The spd-2 gene is required for polarization of the anteroposterior axis and formation of the sperm asters in the Caenorhabditis elegans zygote

In the Caenorhabditis elegans zygote, polarization of the anteroposterior ( AP) axis occurs during a brief period of reorganization that follows fertil ization and results in the establishment of discrete cytoplasmic and cortic al domains. In the cytoplasm, germ-line or P granules are circulated by an actomyosin-driven fountain now of cytoplasm and localize to the posterior, while in the cortex, two proteins required for AP polarity, PAR-2 and PAR-3 , localize to the posterior and the anterior, respectively. The identity of the positional cue that determines AP axis orientation is not known, altho ugh it has been postulated to be a component of the sperm pronucleus/centro some complex (SPCC) as the position of the SPCC correlates with the orienta tion of the AP axis and the direction of the fountain flows. Here, we show that mutations in the spd-e gene disrupt polarization of the AP axis. In mu tant zygotes, the fountain now of cytoplasm and associated asymmetric corti cal contractions are absent, P granules do not localize, and cortical PAR-3 does not become asymmetrically distributed. Interestingly, cortical PAR-2 localizes randomly to either or both poles. The random positioning of PAR-2 requires PAR-3 and indicates that a spd-g-dependent mechanism normally mod ulates PAR-2/PAR-3 interactions to correctly position PAR-P at the posterio r. sgd-2 mutations also disrupt formation of the SPCC by delaying and atten uating the formation of sperm asters until after the period of reorganizati on, suggesting that spd-e mutations disrupt formation of the positional cue . Our results also indicate that sperm asters are not essential for pronucl ear migration but are required for rapid female pronuclear movement and pre mitotic positioning of the pronuclei. (C) 2000 Academic Press.