A screen to isolate genes facilitating co-orientation of meiotic chromosomes at the metaphase plate. M.D. Champion 1, L. Namba 2, R.S. Hawley 1. 1) Stowers Institute, for Medical Research, Kansas City, MO; 2) University of California-Davis, Davis, CA.
Accurate segregation during the first meiotic division of the oocyte requires that chromosomes achieve a stable bipolar orientation. Studies in drosophila melanogaster females have shown that achiasmate (nonrecombinant) chromosomes are properly positioned at the metaphase plate by a chromokinesin-like protein, NOD. Females homozygous for nod exhibit an increased rate of achiasmate chromosome nondisjunction primarily due to loss of the 4th chromosome. A dominant meiotic mutant, a-tubulin 67C (P40), specifically increases achiasmate chromosome nondisjunction and this phenotype was found to be nod+ dosage sensitive. Studies of the P40 mutant provided the basis for a sensitized screen to identify mutations in genes whose protein products functionally balance NOD. We created a background containing three copies of the nod gene and screened through a collection of deficiencies spanning the major autosomes, which is approximately equivalent to 70% of the entire genome. Females carrying three copies of nod+ alone (Dp(nod+)) exhibit an increase of achiasmate chromosome nondisjunction. Females carrying the Dp(nod+) were crossed to the collection of males carrying a deficiency and assayed for enhancement or suppression of the Dp(nod+) phenotype. We identified 17 enhancing genomic intervals and 4 suppressing intervals. In addition, we identified 21 genomic regions that were semi-sterile in the presence of the Dp(nod+). I have further characterized two of these regions using smaller deficiencies and P-element mutations. The dominant genetic interaction with Dp(nod+) has been mapped to the bub1 and Mad (Mothers against dpp) genes. Preliminary cytological analysis of wild-type oocytes has shown that MAD localizes to chromatin and microtubules. In addition, oocytes from females carrying the Dp(nod+) exhibit a compromised meiotic spindle.