Three modes of cytokinesis in Dictyostelium. A series of time-lapse phase contrast images obtained at 30 second intervals are shown. (A) Wild-type AX2 cells were embedded in low melting temperature agarose and cultured without solid surfaces. In this condition, wild-type cells divide using cytokinesis A. (B) mhcA^- cells were cultured on a plastic dish to allow adhesion to a solid surface. Division of mhcA^- cells is driven by cytokinesis B. (C) Multinucleate mhcA^- cells grown in suspension for 3 days were then placed on a plastic dish. These giant cells divide by cytokinesis C. Bars, 10μ m.

Phenotypic analysis of wild-type and mutant cells in which cytokinesis A and/or B has been disrupted. (A,B) Histograms showing the distributions of nuclei/cell among cells grown in suspension (B) or on glass surfaces (A). Cells of all three strains shown were cultured in suspension before being transferred to plastic Petri dishes with thin glass bottoms; fixation, DAPI staining and counting of nuclei were performed immediately (B) or after 3 days of continued growth on the glass surface (A). (C-I) Photomicrographs of cells grown for 3 days on a glass surface. Unlike cells shown in A and B, these cells were precultured on plastic Petri dishes, suspended by streams of medium using micropipettes, diluted, and replated to plastic Petri dishes with thin glass bottoms because cells carrying the mhcA^- mutation were unable to grow in suspension. Wild-type (C) and mhcA^- cells (D) were mostly mononucleate. AmiA^- (E), corA^- (G) and amiA^-/corA^- (I) cells were somewhat larger and flatter than wild-type cells, indicating moderate disruption of cytokinesis. The greater enlargement of amiA^-/mhcA^- (F) and corA^-/mhcA^- (H) double knockout cells suggests cytokinesis is more severely affected in these strains. (J) A histogram showing the distributions of nuclei/cell among the cells shown in C-I. More than half of the amiA^-/mhcA^- and corA^-/mhcA^- cells are multinucleate, and more than 30% of them are highly multinucleate (more than five nuclei per cell), confirming severe disruption of cytokinesis in these cells. Magnifications in panels C-I are the same. Bar, 10 μm.

Sequences of morphological changes during division on a solid substrate. Cells were attached to plastic dishes for 2 days before observation. Each panel shows a series of phase contrast images recorded with intervals of 60 seconds between frames. (A-E) Mitotic wild-type (A), mitotic mhcA^- (B), mitotic amiA^- (C), mitotic corA^- (D) and amiA^-/mhcA^- (E) cells. The cell cycle stage of the cell in E was not determined, but is most probably interphase. Bars, 10 μm. Magnifications of A-D are the same.

Visualization of the cell cycle as a function of GFP-histone expression. (A) Structure of the GFP-tagged histone H1 chimeric gene. (B,C) Confocal images of the nucleus were identified in interphase (B) and mitotic (C) cells by GFP-H1 fluorescence. (D-F) A series of time-lapse micrographs of GFP fluorescence images superimposed on phase contrast images. They show differing cytokinesis processes in three strains of GFP-H1-expressing cells grown on solid substrates: mhcA^- cells undergoing cytokinesis B (D); amiA^- cells failing to divide (E); and amiA^-/mhcA^- cells undergoing cytokinesis C (F). Bars, 10 μm.