Segregation Distorter (SD) is naturally occurring meiotic drive system on the second chromosome of Drosophila melanogaster. Contrary to Mendelian expectations, heterozygous SD/SD+ males transmit the SD chromosome in vast excess over the normal homolog. The basis of this distorted transmission involves the induced dysfunction of the spermatids that receive the SD+ chromosome but the details of the mechanism are unknown. The SD system comprises at least three distinct genetic components: the Sd, the E(SD), and the Rsp loci. We have focused on characterizing the properties of these components with the aim of elucidating the structure, function, and orgin of SD chromosomes and their dynamics in natural populations. We identified a 12 kb tandem duplication uniquely associated with all SD chromosomes, which we believe corresponds to Sd itself, and have identified putative Sd and Sd+ transcripts as well. We will further characterize these transcripts and their relationship to each other by Northern analysis, S1 nuclease mapping, and isolation and sequencing of cDNA clones. Functional tests of the putative Sd clone will be carried out by P-mediated transformation. Portions of the tandem duplication will be sequenced to determine its precise structure and to estimate its evolutionary age. The spatial distribution of the Sd product will be assessed by in situ hybridization and immunolocalization. Additional experiments are aimed at examining allelic variation at the Sd locus, the effect of genetic modifiers on its expression, and its possible homology with other drive systems. We will also carry out further genetic analysis of E(SD) to test the generality of our observation that two doses of E(SD) can produce distortion in the absence of Sd and to examine the basis of this distortion. Genetic tests to assess the possible existence of weak and strong E(SD) alleles and to discriminate between different hypotheses for the role of E(SD) in distortion are also planned. Finally, the presence and frequency of E(SD) on non-SD chromosomes in natural populations will be determined. The work proposed, as well as the work already done, has significant implications for our understanding of basic genetic mechanisms. The presence of SD has a profound effect which can be quantified. Yet, the results of the work from Dr. Ganetzkys laboratory have demonstrated that the mutation resulting in an SD phenotype is a 12 kilobase tandem duplication. Such duplications are not rare so the question is why this particular one has such major effects on the phenotype of organism. The answers also impinge upon our understanding of both evolutionary biology and population genetics.
