M. Syvanen et al., HETEROGENEITY OF THE GLUTATHIONE TRANSFERASE GENES ENCODING ENZYMES RESPONSIBLE FOR INSECTICIDE DEGRADATION IN THE HOUSEFLY, Journal of molecular evolution, 43(3), 1996, pp. 236-240
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.