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Three cDNA clones for cytochrome P450s, CYP6B2, CYP6B6 and CYP6B7 which have 84-87% protein sequence identity have been isolated previously from Helicoverpa armigera, and the CYP6B7 mRNA was found to be over-expressed in a pyrethroid-resistant field population. Subsequent analysis has shown that over-expression is observed in a majority of individuals in all populations tested. Single-pair crosses between resistant and sensitive individuals indicated that the pyrethroid resistance co-segregated with the over-expression of this mRNA. Southern analysis indicated that the over-expression, which was confined to midgut only in many insects, was not related to gene amplification. These observations add weight to the conclusion that CYP6B7 is the cytochrome P450 form involved in pyrethroid resistance, and that over-expression of cytochrome P450 CYP6B7 is a common cause of pyrethroid resistance in H. armigera. The results suggest that specific probes for CYP6B7 protein or mRNA could provide the basis for the development of tools for monitoring pyrethroid resistance due to mixed function oxidase activity in field populations of this insect. |
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