Cross-strain comparison of cypermethrin-induced cytochrome P450 transcription under different induction conditions in diamondback moth

Synergism with piperonyl butoxide indicated that enhanced P450 activity is likely associated with pyrethroid resistance in an isogenic cypermethrin-resistant (CR) strain of diamondback moth (DBM), Plutella xylostella (L.). Eleven P450 genes, including 8 novel ones, were cloned from DBM, and their basal and cypermethrin-induced transcription levels were compared between insecticide-susceptible (Sus) and CR strains. One P450 gene was determined to be constitutively overtranscribed in CR strain. To determine the conditions resulting in maximum levels of P450 gene induction by cypermethrin, several factors, including routes of insecticide administration (topical vs. leaf dip), exposure amounts and durations of exposure, were examined. In general, the leaf dip method resulted in greater levels of induction in a wider array of P450 genes. The conditions of 'low sub-lethal amounts and short exposures' to cypermethrin were more efficient in P450 induction than those of 'high amounts (e.g., LD50 or LC50) and long exposures'. Cross-strain comparisons revealed that 8 of 11 P450 genes were induced (1.5- to 2.2-fold) in CR larvae whereas only a single P450 gene was induced in Sus larvae under optimal induction conditions. This finding demonstrates that selective P450 gene induction by cypermethrin in addition to constitutive overexpression can confer metabolic resistance in the CR strain. Current study also provides a framework for future induction experiments for the transcriptional profiling of insect detoxification genes in response to insecticide exposure.