KNOCKDOWN RESISTANCE (KDR) TO DDT AND PYRETHROID INSECTICIDES MAPS TOA SODIUM-CHANNEL GENE LOCUS IN THE HOUSEFLY (MUSCA-DOMESTICA)

The voltage-sensitive sodium channel is generally regarded as the prim ary target site of dichlorodiphenyl-trichloro-ethane (DDT) and pyrethr oid insecticides, and has been implicated in the widely reported mecha nism of nerve insensitivity to these compounds. This phenomenon is exp ressed as knockdown resistance (kdr) and has been best characterised i n the housefly where several putative alleles, including the more pote nt super-kdr factor, have been identified. We report the isolation of cDNA clones containing part of a housefly sodium channel gene, designa ted Msc, which show close homology to the para sodium channel of Droso phila (99% amino acid identity within the region of overlap). Using So uthern blots of insect DNA, restriction fragment length polymorphisms (RFLPs) at the Msc locus were identified in susceptible, kdr and super -kdr housefly strains. These RFLPs showed tight linkage to resistance in controlled crosses involving these strains, thus providing clear ge netic evidence that kdr, and hence pyrethroid mode of action, is close ly associated with the voltage-sensitive sodium channel.