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

Abstract

Cell division orientation is thought to result from a competition between cell geometry and polarity domains controlling the position of the mitotic spindle during mitosis. Depending on the level of cell shape anisotropy or the strength of the polarity domain, one dominates the other and determines the orientation of the spindle. Whether and how such competition is also at work to determine unequal cell division (UCD), producing daughter cells of different size, remains unclear. Here, we show that cell geometry and polarity domains cooperate, rather than compete, in positioning the cleavage plane during UCDs in early ascidian embryos. We found that the UCDs and their orientation at the ascidian third cleavage rely on the spindle tilting in an anisotropic cell shape, and cortical polarity domains exerting different effects on spindle astral microtubules. By systematically varying mitotic cell shape, we could modulate the effect of attractive and repulsive polarity domains and consequently generate predicted daughter cell size asymmetries and position. We therefore propose that the spindle position during UCD is set by the combined activities of cell geometry and polarity domains, where cell geometry modulates the effect of cortical polarity domain(s). Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=200 SRC="FIGDIR/small/442692v1_ufig1.gif" ALT="Figure 1"> View larger version (75K): org.highwire.dtl.DTLVardef@4b0e87org.highwire.dtl.DTLVardef@1a6fdbdorg.highwire.dtl.DTLVardef@5ca0ddorg.highwire.dtl.DTLVardef@1450e61_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightO_LISpindle tilting in anisotropic cell shape induces unequal cell division C_LIO_LICortical polarity domain can exert attractive or repulsive effect on spindle C_LIO_LICell geometry and polarity domain cooperate to position the spindle C_LIO_LICell geometry modulates the effect of polarity domain C_LI