A QUANTITATIVE-ANALYSIS OF MODIFIER MUTATIONS WHICH OCCUR IN MUTATIONACCUMULATION LINES IN DROSOPHILA-MELANOGASTER

Seven enzyme activities were measured in Drosophila melanogaster lines in which spontaneous mutations had accumulated over about 300 generat ions under the minimum pressure of natural selection. These enzymes in cluded alcohol dehydrogenase (ADH), alpha-glycerol-3-phosphate dehydro genase (alpha GPDH), malate dehydrogenase(MDH), isocitrate dehydrogena se (IDH), glucose-6-phosphate dehydrogenase (G6PD), 6-phosphogluconate dehydrogenase (6PGD) and cr-amylase (AMY). A significant genetic vari ance was observed for some enzyme activities. The mutations which alte r the enzyme activities are called modifier mutations. The magnitudes of the genetic variance in modifier mutations differed greatly among e nzymes but were often similar between two series of mutation accumulat ion lines (AW and JH). This may therefore indicate that the number of modifiers is specific for each enzyme system. The modifier mutation ra te is suggested to be one of the clues for assessing the maintenance m echanism of protein polymorphism in natural populations.