Evolution of starvation resistance in Drosophila melanogaster: Aspects of metabolism and counter-impact selection

An artificial selection experiment for increased female starvation resistan ce employed five selected lines and five control lines of Drosophila melano gaster. Females responded to selection within the first five generations, b ut a substantial male response was not observed until starvation resistance was assessed at generation 15. By measuring respiration rate in selected a nd control lines, it was possible to test the hypothesis that reduced metab olic rate is a general mechanism for stress resistance. There was no associ ation between starvation resistance and respiration rate and thus no suppor t for the hypothesis. Studies using vertebrates have shown that starvation causes a decrease in intermediary metabolism enzyme activity, but this rela tionship is not well documented in invertebrates. In the present study, int ermediary metabolism enzyme activities decreased in response to starvation in control-line females and males, and in selected-line males. However, the selected females showed no overall decrease in enzyme activities in respon se to starvation. One interpretation is that selected females evolved to re sist the phenotypic impact of stress. The concept of "counter-impact select ion" is discussed in relationship to the use of phenotypic manipulations fo r the study of evolution.