Abstract
The genetic component of variation of enzyme activity in natural populations of Drosophila melanogaster was investigated by using two sets of chromosome substitution lines. The constitution of a line of each type is: i(1)/i(1);+(2)/ +(2);i(3)/i(3) and i(1)/i(1);i(2)/ i(2);+(3)/+(3), where i refers to a chromosome from a highly inbred line and + refers to a chromosome from a natural population. The + but not the i chromosomes vary within a set of lines. By use of a randomized block design to test and estimate components of variance, 50 of the second- and 50 of the third- chromosome substitution lines have been screened for variation in the activity levels of seven enzymes. Six of the seven enzymes show a significant genetic component in at least one set of lines, and five of the seven enzymes show activity variations attributable to factors that are not linked to the structural gene. These unlinked activity modifiers identify possible regulatory elements. Analyses of covariance show that most of the genetic variation of enzyme activities cannot be accounted for by genetic variation of live weight or protein content. These results and the lack of strong correlations between the genetic effects on the activities of different enzymes indicate that the effects are mainly specific for individual enzymes.