Mutations in the alpha-tubulin 67C gene specifically impair achiasmate segregation in Drosophila melanogaster

Drosophila melanogaster oocytes heterozygous for mutations in the alpha-tub ulin 67C gene (alpha tub67C) display defects in centromere positioning duri ng prometaphase of meiosis I. The centromeres do not migrate to the polewar d edges of the chromatin mass, and the chromatin fails to stretch during sp indle lengthening. These results suggest that the poleward forces acting at the kinetochore are compromised in the alpha tub67C mutants. Genetic studi es demonstrate that these mutations also strongly and specifically decrease the fidelity of achiasmate chromosome segregation. Proper centromere orien tation, chromatin elongation, and faithful segregation can all be restored by a decrease in the amount of the Nod chromokinesin, These results suggest that the accurate segregation of achiasmate chromosomes requires the prope r balancing of forces acting on the chromosomes during prometaphase.