Modifier gene that affects glucose-6-phosphate dehydrogenase activity in Drosophila melanogaster.

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Abstract
D. melanogaster strains homozygous for X chromosomes sampled from natural populations responded to upward and downward selection for glucose-6-phosphate dehydrogenase activity. Second and 3rd chromosomes sampled from the homozygous strains showing a high enzyme activity as a result of selection have no stimulative effect on the enzyme activity. Flies which were made homozygous for X, 2nd and 3rd chromosomes sampled from high-activity strains also responded to selection. Since the 4th chromosome constitutes only a minor fraction of D. melanogaster genome, these findings suggest the presence of an extrachromosal modifier which is responsible for regulation of the enzyme activity. Implication of these findings in adaptive evolution is discussed.