Isolation and characterization of an unamplified esterase B3 gene from malathion-resistant Culex tarsalis

A malathion-resistant strain of Culex tarsalis has a malathion carboxylesterase which rapidly hydrolyzes the insecticide. This is in contrast to organophosphate-resistant strains of C. quinquefasciatus and C. pipiens, which have elevated levels of general B esterases due to amplification of the corresponding genes, producing increased amounts of enzyme which appear to protect the insects by sequestering the insecticide. The contribution to resistance of the homologous esterase B3 (Est beta 3) gene (est beta 3) in C. tarsalis was investigated by cloning and characterizing sequences from resistant and susceptible strains. est beta 3 is similar to est beta 1, both structurally and in sequence. The first intron of est beta 3, however, has a region of extensive repeats which may be responsible for the inefficient processing of the transcript. Southern blots indicate that the gene is single copy in both strains, and northern blots show that it is not greatly overexpressed in the resistant insects. est beta 3 cDNAs from resistant and susceptible strains have 98% amino acid identity. It appears that, in contrast to other studies, est beta 3 does not play a significant role in insecticide resistance in our strains of C. tarsalis, and the molecular responses of pest insects to organophosphates may be more diverse than has been suggested.