A Screen to Isolate Genes Facilitating Co-orientation of Meiotic Chromosomes at the Metaphase Plate. M.D. Champion , R. Namba , R.S. Hawley. Dept MCB, Univ California, Davis, Davis, CA.

   The nod (no distributive disjunction) gene encodes a chromokinesin that localizes along the arms of homologous chromosomes during prometaphase I of meiosis. Mutations of the nod locus specifically effect the achiasmate system of segregation in Drosophila melanogaster females. Specifically, genetic and cytological analysis of nod mutants has shown that achiasmate chromosomes exhibit an increased rate of nondisjunction and are not properly oriented on the metaphase plate. Therefore, NOD is thought to provide a plateward force, which serves to properly orient achiasmate chromosomes on the metaphase plate during meiosis I. In an effort to further elucidate NOD's role in this biological pathway, we have performed a screen to identify genes interacting with nod⁺. Studies of an a-tubulin67C mutant provided the basis for this screen. Alone, the a-tub67C mutant exhibits chromosome nondisjunction, abnormal chromatin stretching and abnormal centromere orientation. This phenotype is suppressed by decreasing the dosage of nod+, and enhanced by increasing the dosage of nod+. Thus, a sensitized background containing three copies of nod⁺ was used to screen for mutants exhibiting a dominant genetic interaction with nod⁺. Using a collection of deficiencies, we have screened through the bulk of the fly genome. From the screen, I have chosen to concentrate on two regions that enhanced the sensitized background, and one region that suppressed the three copies of nod⁺. I am presently using smaller deletions and P-element mutants in the regions of interest to isolate the specific gene or genes responsible for the phenotype. Known candidate genes in one enhancer region includes toucan, Mothers against dpp, and Protein phosphatase D6.