Polarization of the anterior-posterior axis of C. elegans is a microtubule-directed process

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 embryo(1). Although many factors required for the establishment of A/P polarity have been described (2,3), the nature of the spatial cue provided by the sperm remains unknown. Here we show that a microtubule-organizing centre is necessary and suffici ent to establish several aspects of A/P polarity. In wildtype embryos, appe arance of the first molecular asymmetries along the A/P axis correlates wit h 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 s pindle rather than by the position of the sperm. Together, our data indicat e that the primary spatial cue for A/P polarity in C. elegans derives from microtubules emanating from the sperm asters. Our findings support a parall el(4-7) between C. elegans zygotes and other cells, such as Drosophila oocy tes, which rely on microtubules to regulate polarity.