Implicit to all models for mitotic spindle assembly is the view that centrosomes are essentially permanent structures. Yet, immunofluorescence revealed that spindles in larval brains of urchin mutants in Drosophila were frequently monastral but bipolar; the astral pole contained a centrosome while the opposing anastral pole showed neither gamma tubulin nor a radial array of astral microtubules. Thus, mutations in the urchin gene seem to uncouple centrosome organization and spindle bipolarity in mitotic cells. Hypomorphic mutants showed a high frequency of monastral bipolar spindles but low frequencies of polyploidy, suggesting 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 gonial cells. Prophase gonial cells showed two centrosomes, suggesting cells entered mitosis with the normal number of centrosomes and that centrosomes separated during prophase. Despite a high frequency of monastral bipolar spindles, the end products of the four mitotic divisions were equivalent in size and chromatin content. These results indicate that monastral bipolar spindles are functional and that the daughter cell derived from the anastral pole can assemble a functional bipolar spindle in the subsequent cell cycle. Cell proliferation despite high frequencies of monastral bipolar spindles can be explained if centrosome structure in mitotic cells is dynamic, allowing transient and benign disorganization of pericentriolar components. Since urchin proved to be allelic to KLP61F which encodes a kinesin related motor protein (Heck et al., 1993. J. Cell Biol. 123: 665-671), our results suggest that motors influence the dynamic organization of centrosomes.
| | Tables 1-3 - Mitotic Figures and Cell Proliferation in KLP61F Neuroblasts and Imaginal Discs;
Spindle Organization in KLP61F Larval Neuroblasts;
Prophase Centrosomes and Monastral Bipolar Spindles in KLP61F Gonial Cells
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Figure 1 (61k) - Mitotic figures in KLP61F larval brains.
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Figure 2 (55k) - Monastral bipolar spindles.
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Figure 3 (39k) - Monastral bipolar spindles may resemble monopolar spindles.
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Figure 4
(28k) - Gonial cells in prophase showed 2 centrosomal asters containing gamma tubulin Example of m2 gonial cyst in prophase in KLP61F1 testis.
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Figure 5
(33k) - Gonial cells show monastral bipolar spindles.
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Figure 6
(55k) - Cyst of 16 spermatocytes in KLP61F mutant testis that are equivalent in size and chromatin content.
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Figure 7
(61k) - A. Centriole pairs in wild-type spermatocyte decorated with gamma tubulin. B. Centriole pair distribution in spermatocytes of wild-type and KLP61F mutant animals.
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Figure 8
(94k) - Models for assembly of monastral bipolar spindles.
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