Organophosphate Resistance Mediated by Alterations of Acetylcholinesterase in a Resistant Clone of the Greenbug, Schizaphis graminum (Homoptera: Aphididae)

An organophosphate (OP)-resistant clone (OR-0) of the greenbug, Schizaphis graminum, showed 27.2-, 19.9-, 65.8-, 44.5-, 18.2-, 4.8-, and 3.1-fold greater resistance to dimethoate, omethoate, disulfoton, demeton-S-methyl, parathion, methyl parathion, and chlorpyrifos, respectively, than an OP-susceptible clone (OSS). General esterase and glutathione S-transferase activities were similar in the OSS and OR-0 clones, whereas cytochrome P450 O-demethylase activities were below the detection limit in both greenbug clones. In contrast, acetylcholinesterase (AChE) activity in the OR-0) clone was 2.3-fold higher than that in the OSS clone. Kinetic studies also revealed that AChE from the OR-0 clone was 2.1-, 2.1-, 1.6-, 4.4-, and 1.3-fold less sensitive to inhibition by methyl paraoxon, paraoxon, demeton-S-methyl, omethoate, and demeton S, respectively, than AChE from the OSS clone (P < 0.05). However, Northern blot analysis of mRNA, using a 296-bp AChE cDNA fragment generated by seminested PCR as a homologous probe, indicated that the increased activity of AChE in the OR-0 clone was not due to the overexpression of the AChE gene. Our overall results suggest that the increased AChE activity in the OR-0 clone most likely is due to structural modifications of AChE, leading to both an increased catalytic activity with the model substrate acetylthiocholine and a reduced sensitivity to inhibition by OP compounds.