AbstractDrosophila heteroneura and D differens are closely related, interfertile species of the Hawaiian picture‐winged group. They display marked qualitative and quantitative differences in the pattern of expression of alcohol dehydrogenase (ADH) and an aldehyde oxidase (AO‐1). These presumptive regulatory differences are revealed by comparisons of the relative levels of these enzymes in various tissues in larvae and adults. In hybrids produced between parents carrying different electrophoretic alleles at the structural loci for these two enzymes, each allele is expressed according to the developmental program characteristic of the parent from which it was derived. This result indicates control of the differences in pattern of expression by one or more cis‐acting sites associated with each structural locus. The distribution of activity among all the three forms of these dimeric enzymes produced in hybrids indicates that the pattern differences reflect differential accumulation of enzyme molecules, not altered catalytic properties. As expected, the regulatory differences segregate with the electrophoretic markers in backcr
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