Correlations between development rates, enzyme activities, ribosomal DNA spacer-length phenotypes, and adaptation in Drosophila melanogaster (restriction fragment-length polymorphisms/regulatory gene variation/evolution/population genetics)

Selection for "fast" preadult development rate among the progeny of flies collected in a natural population of Drosophila melanogaster produced a line that developed more rapidly than a line selected for "slow" preadult devel- opment rate. Assays for enzyme activity levels showed that the activities of a-glycerophosphate dehydrogenase, alcohol dehy- drogenase, and malic enzyme were higher in the fast than in the slow line, but that the activity of superoxide dismutase was lower in the fast line. Differences in the frequencies of spacer- length phenotypes of X chromosome-linked rRNA genes (rDNA), which developed between the lines during the selection process, are larger than can be explained on the basis of genetic drift alone. Long rDNA spacers had high frequency in the fast line; short spacers, in the slow line. We conclude that enzyme levels affected adaptation under the selective regimes imposed and that the different X-linked rDNA spacer-length phenotypes are either adaptive in themselves or that they mark chromo- somal segments carrying genes relevant to adaptation.