MITOSIS IN AMEBAS OF THE CELLULAR SLIME-MOLD (MYCETOZOAN) ACYTOSTELIUM-LEPTOSOMUM

We investigated mitosis in amoebae of Acytostelium leptosomum, grown i n liquid culture, by video microscopy of live cells, by indirect immun ofluorescence with antibodies against tubulins, and by transmission el ectron microscopy of ultrathin sections. Amoebae in interphase contain a single microtubule-organizing center (MTOC, [30]) at each nucleus, from which microtubules (MTs) radiate into the cytoplasm. These disapp ear as the intranuclear spindle forms. Concomitantly, the nucleolus di sperses, and the chromosomes that are visible in phase contrast congre ss to the spindle equator. The spindle is closed except for polar fene strae occupied by broad, amorphous spindle pole bodies (SPBs). The chr omosomes at metaphase are joined to form several blocks, each attached to several kinetochore MTs. Anaphase was accomplished within 2.2 min (s.d. = 0.5 min, n = 11). Anaphase A was virtually absent, but anaphas e B contributed substantially to chromosome segregation. The mean velo city of pole separation was 3.2 mum/min (s.d. = 0.8 mum/min) and the m ean elongation factor was 2.8 (range 1.9 to 3.4). The telophase spindl e was a shaft consisting of a few MTs traversing each incipient daught er nucleus and joining in the interzone. The amorphous SPBs were recon verted to compact interphase MTOCs as the chromosomes decondensed and the nucleolus re-formed during cytokinesis. Duration of mitosis and ve locities of its movements are within values typical for lower eucaryot es. In most aspects of mitosis A. leptosomum is very similar to two ot her dictyostelid cellular slime molds, Dictyostelium discoideum and Po lysphondylium violaceum, and the three lower eucaryotes are clearly di stinct from other mycetozoans.