RESUMO
The molecular basis of metabolic resistance to pyrethroids in Helicoverpa armigera is currently under debate. Substantial indirect evidence supports a role for both esterase- and cytochrome-P450-mediated metabolism. However, the relative roles played by these two mechanisms in field-based resistance is uncertain. Our understanding of the importance of P450-mediated metabolism is hindered by the paucity of cloned genes from this species, and the corresponding absence of data on rates of insecticide metabolism by functionally expressed P450s. To facilitate P450 gene isolation from H. armigera we used degenerate primers in the reverse transcriptase-polymerase chain reaction (RT-PCR) to clone P450 gene fragments from the RNA of a pyrethroid-resistant strain. Here we report the isolation of eight new P450 genes: seven from the CYP4 family and one CYP9. One of these genes, CYP4G8, is two-fold over-expressed in the resistant strain, whereas the other CYP4s showed either similar or undetectable levels of expression. CYP9A3 appears to be a homolog of the putatively resistance-associated CYP9A1 of Heliothis virescens. However, no difference in expression between the H. armigera strains was detected. CYP6B2, a gene previously reported to be over-expressed in a different pyrethroid-resistant strain of H. armigera, also revealed non-detectable levels of expression in both strains. These observations suggest that different P450s may be over-expressed in different resistant strains, and emphasize that recombinant expression will be necessary in order to define precisely their individual substrate specificities and ability to metabolize pyrethroids. The gene fragments described here represent an important first step in this direction.
