HIGH-RESOLUTION MAPPING OF GENETIC-FACTORS AFFECTING ABDOMINAL BRISTLE NUMBER IN DROSOPHILA-MELANOGASTER

Factors responsible for selection response for abdominal bristle numbe r and correlated responses in sternopleural bristle number were mapped to the X and third chromosome of Drosophila melanogaster. Lines diver gent for high and low abdominal bristle number were created by 25 gene rations of artificial selection from a large base population, with an intensity of 25 individuals of each sex selected from 100 individuals of each sex scored per generation. Isogenic chromosome substitution li nes in which the high (H) X or third chromosome were placed in an isog enic low (L) background were derived from the selection lines and from the 93 recombinant isogenic (RI) HL X and 67 RI chromosome 3 lines co nstructed from them. Highly polymorphic neutral roo transposable eleme nts were hybridized in situ to the polytene chromosomes of the RI line s to create a set of cytogenetic markers. These techniques yielded a d ense map with an average spacing of 4 cM between informative markers. Factors affecting bristle number, and relative viability of the chromo some 3 RI lines, were mapped using a multiple regression interval mapp ing approach, conditioning on all markers greater than or equal to 10 cM from the tested interval. Two factors with large effects on abdomin al bristle number were mapped on the X chromosome and five factors on the third chromosome. One factor with a large effect on sternopleural bristle number was mapped to the X and two were mapped to the third ch romosome; all factors with sternopleural effects corresponded to those with effects on abdominal bristle number. Two of the chromosome 3 fac tors with large effects on abdominal bristle number were also associat ed with reduced viability. Significant sex-specific effects and epista tic interactions between mapped factors of the same order of magnitude as the additive effects were observed. All factors mapped to the appr oximate positions of likely candidate loci (ASC, bb, emc, h, mab, Dl a nd E(spl)), preciously characterized by mutations with large effects o n bristle number.