Organophosphate resistance mediated by alterations of acetylcholinesterasein 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 grea ter resistance to dimethoate, omethoate, disulfoton, demeton-S-methyl, para thion, methyl parathion, and chlorpyrifos, respectively than an OP-suscepti ble clone (OSS). General esterase and glutathione S-transferase activities were similar in the OSS and OR-0 clones, whereas cytochrome P450 O-demethyl ase activities were below the detection limit in both greenbug clones. In c ontrast, acetylcholinesterase (AChE) activity in the OR-0) clone was 2.3-fo ld higher than that in the OSS clone. Kinetic studies also revealed that AC hE from the OR-0 clone was 2.1-, 2.1-, 1.6-, 4.4-, and 1.3-fold less sensit ive to inhibition by methyl paraoxon, paraoxon, demeton-S-methyl, omethoate , and demeton S, respectively, than AChE from the OSS clone (P < 0.05). How ever. Northern blot analysis of mRNA, using a 296-bp AChE cDNA fragment gen erated by seminested PCR as a homologous probe, indicated that the increase d activity of AChE in the OR-0 clone was not due to the overexpression of t he 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, l eading to both an increased catalytic activity with the model substrate ace tylthiocholine and a reduced sensitivity to inhibition by OP compounds. (C) 2000 Academic Press.