Proper chromosome segregation and spindle assembly require both kinetochore and central spindle components in Drosophila oocytes. Sarah J. Radford, Kim S. McKim. Waksman Institute, Rutgers University, Piscataway, NJ.

   Inaccurate chromosome segregation during oogenesis is a leading cause of spontaneous abortion and birth defects in humans. Proper chromosome segregation is achieved through the regulated interaction of chromosomes with a bipolar array of microtubules that constitute the meiotic spindle. The meiotic spindle in the oocytes of many organisms, including humans and Drosophila, is built in the absence of the classical microtubule-organizing centers known as centrosomes. In the presence of centrosomes, chromosome segregation depends on interactions between the kinetochore, a protein complex that assembles at the centromere, and microtubules that connect to the centrosomes, but how chromosomes interact with the spindle to ensure segregation in the absence of centrosomes remains unclear. We recently showed that Subito, a kinesin-6 family member that binds to microtubules at the central spindle, is required for proper chromosome bi-orientation in Drosophila oocytes. We report here the identification of a kinetochore component, SPC105R, that is also required for chromosome bi-orientation. Loss of SPC105R leads to loss of the microtubules that appear to end at the chromosomes, suggesting that kinetochore-microtubule interactions have been disrupted. In addition, loss of both Subito and SPC105R leads to loss of the oocyte spindle. This suggests that microtubules in the acentrosomal meiotic spindle must be stabilized through either incorporation into the central spindle or interaction with the kinetochore. Both types of interaction can facilitate chromosome bi-orientation, and both may depend on the chromosomal passenger complex (CPC), which is required for spindle assembly in Drosophila oocytes. Indeed, Aurora B, a component of the CPC, is required for both Subito and SPC105R localization. Based on these results, we suggest a model in which the CPC directs the recruitment of the proteins that stabilize the two main types of microtubules that constitute the acentrosomal meiotic spindle, resulting in the proper segregation of chromosomes.