Morphology of the nucleolus in undifferentiated amoebae of Dictyostelium discoideum

We investigated the morphology and behavior of phase-opaque and electron-opaque intranuclear structures in undifferentiated amoebae of the cellular slime mold Dictyostelium discoideum by video microscopy of live amoebae and 3-D reconstruction from serial ultrathin sections. We conclude from our observations and results that these structures, with the possible exception of some small intranuclear granules, represent the nucleolus, which occupies 31 % to 39% of the nuclear volume. The nucleolus thus is a very complex body whose morphology varies from cell to cell. It consists of a single element that is extensively or punctually associated with the nuclear envelope, or of several larger and smaller elements, some of which may be joined. The number of the nucleolar elements in a given nucleus is fixed, i.e. no fusions between elements and no splitting of elements occur. The position of these elements is also fixed, for as the nucleus rotates and changes shape in response to changes in cell shape during amoeboid movement the nucleolar elements maintain their relative position in the nucleus. The morphological variation between nucleoli of different amoebae contrasts remarkably with the constant morphology within any one cell. These features distinguish the nucleolus of D. discoideum from that of higher eukaryotes and pose many questions regarding genetic control over nucleolar morphogenesis and maintenance of its shape.