Segregation Distorter (SD) of Drosophila melanogasteris one of the best studied meiotic drive systems at the chromosomal and population level. We are now ready to study the molecular basis of SD. Molecular cloning will also lead to a better understanding of the population genetics of this system. The target for segregation distortion is the Responder (Rsp) locus: the alleles at the locus are broadly classified as sensitive or insensitive. We have found that a tandem array of 100-1,000 repeats are in all eight sensitive chromosomes so far examined. The array of repeats are absent in all eight insensitive chromosomes examined. Seven copies of the repeats have been sequenced which represent a novel class of satellite DNA and its hypothetical roles in chromatin condensation and segregation. An aberrant process of condensation is a very likely cause of spermiogenic failure, resulting in segregation distortion. We propose to study three aspects of Rsp. I) Further molecular characterization to delineate the limit of the Rsp locus. We plan to use chromosomes of various construct, such as deletions, translocations free duplications as well as naturally-occurring variants, to achieve this goal. The sequences sufficient as well as necessary for sensitivity will be identified. Our data suggest such sequences are none other than the large array of repeats themselves and we will further test this hypothesis that sensitivity is a direct consequence of the size of the tandem array. Sequencing a larger number of repeats will be carried out for molecular evolution studies. II) Evolutionary and population genetic aspects of SD will be studied. We will characterize the structure of Rsp in closely related species. Rsp alleles in natural populations will be surveyed. Four experimental populations will be set up and monitored. These populations will represent competition between sensitive and insensitive chromosomes both in the presence and absence of SD chromosomes. A satellite amplification hypothesis, proposed to explain perplexing observations on SD, will be tested. III) Molecular mechanisms underlying distortion will be explored. We will investigate if the Rsp alleles are transcribed under distorting and nondistorting conditions. We have developed a long term plan to study the chromatin structure associated with the Rsp satellite DNA. Assays for proteins that bind to such DNA in cell lines and in different tissues are planned.
