive with insecticides than the non-elevated Estβ13. This supports the hypothesis that the elevated esterase-based mechanism confers resistance through amplification of alleles coding for esterases which nave a greater specificity for the insecticides they sequester than the esterases coded by their non-amplified counterparts.