The complete amino acid sequence of a mammalian acetylcholinesterase from fetal bovine serum (FBS ACHE) is presented. This enzyme has a high degree of sequence identity with other cholinesterases, liver carboxyesterases, esterase-6, lysophospholipase, and thyroglobulin. The locations of 191 amino acids in 10 regions of the FBS enzyme were compared with corresponding sequences of Torpedo, human, and Drosophila AChEs and human serum butyrylcholinesterase (BChE). In one region there is a marked difference in both the number of amino acids and their sequence between mammalian AChE and other AChEs and the human serum BChE. The amino acid sequence of FBS AChE showed overall homologies of 90% with human ACHE, 60% with T. californica ACHE, 50% with human serum BChE, and 39% with Drosophila AChE in these regions.

After ethyl methanesulfonate (EMS) mutagenesis of a susceptible strain (SWT), selective screening of Lucilia cuprina (Wiedemann) resulted in four strains that were resistant to the insecticide dieldrin. Concentrations used for selection were greater than LC99 of susceptible phenotypes. No resistant variants were screened from the standard laboratory strain (SWT) not treated with EMS. The resistance phenotypes of the four resistant strains were similar to each other and to that of a field-selected resistant strain. The genetic basis of resistance is monogenic in all strains and the data are consistent with the same locus, Rdl, determining resistance status in each strain. The Rdl locus maps to chromosome V, approximately 3.5 map units distal to the Sut locus. Dieldrin resistance may be caused by less effective blocking of insect neuronal GABA receptors by the chemical in resistant strains. The data indicate that the evolution of resistance to an insecticide in the field may be constrained by a limited number of genetical and biochemical options if a monogenic response is selected for and that the spontaneous mutation rate to the Rdl allele is less than 1 in 10(6) in the laboratory.

We identify an esterase isozyme in Drosophila melanogaster, EST 23, which shares biochemical, physiological, and genetic properties with esterase E3, which is involved in resistance to organophosphate insecticides in Lucilia cuprina. Like E3, the D. melanogaster EST 23 is a membrane-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 sensitivity to inhibition by paraoxon and their insensitivity to inhibition by eserine sulfate, both enzymes were classified as subclass I carboxylesterases. The activity of each enzyme peaks early in development and, again, in the adult stage. Both enzymes are found in the male reproductive system and larval and adult digestive tissues, the latter being consistent with a role for these enzymes in organophosphate resistance. Fine structure deficiency mapping localized Est 23 to cytological region 84D3 to E1-2 on the right arm of chromosome 3. Moreover, we show that the genes encoding three other esterase phenotypes also map to the same region; these phenotypes involve allozymic differences in EST 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 corresponds closely to that encompassing E3 and malathion carboxylesterase on chromosome 4 in L. cuprina, the homologue of chromosome 3R in D. melanogaster.

Esterase activities associated with organophosphate insecticide resistance in the Australian sheep blowfly, Lucilia cuprina, are compared with similar activities in other Diptera. The enzymes making the major contribution to methyl butyrate hydrolysis ("ali-esterase") in L. cuprina, M. domestica, and D. melanogaster comigrate during electrophoresis. The enzymes in L. cuprina and D. melanogaster correspond to the naphthyl acetate hydrolyzing E3 and EST23 isozymes of those species. These and previously published data suggest that the ali-esterases of all three species are orthologous. Strains of L. cuprina fall into four groups on the basis of quantitative determinations of their ali-estesterase, OP hydrolase, and malathion carboxylesterase activities and these groups correspond to their status with respect to two types of OP resistance. Strains susceptible to OP's have high ali-esterase, low OP hydrolase, and intermediate MCE activities; those resistant to malathion but not diazinon have low ali-esterase, intermediate OP hydrolase, and high MCE activities; those resistant to diazinon but not malathion have low ali-esterase, high OP hydrolase, and low MCE activities; those resistant to both OPs have low ali-esterase, high OP hydrolase, and high MCE activities. The correlated changes among the three biochemical and two resistance phenotypes suggest that they are all properties of one gene/enzyme system; three major allelic variants of that system explain OP susceptibility and the two types of OP resistance. Models are proposed to explain the joint contribution of OP hydrolase and MCE activities to malathion resistance and the invariant association of low ali-esterase and elevated OP hydrolase activities in either type of resistance.