| High resolution mapping of the QTLs responsible for naturally occurring variation in bristle number in a population of Drosophila melanogaster was carried out on recombinant isogenic (RI) X and third chromosomes derived from high and low sternopleural (SB) bristle number artificial selection lines. SB and abdominal (AB) bristle number phenoytpes and cytogenetic locations of roo transposable element markers were determined for each RI line. QTLs affecting bristle numbers were located by interval mapping, combined with conditioning on flanking markers. Four factors with significant effects on SB bristle number mapped to the X and five to the third chromosome. Epistatic interaction was detected among four interactions between significant intervals on C-1. To test the hypothesis that the factors contributing to selection response interact with candidate loci, we used quantitative complementations tests. Such tests can determine if putative QTLs are allelic or epistatic to the candidate loci. Quantitative complementation tests revealed genetic interactions for SB at the candidate loci emc, qm, hairy, abd, pyd, scr, Delta, and Enhancer-of-split. This work suggests that several genes of large effect may cause quantitative variation and that future approaches to quantitative genetics using candidate genes are promising. A second mapping experiment was carried out on unselected recombinant inbred lines from a laboratory population. The RI lines were grown up in three different temperature environments and scored for bristle number. Nine QTLs were found affecting SB and eleven affecting AB. Of these QTLs, four affecting SB and five affecting AB had a significant marker x temperature or marker x sex x temperature interaction. G x E effects were as large as main effects. Two instances of overdominance were also found and they were both sex and temperature sensitive. |
