Reduction of cortical pulling at mitotic entry facilitates aster centration.

Equal cell division relies upon astral microtubule-based centering mechanisms, yet how the interplay between mitotic entry, cortical force generation, and long astral microtubules leads to symmetric cell division is not resolved. We report that a cortically-located sperm aster displaying long astral microtubules that penetrate the whole zygote does not undergo centration until mitotic entry. At mitotic entry we find that microtubule-based cortical pulling is lost. Quantitative measurements of cortical pulling and cytoplasmic pulling together with physical simulations suggested a wavelike loss of cortical pulling at mitotic entry led to aster centration based on cytoplasmic pulling. Cortical actin is lost from the cortex at mitotic entry coincident with a fall in cortical tension from around 300pN/µm to 100pN/µm. Following the loss of cortical force generators at mitotic entry long microtubule-based cytoplasmic pulling is sufficient to displace the aster towards the cell center. These data reveal how mitotic aster centration is coordinated with mitotic entry in chordate zygotes.