The early development of Caenorhabditis elegans embryos is characterized by a series of asymmetric divisions in which the mitotic spindle is repeatedly oriented on the same axis due to a rotation of the nuclear-centrosome complex. To identify genes involved in the control of spindle orientation, we have screened maternal-effect lethal mutants for alterations in cleavage pattern. Here we describe mutations in ooc-5 and ooc-3, which were isolated on the basis of a nuclear rotation defect in the P(1) cell of two-cell embryos. These mutations are novel in that they affect the asymmetric localization of PAR proteins at the two-cell stage, but not at the one-cell stage. In wild-type two-cell embryos, PAR-3 protein is present around the entire periphery of the AB cell and prevents nuclear rotation in this cell. In contrast, PAR-2 functions to allow nuclear rotation in the P(1) cell by restricting PAR-3 localization to the anterior periphery of P(1). In ooc-5 and ooc-3 mutant embryos, PAR-3 was mislocalized around the periphery of P(1), while PAR-2 was reduced or absent. The germ-line-specific P granules were also mislocalized at the two-cell stage. Mutations in ooc-5 and ooc-3 also result in reduced-size oocytes and embryos. However, par-3 ooc double-mutant embryos can exhibit nuclear rotation, indicating that small size per se does not prevent rotation and that PAR-3 mislocalization contributes to the failure of rotation in ooc mutants. We therefore postulate that wild-type ooc-5 and ooc-3 function in oogenesis and in the reestablishment of asymmetric domains of PAR proteins at the two-cell stage.
Copyright 1999 Academic Press.