Microtubules Are Involved in Anterior-posterior Axis Formation in C. Elegans Embryos

Microtubules deliver positional signals and are required for establishing polarity in many different organisms and cell types. In Caenorhabditis elegans embryos, posterior polarity is induced by an unknown centrosome-dependent signal. Whether microtubules are involved in this signaling process has been the subject of controversy. Although early studies supported such an involvement (O'Connell, K.F., K.N. Maxwell, and J.G. White. 2000. Dev. Biol. 222:55-70; Wallenfang, M.R., and G. Seydoux. 2000. Nature. 408:89-92; Hamill, D.R., A.F. Severson, J.C. Carter, and B. Bowerman. 2002. Dev. Cell. 3:673-684), recent work involving RNA interference knockdown of tubulin led to the conclusion that centrosomes induce polarity independently of microtubules (Cowan, C.R., and A.A. Hyman. 2004. Nature. 431:92-96; Sonneville, R., and P. Gonczy. 2004. Development. 131: 3527-3543). In this study, we investigate the consequences of tubulin knockdown on polarity signaling. We find that tubulin depletion delays polarity induction relative to wild type and that polarity only occurs when a small, late-growing microtubule aster is visible at the centrosome. We also show that the process of a normal meiosis produces a microtubule-dependent polarity signal and that the relative levels of anterior and posterior PAR (partitioning defective) polarity proteins influence the response to polarity signaling. Our results support a role for microtubules in the induction of embryonic polarity in C. elegans.