2 SITES IN THE DELTA-GENE REGION CONTRIBUTE TO NATURALLY-OCCURRING VARIATION IN BRISTLE NUMBER IN DROSOPHILA-MELANOGASTER

A restriction enzyme survey of a 57-kb region including the gene Delta uncovered 53 polymorphic molecular markers in a sample of 55 naturall y occurring chromosomes. A permutation test, which assesses the signif icance of the molecular marker with the largest effect on bristle vari ation in four genetic backgrounds relative to permuted data-sets, foun d two sites that were independently associated with variation in brist le number. A common site in the second intron of Delta affected only s ternopleural bristle number, and another common site in the fifth intr on affected only abdominal bristle number in females. Under an additiv e genetic model, the polymorphism in the second intron may account for 12% of the total genetic variation in sternopleural bristle number du e to third chromosomes, and the site in the fifth intron may account f or 6% of the total variation in female abdominal bristle number due to the third chromosomes. These results suggest the following: (1) model s that incorporate balancing selection are more consistent with observ ations than deleterious mutation-selection equilibrium models, (2) map ped quantitative trait loci of large effect may not represent a single variable site at a genetic locus, and (3) linkage disequilibrium can be used as a tool for understanding the molecular basis of quantitativ e variation.