NONLINEAR SELECTION RESPONSE IN DROSOPHILA - A STRATEGY FOR TESTING THE RARE-ALLELES MODEL OF QUANTITATIVE GENETIC-VARIABILITY

Quantitative genetic theory predicts that variation due to rare allele s at many loci will generate a transient acceleration in the response to directional selection. We have tested this prediction by constructi ng experimental lines of Drosophila melanogaster that carry positively selected ethanol resistance alleles at low frequencies, and then subj ecting the lines to directional selection for ethanol resistance. Appr oximately 468,000 files were subjected to artificial selection over 30 generations. The predicted non-linear selection responses were observ ed in all experimental lines and replicates, on three genetic backgrou nds. In contrast, un-selected controls and lines carrying random allel es at low frequencies on the same genetic backgrounds exhibited linear selection responses. These results demonstrate that non-linearities d ue to rare alleles are detectable and repeatable, provided that experi ments are done on a sufficiently large scale. The results suggest that it may be possible to test for rare-alleles as a component of natural ly occurring genetic variation by careful examination of selection res ponse curves.