Substrate-preference polymorphism at an esterase locus of Drosophila mojavensis

In a larval esterase of Drosophila mojavensis there are alleles whose products preferentially hydrolyze alpha-naphthyl esters, whereas the majority of the alleles hydrolyze preferentially beta-naphthyl esters. In a collection of laboratory stocks alpha alleles have a frequency of 15%. Three different mobilities of alpha alleles were discovered, suggesting a polymorphism rather than a single mutation event. If substrate-preference polymorphisms are common among "multiple-substrate" enzymes (category II of Gillespie and Langley 1974), allozyme variation at these enzyme loci may well be maintained by balancing selection.