Molecular and biochemical mechanisms of organophosphate resistance in laboratory-selected lines of the oriental fruit fly (Bactrocera dorsalis)

Genetic and biochemical factors leading to resistance to various organophosphate (OP) based insecticides were studied in lines selected for OP-resistance in the oriental fruit fly Bactrocera dorsalis. Lines were separately selected for resistance to naled, trichlorfon, fenitrothion, fenthion, formothion, and malathion. Overall, these lines showed increased resistance ratios ranging from 13.7- to 814-fold relative to a susceptible (S) line. Also, in these newly selected lines the same three point mutations in the ace gene, previously identified in resistance studies and designated as I214V, G488S and Q643R, were found. As expected, the enzyme from the resistant lines showed lower overall activity and reduced sensitivity to inhibition by fenitrothion, methyl-paraoxon and paraoxon compared to the wild type acetylcholinesterase (AChE) enzyme. The apparent V-max values for esterase from the resistant lines were 1.2-3.69 times higher than that of the S line. Although only the naled-, trichlorfon- and fenthion-r lines showed lower esterase affinities (based on apparent Km values) compared with the S line, all of the V-max/K-m ratios were higher in the resistant lines compared to that of the S line. The OP-resistant lines also displayed an overall similar pattern of isozyme expression, except for one additional band found only in the naled-r line and one band that was absent in the trichlorfon-, malathion-, and fenthion-r lines. Our results also show that overall, multiple examples of high OP resistance in selected lines of B. dorsalis exhibiting the same genetic alterations in the ace gene seen previously resulted in different effects on esterase enzyme activity in relation to various OP compounds. (C) 2011 Elsevier Inc. All rights reserved.