6-Phosphogluconate dehydrogenase activity variants in Musca domestica L.: A further allele at the Pgd locus as proved by densitometric assay.

A new electrophoretic variant of 6-phosphogluconate dehydrogenase (6PGD) has been detected in flies of a laboratory Musca domestica strain. This variant is to be added to the two already described, PGD-A and PGD-B, identified by a fast-weak and a slow-thick electrophoretic band, respectively. The new variant, PGD-C, has the same mobility as PGD-A but provides a more intensely stained band; therefore it can be described as a fast-thick phenotype. The staining intensity of PGD-C is slightly lower than that of PGD-B. Genetic and densitometric tests have shown that the different levels of enzymatic activity of the two fast variants A and C are inherited as alternative genetic units, and they have been interpreted as one aspect of the phenotypic expression of two Pgd alleles, namely, PgdA and PgdC. These alleles determine both the rates of electrophoretic mobility (fast in both cases) and the levels of activity (low for A, strong for C; shown by weak or thick stained electrophoretic bands). Similarly, the two distinctive features of PGD-B, namely, slow mobility and high activity level, are always jointly inherited and appear as two pleiotropic aspects of the phenotype coded for by the PgdB allele. The PgdB/PgdC heterozygous flies provide a slightly asymmetrical three-banded zymogram, while the PgdA/PgdC combination leads to a single-banded pattern, showing the same mobility as the parents and an intermediate staining intensity. The quantitative analysis of enzyme activity of 6PGD zymograms, performed through densitometric methods, has led to the recognition of three different activity levels coded for by Pgd alleles, one of which, namely, PgdC, would not have been detected using electrophoretic methods alone.