UC Davis

Microtubule-based spindle formation is essential to faithful chromosome segregation during cell division. In many animal species, the oocyte meiotic spindle forms without centrosomes (the major microtubule organizing centers), unlike most mitotic cells. Even in mitotic cells, centrosomes are sometimes dispensable for bipolar spindle formation, indicating a redundant pathway initiating spindle assembly. In this study, we examined meiotic spindle assembly in C. elegans oocytes. We have demonstrated: First, metaphase I spindle formation is Ran-GEF and Ran-GAP independent in meiotic embryos. Second, free tubulin, also called soluble tubulin, concentrates in the nuclear volume during Germinal Vesicle Breakdown (GVBD) as well as in the spindle region during metaphase I and metaphase II. We then showed that the concentration of free tubulin at metaphase II spindle region is enclosed by dense ER sheets which exclude cytoplasmic organelles including mitochondria and yolk granules from the meiotic spindle. Similar observations are also shown in early mitotic cells. Together, this suggests free tubulin concentrating in the nuclear region might be a common mechanism promoting spindle formation through volume exclusion in both meiotic and early mitotic embryos in C. elegans. Moreover, the movement of molecules during GVBD depends on the size and the charge of the molecules.