Recruitment of maternal material during assembly of the zygote centrosome in fertilized sea urchin eggs

Spindle poles of sea urchin embryos contain centrosomal material derived from maternal as well as paternal sources. To examine how maternal centrosomal material becomes recruited into spindle poles during the first cell cycle, fertilized sea urchin eggs were fixed and labeled with an anti-centrosomal antibody at sequential timepoints after insemination. Immunolabeling patterns demonstrate that the unfertilized egg contains small foci of immunoreactive material dispersed throughout the cytoplasm. Shortly after insemination, the diffuse foci coalesce to form a dense aggregate close to the sperm nucleus. Subsequently, centrosomal material spreads over the surface of the zygote nucleus and becomes partitioned into two masses during spindle pole formation. The involvement of the cytoskeleton in the translocation and targeting of maternal centrosomal material through the first cell cycle was examined by treating eggs with cytoskeletal disrupting agents, a general kinase inhibitor, and by re-inseminating fertilized eggs. These experiments indicate that the initially diffuse centrosomal material is transported centripetally to the sperm nucleus by the sperm aster and the centrosomal material is subsequently sequestered around the zygote nucleus by a microtubule-mediated mechanism. Remarkably, 6-dimethylaminopurine treatment shifted the targeting of maternal centrosomal material from the sperm nucleus to the female pronucleus; upon recovery, some of these zygotes formed spindle poles that flanked only the maternal chromosomes.