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 that a wavelike loss of cortical
pulling at mitotic entry leads 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 ∼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.