Biophysics of Mitotic Spindle Positioning in Caenorhabditis elegans Early Embryos

Abstract

In the C. elegans early embryo, the pronuclei complex migrates to the cell center and rotates such that the mitotic spindle forms along the anterior-posterior axis. Subsequently, the spindle undergoes transverse oscillations as it moves toward the posterior. This asymmetric spindle positioning is crucial for asymmetric cell division and proper development. The contributions of pushing, cortical pulling, and cytoplasmic pulling forces to those dynamic centrosome positioning events are not fully understood. To study these processes, we constructed a novel laser ablation system capable of creating nearly arbitrary 3D cuts of astral microtubules at any desirable timing. We used this system to dissect the relative contribution of pushing and pulling forces throughout pronuclei and spindle motions. Our results suggest that all of these motions are dominated by net pulling. We used microinjected fluorescent nanodiamonds to track cytoplasmic fluid flow, which indicates that cortical pulling forces dominate over cytoplasmic pulling at all stages. We used mathematical modeling and computer simulations to interpret our experimental data. Taken together, our results strongly argue that cortical pulling drives pronuclear migration and rotation, metaphase spindle positioning, asymmetric spindle positioning and all aspects of spindle oscillations.