MOLECULAR STUDIES OF KNOCKDOWN RESISTANCE TO PYRETHROIDS - CLONING OFDOMAIN-II SODIUM-CHANNEL GENE-SEQUENCES FROM INSECTS

Knockdown resistance (kdr) is a target-site resistance mechanism that confers nerve insensitivity to DDT and pyrethroid insecticides. In the housefly, Musca domestica, molecular cloning of the para-type sodium channel gene has revealed two amino acid mutations that are associated with kdr and super-kdr resistance phenotypes. Both mutations are loca ted in the domain II region of the channel; Leu1014 to Phe in the hydr ophobic segment IIS6 and Met918 to Thr in the IIS4-IIS5 linker. To inv estigate whether these mutations also occur in other insects, we have designed degenerate primers based on conserved sequences in the domain II region of the sodium channel and used these to PCR amplify this re gion from insecticide-susceptible strains of eight diverse insect spec ies representing four different insect Orders: Helicoverpa armigera, P lutella xylostella, Spodoptera littoralis (Lepidoptera), Blattella ger manica (Dictyoptera), Tribolium castaneum (Coleoptera), Myzus persicae , Aphis gossypii and Phorodon humuli (Hemiptera). The primers amplifie d closely related para-type sodium channel sequences from each insect with a minimum of 85% amino acid identity between species. All of the sequences contained 'susceptible' Leu and Met residues at the position s associated with kdr and super-kdp resistance in the housefly. Recent results detailing the presence of a kdr-type Leu to Phe mutation in p yrethroid-resistant strains of two important agricultural pests, P. xy lostella and M. persicae, are discussed.