The perturbation ofDrosophila melanogaster esterase-6 allozyme frequencies by selection for malathion resistance

The effects of selection for resistance to the organophosphate insecticide malathion on esterase-6 polymorphism was studied in laboratory populations ofD. melanogaster. A genetically well-mixed population was constructed from 40 locally-caught, iso-female lines, divided into control and malathion-selected lines and the frequency of the majorEst-6 alleles was followed for more than 100 generations. The main findings were: (1) The allele frequency in control replicates remained stable for over one hundred generations; (2) the allele frequency in the populations exposed to malathion changed dramatically and in the opposite direction between replicates during the early generations of the selection experiment; (3) all selected populations eventually returned to the controlEst-6 allele frequencies. This return was more rapid in populations exposed to lower selection intensity compared to those exposed to higher selection intensity; (4) the convergence ofEst-6 allele frequencies to control values was also observed in three populations obtained by mixing flies of appropriate genotypes from the control population to give different initial frequencies. These results have been interpreted to mean that esterase-6 does not have a direct role in malathion resistance, and that theEst-6 polymorphism in our experimental population was maintained by balancing selection.