A molecular mechanism of meiotic spindle translocation in Caenorhabditis elegans oocytes

In animals, female meiosis is a highly asymmetric cell division, in which big oocytes and small polar bodies are generated by an asymmetric localization of meiotic spindles at the oocyte cortex. Spindle translocation is essential for the cortical attachment and chromosome segregation, but the molecular mechanism is unclear. By filming C. elegans expressing GFP-tubulin or GFP-histone, I found that the meiosis I spindle translocates to the cortex, where it adopts a parallel orientation, before the metaphase anaphase transition. Wild-type spindles sequentially begin shortening, rotate perpendicular to the cortex, and undergo chromosome separation. This process repeats as the meiosis II spindle assembles. To test whether microtubules are required for spindle movements, tubulin was deleted. Cortical translocation of chromosomes was blocked in these worms, but not in F-actin depleted worms. Translocation of bivalents was also defective in oocytes null for MEI-1, a microtubule disassembly factor. These results suggest that microtubules are involved in translocation of chromosomes to the cortex. To test whether a microtubule motor is required, meiosis was filmed in worms depleted of several kinesins. Depletion of UNC-116/kinesin-1 blocked the early, pre-anaphase spindle translocation and unc-116 (RNAi) spindles moved to the cortex at late time points at a faster rate and in a different orientation than wild-type spindles after spindle shortening, which requires the anaphase promoting complex (APC). This suggests that the delayed spindle movement in unc-116 (RNAi) embryos may be driven by an APC-dependent mechanism, which might mediate spindle rotation in wild type worms. Indeed, spindle translocation was completely blocked in embryos depleted of both UNC-116 and an APC subunit. These results indicate that UNC-116 mediates the early spindle translocation in wild type worms and that a discrete, post-anaphase mechanism is responsible for the delayed movement of unc-116 (RNAi) spindles.;I also found that depletion of kinesin light chains, or a light chain interacting protein, KCA-1, causes the same translocation defect as UNC-116 depletion. These observations reveal redundant pathways for achieving the asymmetry of spindle attachment at the cortex and suggest a novel function of UNC-116/kinesin-1 complex and in KCA-1 translocating meiotic spindles to the oocyte cortex in C. elegans.