Genetic variation in the activities of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in Drosophila melanogaster. Evidence for an autosomal modifier system.

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Abstract
Considerable variations in activities of 2 X-linked enzymes, G6PD [glucose-6-phosphate dehydrogenase] and 6PGD [6-phosphogluconate dehydrogenase] were observed within and between various strains of D. melanogaster which were made homozygous for different X chromosomes sampled from natural populations. The loci coding for these 2 enzymes were both found to be segregating for 2 electrophoretically distinguishable alleles in the populations studied, but the enzymes purified to homogeneity from the different genotypes were mutally indistinguishable with respect to several enzymological and immunological parameters. The observed variations in enzyme activity would be attributable to variations in enzyme quantity, but not in enzyme quality. This was confirmed immunologically. Five isogenic strains tested all responded immediately and rapidly to upward and downward selection for G6PD activity. The presence of a modifier gene system outside the X chromosome is suggested. The number of genes constituting the system is not known at present, but it is likely that there may be a major gene segregating for at least 2 alleles, high- and low-activity, to which the observed variations in enzyme amount would be primarily ascribed. This autosomal modifier system is functioning throughout development, and has influence also on activities of 6PGD and HAD [hydroxy acid dehydrogenase].