MONASTRAL BIPOLAR SPINDLES - IMPLICATIONS FOR DYNAMIC CENTROSOME ORGANIZATION

Implicit to all models for mitotic spindle assembly is the view that c entrosomes are essentially permanent structures, Yet, immunofluorescen ce revealed that spindles in larval brains of urchin mutants in Drosop hila were frequently monastral but bipolar; the astral pole contained a centrosome while the opposing anastral pole showed neither gamma tub ulin nor a radial array of astral microtubules, Thus, mutations in the urchin gene seem to uncouple centrosome organization and spindle bipo larity in mitotic cells, Hypomorphic mutants showed a high frequency o f monastral bipolar spindles but low frequencies of polyploidy, sugges ting that monastral bipolar spindles might be functional, To test this hypothesis, we performed pedigree analysis of centrosome distribution and spindle structure in the four mitotic divisions of genial cells. Prophase genial cells showed two centrosomes, suggesting cells entered mitosis with the normal number of centrosomes and that centrosomes se parated during prophase, Despite a high frequency of monastral bipolar spindles, the end products of the four mitotic divisions were equival ent in size and chromatin content, These results indicate that monastr al bipolar spindles are functional and that the daughter cell derived from the anastral pole can assemble a functional bipolar spindle in th e subsequent cell cycle, Cell proliferation despite high frequencies o f monastral bipolar spindles can be explained if centrosome structure in mitotic cells is dynamic, allowing transient and benign disorganiza tion of pericentriolar components. Since urchin proved to be allelic t o KLP61F which encodes a kinesin related motor protein (Heck et al, (1 993) J. Cell Biol, 123, 665-671), our results suggest that motors infl uence the dynamic organization of centrosomes.