The Genetic Basis Of Variation in a Polygenic Character: Bristle Number in Drosophila melanogaster

Abstract

<p>Quantitative traits are important in medicine, evolution, and plant and animal breeding, yet very little is known about the nature of standing variation at the loci that affect quantitative traits. This thesis describes experiments designed to determine the nature of standing genetic variation for a model train; bristle number in Drosophila melanogaster. The first experiment mapped factors responsible for response to short term selection on abdominal bristle number. Eight factors of large effect which accounted for much of the difference between the parental lines were mapped to the X and third chromosomes. Sex specific and epistatic effects of the same order of magnitude as the allelic effects were associated with the mapped factors. In addition, factors mapped to the approximate positions of likely candidate loci (ASC, bb, emc, h, mab, Dl, and E(spl)), previously characterized by mutations with large effect on bristle number. In a second experiment restriction variation among 47 naturally occurring second chromosomes was associated with variation in abdominal and sternopleural bristle number. Unlike previous studies, the molecular variants associated with phenotypic variation were not large insertion and deletions, but intermediate frequency restriction site variants. A new permutation test based statistical method was developed in this work in order to assess the significance of the observed associations between molecular and phenotypic variation. The experiments described in this thesis do not support the infinitesimal model of quantitative variation, and instead suggest that a great deal of quantitative genetic variation in nature may be due to alleles of large effect at candidate loci that are at intermediate frequency.</p>