  During the asymmetric division of the Caenorhabditis elegans nematode zygote,
the polarity cues distribution and daughter cell fates depend on the correct
positioning of the mitotic spindle, which results from both centering and
cortical pulling forces. Revealed by anaphase spindle rocking, these pulling
forces are regulated by the force generator dynamics, which are in turn
consequent of mitotic progression. We found a novel, additional, regulation of
these forces by the spindle position. It controls astral microtubule
availability at the cortex, on which the active force generators can pull.
Importantly, this positional control relies on the polarity dependent LET-99
cortical band, which restricts or concentrates generators to a posterior
crescent. We ascribed this control to the microtubule dynamics at the cortex.
Indeed, in mapping the cortical contacts, we found a correlation between the
centrosome-cortex distance and the microtubule contact density. In turn, it
modulates pulling force generator activity. We modelled this control,
predicting and experimentally validating that the posterior crescent extent
controlled where the anaphase oscillations started, in addition to mitotic
progression. Finally, we propose that spatially restricting force generator to
a posterior crescent sets the spindle's final position, reflecting polarity
through the LET-99 dependent restriction of force generators to a posterior
crescent. This regulation superimposes that of force generator processivity.
This novel control confers a low dependence on microtubule and active force
generator exact numbers or dynamics, provided that they exceed the threshold
needed for posterior displacement. Interestingly, this robustness originates in
cell mechanics rather than biochemical networks.
