Abstract
Summary We investigated mitosis in trophic amoebae of Acrasis rosea (Olive and Stoianovitch) by video microscopy of live cells, by immunofluorescence with an antibody against tubulin, and by transmission electron microscopy. As interphase cells contain neither microtubules (MTs) nor microtubule-organizing centers (MTOCs) the mitotic spindle forms entirely de novo from presumptive diffuse intranuclear MTOCs. Mitosis is closed and devoid of any visible pole organelles during all its stages. The spindle axis is probably established by parallel alignment of MTs due to spatial constraints or lateral interaction. Chromosomes condense only when spindle formation is well advanced; their kinetochores presumably acquire MTs (kMTs) by capture. The two main elements of the nucleolus, viz. granular body and dense granules, disperse, but do not disintegrate. They later arrange in the center of the spindle along its axis and eventually segregate as two granular bodies and two groups of granules. Reconstruction of the nucleolus during telophase involves the compaction of the granular bodies and coaggregation of the dense granules. At metaphase the chromosomes are aligned at the equator of a spindle that tapers towards the blunt spindle poles. Chromosomes are small, but they have distinct layered kinetochores with two MTs each that terminate in their outer layer. Cytokinesis is accomplished ca. eight minutes after the first signs of prophase. Chromosome segregation during anaphase is effectuated almost solely by spindle elongation, which begins when chromosomes are still undivided and aligned at the equator. The velocity of spindle elongation and chromosome segregation was 6 μm/min. Chromosome congression and segregation are most likely driven by interactions, static or dynamic, between kMTs and non-kinetochore MTs (nkMTs), whereas the mechanism of spindle elongation is probably based on movements between staggeredly overlapping nkMTs. At the end of telophase the closure of the nuclear envelope around the daughter nuclei pinches off a membrane tube containing remnant nkMTs. Mitosis in A. rosea differs markedly from that in other cellular slime molds and has much in common with that in several protozoa.
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