The genetic component of variation of enzyme activity in natural populations of Drosophila melanbgaster was investigated by using two sets of chromosome substitution lines. The constitution 6f a line of-each type is: ii/ii;+s/±s;is/is and iI/iI;i2/'i2;+3/+3, where i refers to a chromnosome from'a highly inbred line and + refers to a chromosome from a natural population. The These studies and recent models of the regulation of gene expression in eukaryotes (9, 10) suggest that in natural populations there may be several polymorphic loci distributed throughout the genome that affect the expression of a given structural gene and therefore contribute to variation in the activity of an enzyme. However, at present there is very little quantitative information about the extent of genetic variation of enzyme activities, the relative importance of structural, regulatory, or other types of genes in causing this variation, and the number, organization, and types of activity modifiers that are polymorphic in natural populations. Here we report some preliminary results of a study designed to investigate these questions. This study may ultimately prove useful in the design of experiments to test the adaptive significance of enzyme variability and will also have a bearing on the suggestion that regulatory variation of enzyme activity levels is a more important source of adaptive variation than structural variability (9, 11).Our basic approach to quantifying the amount of genetic variation of enzyme detivity in natural populations of Drosophila is to view activity as a quantitative trait and to use standard biometrical methods to partition its variance into genetic and environmental components. In order to localize activity variants, two sets of homozygous lines were constructed in which either second of third chromosomes from natural populations were substituted onto an isogenic background.Within the set of second-chromosome substitution lines, for example, all X and third-chromosome loci are constant but second-chromosome loci vary. This design permits detection of activity variants that are not linked to the structural locus of the enzyme and can therefore easily identify modifier loci.

MATERIALS AND METHODSThe populations.-Two sets of populations were used in this study; one set has been selected for resistance to DDT (2,2-bis(p-chlorophenyl)-I, 1, l-trichloroethane) and the other consists of the corresponding control populations. They are designated as follows:ORC and ORR were derived in 1952 from the well-known stock Oregon R, which has been kept in the laboratory since at least 1925 (Bridges and Brehme, 1944). The pairs 731C and 731R and 91C and 91R were established in 1952 from collections of several hundred wild flies made in a residential area of St. Paul, Minnesota, about 1 month apart. The J populations were established from another studies of these populations have shown that a very high level of resistance has been achieved (Merrell, 1960;Dapkus and Merrell, 1977). The purpose of this report is to present evidence that (a) frequency changes of aldehyde oxidase (Ao) allozymes and the Payne inversion (In(3R)P) with which they are associated have occured in response to selection for DDT resistance and (b) the maintenance of the In(3R)P polymorphism at intermediate frequencies in selected populations is due to heterokaryotype advantage in survival that is specifically dependent on DDT in the environment.

Genetic variation in the modulating effect of dietary sucrose was assessed in Drosophila melanogaster by examining 27 chromosome substitution lines coisogenic for the X and second chromosomes and possessing different third isogenic chromosomes derived from natural populations. An increase in the concentration of sucrose from 01 % to 5 % in modified Sang's medium C significantly altered the activities of 11 of 15 enzyme activities in third instar larvae, indicating that dietary sucrose modulates many, but not all, of the enzymes of D. melanogaster. A high sucrose diet promoted high activities of enzymes associated with lipid and glycogen synthesis and low activities of enzymes of the glycolytic and Krebs cycle pathways, reflecting the physiological requirements of the animal. Analyses of variance revealed significant genetic variation in the degrees to which sucrose modulated several enzyme activities. Analysis of correlations revealed some relationships between enzymes in the genetic effects on the modulation process. These observations suggest that adaptive evolutionary change may depend in part on the selection of enzyme activity modifiers that are distributed throughout the genome.

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Genetic variation among second and third chromosomes from natural populations of Drosophila melanogaster affects the activity level of sn-glycerol-3-phosphate dehydrogenase (EC 1.1.1.8; GPDH) at both the larval and the adult stages. The genetic effects, represented by differences among chromosome substitution lines with coisogenic backgrounds, are very repeatable over time and are generally substantially larger than environmental and measurement error effects. Neither the GPDH allozyme, the geographic origin, nor the karyotype of the chromosome contributes significantly to GPDH activity variation. The strong relationship between GPDH activity level and GPDH-specific CRM level, as well as our failure to find any thermostability variation among the lines, indicates that most, if not all, of the activity variation is due to variation in the steady-state quantity of enzyme rather than in its catalytic properties. The lack of a strong relationship between adult and larval activity levels suggests the importance of stage- or isozyme-specific effects.