A NOVEL TYPE OF UNORTHODOX MITOSIS IN AMEBAS OF THE CELLULAR SLIME-MOLD (MYCETOZOAN) ACRASIS-ROSEA

We investigated mitosis in trophic amoebae of Acrasis rosea (Olive and Stoianovitch) by video microscopy of live cells, by immunofluorescenc e with an antibody against tubulin, and by transmission electron micro scopy. As interphase cells contain neither microtubules (MTs) nor micr otubule-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 on ly when spindle formation is well advanced; their kinetochores presuma bly acquire MTs (kMTs) by capture. The two main elements of the nucleo lus, viz. granular body and dense granules, disperse, but do not disin tegrate. They later arrange in the center of the spindle along its axi s and eventually segregate as two granular bodies and two groups of gr anules. Reconstruction of the nucleolus during telophase involves the compaction of the granular bodies and coaggregation of the dense granu les. At metaphase the chromosomes are aligned at the equator of a spin dle that tapers towards the blunt spindle poles. Chromosomes are small , but they have distinct layered kinetochores with two MTs each that t erminate in their outer layer. Cytokinesis is accomplished ca. eight m inutes after the first signs of prophase. Chromosome segregation durin g anaphase is effectuated almost solely by spindle elongation, which b egins when chromosomes are still undivided and aligned at the equator. The velocity of spindle elongation and chromosome segregation was 6 m u m/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 b ased on movements between staggeredly overlapping nkMTs. At the end of telophase the closure of the nuclear envelope around the daughter nuc lei pinches off a membrane tube containing remnant nkMTs. Mitosis in A . rosea differs markedly from that in other cellular slime molds and h as much in common with that in several protozoa.