HETEROGENEITY OF THE GLUTATHIONE TRANSFERASE GENES ENCODING ENZYMES RESPONSIBLE FOR INSECTICIDE DEGRADATION IN THE HOUSEFLY

One of the four glutathione-S-transferases (GST) that is overproduced in the insecticide-resistant Cornell-R strain of the housefly (Musca d omestica) produces an activity that degrades the insecticide dimethyl parathion and conjugates glutathione to lindane. In earlier work, it w as shown that the resistant Cornell-R carries an amplification, probab ly a duplication, of one or more of its GST loci and that this amplifi cation is directly related to resistance. Using polymerase chain react ion (PCR) amplification with genomic DNA, multiple copies of the gene encoding the parathion-degrading activity (called MdGst-3) were subclo ned from both the ancestral, insecticide-susceptible strain BPM and fr om the insecticide-resistant Cornell-R. In BPM, three different MdGst- 3 genes were identified while in Cornell-R, 12 different MdGst-3 seque nces were found that, though closely related to ancestral genes, had d iverged by a few nucleotides. This diversity in MdGst3 genomic sequenc es in Cornell-R is reflected in the expressed sequences, as sampled th rough a cDNA bank. Population heterozygosity cannot account for these multiple GST genes. We suggest that selection for resistance to insect icides has resulted in not only amplification of the MdGst-3 genes but also in the divergence of sequence between the amplified copies.