SUMMARY: In , microscopic uninucleate amoebae develop into macroscopic multinucleate plasmodia. In the mutant strain, RA614, plasmodium development is blocked. RA614 carries a recessive mutation () in a gene that functions in sexual as well as apogamic development. In apogamic development, binucleate cells arise from uninucleate cells by mitosis without cytokinesis at the end of an extended cell cycle. In cultures, apogamic development became abnormal at the end of the extended cell cycle. The cells developed a characteristic rounded, vacuolated appearance, nuclear fusion and vigorous cytoplasmic motion occurred, and the cells eventually died. Nuclei were not visible by phase-contrast microscopy in most of the abnormally developing cells, but fluorescence microscopy after DAPI staining revealed intensely staining, condensed nuclei without nucleoli. Studies of tubulin organization during development indicated a high frequency of abnormal mitotic spindles and, in some interphase cells, abnormally thick microtubules. Some of these features were observed at low frequency in the parental strain and may represent a pathway of cell death, resembling apoptosis, that may be triggered in more than one way. Nuclear fusion occurred during interphase and mitosis in cells, and multipolar spindles were also observed. None of these features were observed in cells, suggesting that a specific effect of the mutation may be an incomplete alteration of nuclear structure from the amoebal to the plasmodial state.


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