Biochemical Genetics, Vol. 35, Nos. 3/4, 1997 Isolation and Characterization of an Unamplified Esterase B3 Gene from Malathion-Resistant Culex tarsalis Claus Tittiger 1 and Virginia K. Walker 1,2 Received 11 Oct. 1996—Final 21 Feb. 1997 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 (Estf33) gene (estp3) in C. tarsalis was investigated by cloning and characterizing sequences from resistant and susceptible strains. est(33 is similar to est(31, both structurally and in sequence. The first intron of est(33, 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|33 cDNAs from resistant and susceptible strains have 98% amino acid identity. It appears that, in contrast to other studies, est|33 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. KEY WORDS: insecticide resistance; mosquitoes; esterase; cDNA. INTRODUCTION Resistance to organophosphate insecticides is common in insect populations (Oppenoorth, 1985) and often is correlated with an increase in general esterase 1 Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6. 2 To whom correspondence should be addressed. 119 0006-2928/97/0400-0119$1250/0 @ 1997 Plenum Publishing Corporation