Astral microtubule dynamics regulate anaphase oscillation onset and set a robust final position of the C. elegans zygote spindle
Abstract
Background:
The correct positioning of the mitotic spindle during the asymmetric division of the
nematode
C. elegans
zygote relies on the combination of centering and cortical–pulling forces. These
forces, revealed by centrosome anaphase oscillations, are regulated through the dynamics of force
generators, related to mitosis progression. Recently, we reported the control of oscillation onset by the
posterior spindle pole position in related species
C. briggsae
, necessitating a re-evaluation of the role of
astral microtubules dynamics.
Results:
After exhibiting such a positional switch in
C. elegans
, we mapped
the microtubule ends at the cortex and observed a correlation between the proximity of the centrosomes
and the density of microtubule contacts. To explore the functional consequences, we extended the “tug–
of–war” model and successfully accounted for the positional switch. We predicted and experimentally
validated that the control of oscillation onset was robust to changes in cell geometry or maximum number
of attached force generators. We also predicted that the final position of the posterior centrosome and
thus the spindle has a reduced dependence upon the force generator dynamics or number.
Conclusion:
The outburst of forces responsible of spindle anaphase oscillations and positioning is regulated by the
spindle position through the spatial modulation of microtubule contacts at the cortex. This regulation
superimposes that of force generator processivity putatively linked to the cell cycle. This novel control
provides robustness to variations in zygote geometry or detailed properties of cortical force generators.