RECRUITMENT OF MATERNAL MATERIAL DURING ASSEMBLY OF THE ZYGOTE CENTROSOME IN FERTILIZED SEA-URCHIN EGGS

Spindle poles of sea urchin embryos contain centrosomal material deriv ed from maternal as well as paternal sources. To examine how maternal centrosomal material becomes recruited into spindle poles during the f irst cell cycle, fertilized sea urchin eggs were fixed and labeled wit h an anti-centrosomal antibody at sequential timepoints after insemina tion. Immunolabeling patterns demonstrate that the unfertilized egg co ntains small foci of immunoreactive material dispersed throughout the cytoplasm. Shortly after insemination, the diffuse foci coalesce to fo rm a dense aggregate close to the sperm nucleus. Subsequently, centros omal material spreads over the surface of the zygote nucleus and becom es partitioned into two masses during spindle pole formation. The invo lvement of the cytoskeleton in the translocation and targeting of mate rnal centrosomal material through the first cell cycle was examined by treating eggs with cytoskeletal disrupting agents, a general kinase i nhibitor, and by re-inseminating fertilized eggs. These experiments in dicate that the initially diffuse centrosomal material is transported centripetally to the sperm nucleus by the sperm aster and the centroso mal material is subsequently sequestered around the zygote nucleus by a microtubule-mediated mechanism. Remarkably, 6-dimethylaminopurine tr eatment shifted the targeting of maternal centrosomal material from th e sperm nucleus to the female pronucleus; upon recovery, some of these zygotes formed spindle poles that flanked only the maternal chromosom es.