A CLUSTER OF ESTERASE GENES ON CHROMOSOME 3R OF DROSOPHILA-MELANOGASTER INCLUDES HOMOLOGS OF ESTERASE GENES CONFERRING INSECTICIDE RESISTANCE IN LUCILIA-CUPRINA

We identify an esterase isozyme in Drosophila melanogaster, EST 23, wh ich shares biochemical, physiological, and genetic properties with est erase E3, which is involved in resistance to organophosphate insectici des in Lucilia cuprina. Like E3, the D. melanogaster EST 23 is a membr ane-bound alpha-esterase which migrates slowly toward the anode at pH 6.8. Both enzymes have similar preferences for substrates with shorter acid side chain lengths. Furthermore, on the basis of their high sens itivity to inhibition by paraoxon and their insensitivity to inhibitio n by eserine sulfate, both enzymes were classified as subclass I carbo xylesterases. The activity of each enzyme peaks early in development a nd, again, in the adult stage. Both enzymes are found in the male repr oductive system and larval and adult digestive tissues, the latter bei ng consistent with a role for these enzymes in organophosphate resista nce. Fine structure deficiency mapping localized Est 23 to cytological region 84D3 to E1-2 on the right arm of chromosome 3. Moreover, we sh ow that the genes encoding three other esterase phenotypes also map to the same region; these phenotypes involve allozymic differences in ES T 9 (formerly EST C), ali-esterase activity, defined by the hydrolysis of methyl butyrate, and malathion carboxylesterase activity, defined by hydrolysis of the organophosphate malathion. This cluster correspon ds closely to that encompassing E3 and malathion carboxylesterase on c hromosome 4 in L. cuprina, the homologue of chromosome 3R in D. melano gaster.