In Caenorhabditis elegans, polarity along the anterior–posterior (A/P) axis is established shortly after fertilization and is determined by the sperm, whose position specifies the posterior end of the embryo1. Although many factors required for the establishment of A/P polarity have been described2,3, the nature of the spatial cue provided by the sperm remains unknown. Here we show that a microtubule-organizing centre is necessary and sufficient to establish several aspects of A/P polarity. In wild-type embryos, appearance of the first molecular asymmetries along the A/P axis correlates with and requires nucleation of microtubules by the sperm-derived centrosomes (sperm asters). In mutant embryos arrested in meiosis, sperm asters fail to form, and posterior is defined by the position of the persistent meiotic spindle rather than by the position of the sperm. Together, our data indicate that the primary spatial cue for A/P polarity in C. elegans derives from microtubules emanating from the sperm asters. Our findings support a parallel4,5,6,7 between C. elegans zygotes and other cells, such as Drosophila oocytes, which rely on microtubules to regulate polarity.
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We are grateful to A. Golden, P. Sadler, J. Schumacher and D. Shakes for their characterization of the mat mutants. We also thank K. O'Connell and J. White for sharing results before publication; K. Kemphues and L. Boyd for antibodies; L. Timmons and A. Fire for RNAi feeding materials; and A. Golden, P. Sadler, D. Shakes, K. Kemphues, K. O'Connell, Y. Zheng and members of the Seydoux lab for comments on the manuscript. Some strains used in this study were provided by the Caenorhabditis Genetics Center (University of Minnesota). M.R.W. was an NSF predoctoral fellow. This work was supported by grants from the Searle and Packard Foundations to G.S.
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