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

Abstract

Considerable variations in activities of two X-linked enzymes, G6PD and 6PGD, have been observed within and between various strains of Drosophila melanogaster which were made homozygous for different X chromosomes sampled from natural populations. Although the loci coding for these two enzymes were both found to be segregating for two electrophoretically distinguishable alleles in the populations studied, the enzymes purified to homogeneity from the different genotypes were mutually indistinguishable with respect to several enzymological and immunological parameters. It seems therefore that 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. This suggests the presence of a modifier gene system outside the X chromosome. Although the number of genes constituting the system is not known at present, it is likely that there may be a major gene segregating for at least two 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.