Both naturally occurring insertions of transposable elements and intermediate frequency polymorphisms at the achaete-scute complex are associated with variation in bristle number in Drosophila melanogaster

A restriction enzyme survey of a 110-kb region including the achaete scute complex (ASC) examined 14 polymorphic molecular markers in a sample of 56 n aturally occurring chromosomes. Large insertions as a class were associated with a reduction in both sternopleural and abdominal bristle number, suppo rting deleterious mutation-selection equilibrium models Tol the maintenance of quantitative generic variation. Two polymorphic sites were independentl y associated with variation in bristle number measured in two genetic backg rounds as assessed by a permutation test. A 6-bp deletion near sc alpha is associated with sternopleural bristle number variation in both sexes and a 3.4-kb insertion between sc beta and sc gamma is associated with abdominal bristle number variation in females. Under an additive genetic model, the s mall deletion polymorphism near sc alpha accounts for 25% of the total X ch romosome genetic variation in sternopleural bristle number, and the 3.4 kb insertion accounts for 22% of the total X chromosome variation in female ab dominal bristle number. The observation of common polymorphisms associated with variation in bristle number is more parsimoniously; explained by model s that incorporate balancing selection or assume variants affecting bristle number are neutral, than mutation-selection equilibrium models.