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. 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.

The one-cell Caenorhabditis elegans embryo divides asymmetrically into a larger and smaller blastomere, each with a different fate. How does such asymmetry arise? The sperm-supplied centrosome establishes an axis of polarity in the embryo that is transduced into the establishment of anterior and posterior cortical domains. These cortical domains define the polarity of the embryo, acting upstream of the PAR proteins. The PAR proteins, in turn, determine the subsequent segregation of fate determinants and the plane of cell division. We address how cortical asymmetry could be established, relying on data from C. elegans and other polarized cells, as well as from applicable models. We discuss how cortical polarity influences spindle position to accomplish an asymmetric division, presenting the current models of spindle orientation and anaphase spindle displacement. We focus on asymmetric cell division as a function of the actin and microtubule cytoskeletons, emphasizing the cell biology of polarity.

In many organisms, a synaptonemal complex (SC) intimately connects each pair of homologous chromosomes during much of the first meiotic prophase and is thought to play a role in regulating recombination. In the yeast Saccharomyces cerevisiae, the central element of each SC contains Zip1, a protein orthologous to mammalian SYCP1. To study the dynamics of SCs in living meiotic cells, a functional ZIP1::GFP fusion was introduced into yeast and analyzed by fluorescence video microscopy. During pachytene, SCs exhibited dramatic and continuous movement throughout the nucleus, traversing relatively large distances while twisting, folding, and unfolding. Chromosomal movements were accompanied by changes in the shape of the nucleus, and all movements were reversibly inhibited by the actin antagonist Latrunculin B. Normal movement required the NDJ1 gene, which encodes a meiosisspecific telomere protein needed for the attachment of telomeres to the nuclear periphery and for normal kinetics of recombination and meiosis. These results show that SC movements involve telomere attachment to the nuclear periphery and are actindependent and suggest these movements could facilitate completion of meiotic recombination.actin ͉ meiosis ͉ recombination ͉ synaptonemal complex ͉ yeast S porulation of diploid cells in the ascomycete Saccharomyces cerevisiae is accompanied by a typical meiotic cell cycle that culminates with the production of four haploid ascospores. Chromosomes pair, undergo recombination, and then segregate from each other in two successive divisions. Reciprocal recombination between homologous chromosomes takes place during prophase of the first meiotic division (prophase I) and is essential for proper segregation. In S. cerevisiae, prophase I can be divided into leptotene-, zygotene-, pachytene-, and diplotene-like substages defined by the state of chromosome pairing and condensation. During leptotene, chromosomes organize on proteinaceous axial elements, and double-strand breaks (DSBs) begin to appear in DNA, initiating recombination. At the same time, perinuclear telomeres begin to cluster near the spindle pole body (the yeast centrosome), pushing chromosomes into a bouquetlike configuration at the leptotene/zygotene transition. At zygotene, the axial elements pair or synapse at the sites of DSBs, and tripartite proteinaceous structures called synaptonemal complexes (SCs) begin to form between homologous chromosomes (1-3). At pachytene, the SCs have matured by a zipper-like mechanism into ribbon-like structures that are embedded at each end in the nuclear envelope and intimately connect each pair of homologues from end to end (3-5). Mature SCs are present for at least 1 h in strain SK1 and longer in other strains, so that pachytene accounts for a relatively large part of prophase I (6-8). In S. cerevisiae, DNA strand invasion and Holliday junction recombination intermediates are observed throughout zygotene and pachytene (9-12), suggesting that recombination is completed during pachytene, presumably when chromosomes...

Taken together, our results suggest that SPD-2 may link PCM recruitment and centriole duplication in C. elegans. SPD-2 shares homology with a human centrosome protein, suggesting that this key component of the C. elegans centrosome is evolutionarily conserved.

At anaphase, the mitotic spindle positions the cytokinesis furrow [1]. Two populations of spindle microtubules are implicated in cytokinesis: radial microtubule arrays called asters and bundled nonkinetochore microtubules called the spindle midzone [2-4]. In C. elegans embryos, these two populations of microtubules provide two consecutive signals that position the cytokinesis furrow: The first signal is positioned midway between the microtubule asters; the second signal is positioned over the spindle midzone [5]. Evidence for two cytokinesis signals came from the identification of molecules that block midzone-positioned cytokinesis [5-7]. However, no molecules that are only required for, and thus define, the molecular pathway of aster-positioned cytokinesis have been identified. With RNAi screening, we identify LET-99 and the heterotrimeric G proteins GOA-1/GPA-16 and their regulator GPR-1/2 [10-12] in aster-positioned cytokinesis. By using mechanical spindle displacement, we show that the anaphase spindle positions cortical LET-99, at the site of the presumptive cytokinesis furrow. LET-99 enrichment at the furrow depends on the G proteins. GPR-1 is locally reduced at the site of cytokinesis-furrow formation by LET-99, which prevents accumulation of GPR-1 at this site. We conclude that LET-99 and the G proteins define a molecular pathway required for aster-positioned cytokinesis.