Anastral spindle assembly and γ-tubulin in Drosophila oocytes

Background: Anastral spindles assemble by a mechanism that involves microtubule nucleation and growth from chromatin. It is still uncertain whether gamma-tubulin, a microtubule nucleator essential for mitotic spindle assembly and maintenance, plays a role. Not only is the requirement for gamma-tubulin to form anastral Drosophila oocyte meiosis I spindles controversial, but its presence in oocyte meiosis I spindles has not been demonstrated and is uncertain. Results: We show, for the first time, using a bright GFP fusion protein and live imaging, that the Drosophila maternally-expressed gamma Tub37C is present at low levels in oocyte meiosis I spindles. Despite this, we find that formation of bipolar meiosis I spindles does not require functional gamma Tub37C, extending previous findings by others. Fluorescence photobleaching assays show rapid recovery of gamma Tub37C in the meiosis I spindle, similar to the cytoplasm, indicating weak binding by gamma Tub37C to spindles, and fits of a new, potentially more accurate model for fluorescence recovery yield kinetic parameters consistent with transient, diffusional binding. Conclusions: The FRAP results, together with its mutant effects late in meiosis I, indicate that gamma Tub37C may perform a role subsequent to metaphase I, rather than nucleating microtubules for meiosis I spindle formation. Weak binding to the meiosis I spindle could stabilize pre-existing microtubules or position gamma-tubulin for function during meiosis II spindle assembly, which follows rapidly upon oocyte activation and completion of the meiosis I division.