Mitotic Segregation of the Nucleolar Ribosomal RNA in Physarum polycephalum

In the naturally synchronous mitosis of the syncytial plasmodium of Physarum polycephalum , the nucleolus disintegrates in prophase, releasing a large amount of ribosomal RNA. Using biotinylated rDNA probes, we studied by high-resolution in situ hybridization the behavior of this nucleolar RNA throughout mitosis. Our results demonstrate that this rRNA is stable and maintained within the mitotic nucleus mainly, but not exclusively, associated with fibrillar nucleolar remnants. The distribution of these rRNA molecules on both sides of the cleavage plane in telophase is indicative of a precise mechanism of mitotic partition of the nucleolar components, supporting our recent findings concerning the rDNA minichromosomes (Puvion-Dutilleul and Pierron, 1992, Exp. Cell Res. 203, 354-364). Taking advantage of the stability of this RNA component in mitosis, we unambiguously demonstrate that the nucleolar remnants are the precursors of the prenucleolar bodies appearing in the newly divided nuclei which, by fusion, reconstitute a single nucleolus. Our data exemplify the persistence of the nucleolar rRNA in mitosis and demonstrate that in Physarum, following its disintegration, the nucleolus is segregated and inherited.