Characterization of carboxylesterases involved in the insecticide resistance of Culex quinquefasciatus from the Caribbean and South America
The organophosphate resistance-associated elevated esterases Estα2, Estβ1 and Estβ2 were purified to homogeneity from larvae of the Cuban Habana strain. The bimolecular rate constants (kas) of Habana Estβ1 with a range of organophosphates were not significantly different to those of PelRR Estβ21 , and were higher with some organophosphates than PelRR Estα21 (Karunaratne et al, 1993). The relative insecticide binding efficiency of these esterases could not, therefore, explain why co-amplified estα2 and estβ2 are out competing estβ1 in the field. On the basis of their kas, both Habana Estα2 and Estβ2 could be distinguished from their equivalents purified from other strains. In two organophosphate resistant strains of Culex quinquefasciatus from Colombia and Trinidad, possessing the amplified esterase genes estα3 and estβl, the EcoRI restriction fragment lengths of the estβl genes and their flanking regions were different both to each other and to those previously reported for TEM-R estβ11 (Raymond et aL, 1991) and MRES estβ12 (Vaughan et aL, 1995). There were a number of significant differences between the kas of purified Colombia, Trinidad and Habana Estβ1s. The low kas and high k3s for the interaction of Colombia Estβ1 with several insecticides confirmed that, as for Estα21 and Estβ21, the main role of Estβ1 is sequestration. The kas of Habana, Colombia and Trinidad Estβ1s were higher than that of the electrophoretically identical Est'β13 purified from the susceptible PelSS strain (Karunaratne et al, 1995a). This suggests that the elevated esterase-based mechanism confers resistance through amplification of alleles coding C for esterases having a higher reactivity with the insecticides they sequester than esterases coded for by their non-amplified counterparts. A PelRR Estα21 antiserum had the same cross-reactivity with Habana Estα2 as with Estα21. However, both Habana Estβ1 and Estβ2 had a cross-reactivity of approximately 150-fold less than the Estα2s.