Increased expression of an acetylcholinesterase gene may confer organophosphate resistance in the greenbug, Schizaphis graminum (Homoptera: Aphididae)
2002
Gao, Jian-Rong | Zhu, Kun Yan
Acetylcholinesterases (AChE, EC 3.1.1.7) were purified from an organophosphate (OP)-susceptible (OSS) clone and an OP-resistant (OR-0) clone of the greenbug, Schizaphis graminum (Rondani). Enzyme inhibition kinetics showed that AChE from the OR-0 clone was 1.1- to 12.8-fold less sensitive to inhibition by chlorpyrifos oxon, paraoxon, methyl paraoxon, malaoxon, demeton-S-methyl and omethoate than AChE from the OSS clone based on their bimolecular rate constants (ki). Analyses of DNA sequences of PCR-amplified fragments from the AChE coding regions did not reveal any differences between the OSS and the three OP-resistant clones (OR-0, OR-1, and OR-2). Northern blot analysis, however, showed that the amount of AChE mRNA in the resistant clones was approximately 1.5-fold higher than that in the OSS clone. Southern blots did not provide any evidence of gene amplification for the increased mRNA. The increased AChE mRNA appears to be positively correlated with the amount of AChE in crude enzyme preparations. These results indicated that the increased AChE activity in OP-resistant clones was due to increased expression of the AChE gene. It is possible that an increased transcription rate and/or increased stability of the mRNA results in the increase of AChE mRNA in the OP-resistant clones of the greenbug.
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