Centrosomes direct cell polarity independently of microtubule assembly in C-elegans embryos

Polarity establishment requires a symmetry-breaking event, resulting in an axis along which determinants are segregated. In Caenorhabditis elegans, oocytes are apolar and are triggered to polarize rapidly along one axis after fertilization. The establishment of this first polarity axis is revealed by the asymmetric distribution of PAR proteins and cortical activity in the one-celled embryo. Current evidence suggests that the centrosome pronucleus complex contributed by the sperm is involved in defining the polarization axis(1-6). Here we directly assess the contribution of the centrosome to polarity establishment by laser ablating the centrosome before and during polarization. We find that the centrosome is required to initiate polarity but not to maintain it. Initiation of polarity coincides with the proximity of the centrosome to the cortex and the assembly of pericentriolar material on the immature sperm centrosome. Depletion of microtubules or the microtubule nucleator gamma-tubulin did not affect polarity establishment. These results demonstrate that the centrosome provides an initiating signal that polarizes C. elegans embryos and indicate that this signalling event might be independent of the role of the centrosome as a microtubule nucleator.