Relationship between alpha-glycerophosphate dehydrogenase activity and metabolic rate during flight in Drosophila melanogaster.

Measurements of wing-beat frequency (WBF) have been used to characterize flight muscle metabolic rate in Drosophila melanogaster during tethered flight. Progeny of crosses between 17 X-chromosome substitution lines and three null-activity stocks have been studied in order to determine the effect on flight metabolism of sharply reduced activity of alpha-glycerophosphate dehydrogenase (alpha GPDH). It was found that flies with an approximate 50% reduction in alpha GPDH activity have a metabolic rate that is, in most cases, indistinguishable from that of wild-type flies and, in the most extreme cases, reduced by only 4%. These results demonstrate that alpha Gpdh is not a "major gene" for flight metabolism, in the quantitative genetic sense of the term. These results are in agreement with the Kacser and Burns (1973, 1979, 1981) theory of flux, which postulates that the activity of an enzyme embedded in a multienzyme pathway can sometimes vary from wild-type to very low levels (perhaps 5-10% wild type) with no significant effect on flux through the total pathway.