Combined effect of cell geometry and polarity domains determines the orientation of unequal division

Cell division orientation is thought to result from a competition between cell geometry and 16 polarity domains controlling the position of the mitotic spindle during mitosis. Depending on the 17 level of cell shape anisotropy or the strength of the polarity domain, one dominates the other 18 and determines the orientation of the spindle. Whether and how such competition is also at 19 work to determine unequal cell division (UCD), producing daughter cells of different size, 20 remains unclear. Here, we show that cell geometry and polarity domains cooperate, rather 21 than compete, in positioning the cleavage plane during UCDs in early ascidian embryos. We 22 found that the UCDs and their orientation at the ascidian third cleavage rely on the spindle 23 tilting in an anisotropic cell shape, and cortical polarity domains exerting different effects on 24 spindle astral microtubules. By systematically varying mitotic cell shape, we could modulate 25 the effect of attractive and repulsive polarity domains and consequently generate predicted 26 daughter cell size asymmetries and position. We therefore propose that the spindle position 27 during UCD is set by the combined activities of cell geometry and polarity domains, where cell 28 geometry modulates the effect of cortical polarity domain(s).