Two dynamical behaviours of the microtubules at cell cortex reveal pulling and pushing forces that position the spindle in C. elegans embryo

In the Caenorhabditis elegans zygote, astral microtubules generate forces, pushing against and pulling from the cell periphery. They are essential to position the mitotic spindle and in turn the cytokinesis furrow, ensuring the proper distribution of fate determinants to the daughter cells. By measuring the dynamics of astral microtubules, we revealed the presence of two populations, residing at the cortex during 0.4 s and 1.8 s, proposed to reflect the pulling and pushing events, respectively. This is a unique opportunity to unravel the time and space variations of these both spindle-positioning forces, to study their regulation under physiological conditions. By an investigation at the microscopic level, we first confirmed that the asymmetry in pulling forces was encoded by an anteroposterior imbalance in dynein-engaging rate, and that this asymmetry exists from early metaphase on and accounts for the final spindle position. More importantly, we obtained direct proof of the temporal control of pulling forces through the forcegenerator processivity increase during anaphase. Lastly, we discovered an anti-correlation between the long-lived population density and the stability of the spindle position during metaphase, which strongly suggests that the pushing forces contribute to maintaining thespindle at the cell centre