Gene regulation in adaptive evolution.

It has been suggested that gene regulation may play a critical role in adaptive evolution. However, gene regulation has proved to be most refractory to experimental investigation in multicellular organisms. Using specially constructed stocks of Drosophila melanogaster, we have demonstrated the following. (1) The existence in natural populations of ample variation in regulatory genes that modify the activity of alcohol dehydrogenase (ADH), an enzyme coded by a structural gene locus, Adh, located on the second chromosome; the regulatory genes are located on the third chromosome, and thus are not adjacent to the structural locus. (2) The regulatory genes act not by means of post-transcriptional or post-translational modification of the gene product, but rather by controlling the number of ADH molecules; this is consistent with the hypothesis of gene regulation by means of macromolecules specifically binding at control sites adjacent to the structural gene locus. (3) The variation in regulatory genes is adaptively significant; adaptation to higher levels of environmental alcohol takes place not by changes in the Adh structural locus, but by changes in regulatory genes that control the number of ADH molecules in the organisms. Our results provide direct evidence of the importance of gene regulation in eucaryotic evolution.