The overexpression of C. pipiens or A. gossypii esterases in Drosophila, no matter wild-type or mutant enzymes, produced at most 2.5-fold resistance to OP insecticides in this study, much lower than the resistance levels generally associated with the gene amplifications seen in the aphids and mosquitoes [4], [7] and [9]. Therefore, the insecticide resistance levels observed in this study could be underestimated if the mutations were observed in field populations of C. pipiens or A. gossypii. Another possible reason for this underestimation is that the resistance levels presented in this study were manifested in heterozygous flies or those with a mutant allele and a wild-type allele as a result of the cross between transgenic flies and Gal4 flies.
The insecticide-resistant phenotypes of the mutated carboxylesterases from C. pipiens and A. gossypii presented in this study differed from those of L. cuprina E3. The G151D mutation of E3 generated 10-fold to 16-fold increase in resistance to diethyl OP diazinon; by contrast, no evident resistance to dimethyl OP malathion was observed. The W271L mutation of E3 resulted in 3-fold to 5-fold resistance to diazinon and 600-fold resistance to malathion in L. cuprina [14]. No evident resistance to the tested insecticides was induced by G151D or W271L mutation of C. pipiens carboxylesterase. The A151D mutation of A. gossypii carboxylesterase induced comparable resistance to diethyl OP chlorfenvinphos and dimethyl OP monocrotophos. The W271L mutation caused only marginal resistance to chlorfenvinphos. Such different insecticide-resistant phenotypes could be attributed to species specificity or large phylogenetic differences between the two esterases (C. pipiens esterase B1 and A. gossypii esterase EU783916) and L. cuprina E3 [17]. However, mutations in carboxylesterases in these systems resulted in insecticide resistance.
The resistance phenotype to malathion and parathion from transgenic flies is not consistent with that of in vitro expressed enzymes. CpEST-151D transgenic flies were more susceptible to parathion, and CpEST-271L transgenic flies were more susceptible to malathion and parathion than the wild-type CpEST transgenic flies, whereas the mutant CpEST enzymes showed either no or increased hydrolysis activities to malathion and methyl parathion in vitro [16]. Both AgEST-151D and AgEST-271L transgenic flies were more susceptible to parathion while the two mutations conferred hydrolysis activity to paraoxon (the effective form of parathion) for AgEST in vitro [17]. This inconsistency between in vivo and in vitro experimental results could arise from multiple reasons, one of which obviously is the controllability of hydrolysis reaction in vitro and the complex integrative effects of an exogenous gene knock-in.