Down-regulated transcriptional level of Ace1 combined with mutations in Ace1 and Ace2 of Aphis gossypii are related with omethoate resistance.

The degree of insecticide resistance, acetylcholinesterase (AChE) activity kinetics, AChE gene expression and the cDNA sequence of AChE gene were investigated in resistant and relatively susceptible strains of the cotton aphids, Aphis gossypii (Glover). The resistant strain (ORR) exhibited 53.28-fold resistance to omethoate compared to the relatively susceptible strain (OSS) in cotton aphids. AChE activity, V(max) and K(m) were significantly lower in the ORR strain than in the OSS strain (0.13-, 0.04- and 0.31-fold, respectively). Based on analysis of IC(50) indices, enzyme inhibition experiments showed that AChE from the ORR strain was 7.99-, 4.12-, 4.27-, 8.71- and 4.57-fold insensitive to inhibition by eserine, omethoate, paraoxon, paraoxon-methyl and malaoxon than the OSS strain. Sequence analysis indicated that there were no amino acid substitutions in AChEI (Ace1) and AChEII (Ace2) between the OSS and ORR strain. However, when compared with the 81-171B strain (GenBank No. AF502081), we detected two site mutations (S146N and L532P) in Ace1 with high frequency in both the ORR and OSS strains. One conserved mutation (S431F) in Ace2 was also found in both strains when compared with the 171B strain (GenBank No. AJ748114). Measurements of the levels of gene expression for Ace1 and Ace2 in ORR and OSS, as determined by real-time quantitative PCRs, revealed that the relative transcription levels of Ace1 and Ace2 were 0.26- and 1.07-fold, respectively, in the ORR strain as compared to the OSS strain. These results indicate that the altered AChE sensitivity brought about by a decreased transcriptional level of Ace1 mRNA and combined with the site mutants in both Ace1 and Ace2 might be related to omethoate resistance in cotton aphids.