Ggpd Polymorphism in Drosophila Melanogaster

Several biochemical studies have suggested that in Drosophila melanoguster the two common allozymes of G6PD differ in their in vitro activities and thermal stabilities. Yei, it remains to be shown that these characterizations reflect actual in vivo differences and are not artifacts of the biochemical approach. In this study it is shown that in vivo activity differences must exist between these two variants. This conclusion arises from the observation that the viability of flies bearing a low activity allele of GPGD is strongly dependent on the genotype at the G6PD (Zw) locus, whereas no measurable difference in viability can be detected between Zw genotypes in a normal activity GPGD background. These viability interactions are in the direction predicted by the reported in vitro activities of the allozymes and the proposed deleterious effects of 6-phosphogluconate accumulation.-In addition, a genetic scheme is used that uncouples and quantities the effects of viability modifiers in the region of the Zw locus, while homogenizing 98% of the X chromosome. The viability of different Zw genotypes is measured by examining whole chromosome viabilities relative to the FM6 balancer chromosome. The advantages of this particular scheme are discussed. N recent years a promising approach to understanding the potential adaptive I nature of enzyme polymorphism has involved the biochemical characterization of naturally occurring electrophoretic variants (KOEHN 1969; MERRITT 1972; WATT 1977; PLACE and POWERS 1979; MACDONALD, ANDERSON and SANTOS 1980; ZERA, KOEHN and HALL 1984). As direct as this approach may appear, a difficulty arises from the implicit assumption that in vitro characterizations reflect in vivo function. If the number of experimental conditions which can be varied is considered (such as pH and cofactor concentrations), and given our ignorance of the cellular microenvironment, this assumption becomes problematic. Most reported kinetic differences between naturally occurring polymorphic variants are relatively subtle, without obvious physiological effects. It is not clear that these biochemical differences reflect real changes in pathway flux; yet, this remains the principle argument for using the biochemical approach. However, in the last couple of years, several studies have linked allozyme phenotypes with physiological consequences related to the presumed function of the study enzyme (DIMICHELE and POWERS 1982a, b; HILBISH, Genetics 106 95-107 January, 1984.