MITOSIS IN AMEBAS OF THE CELLULAR SLIME-MOLD (MYCETOZOAN) PROTOSTELIUM-MYCOPHAGA

We investigated mitosis in amoebae of Protostelium mycophaga by video microscopy of live cells, by indirect immunofluorescence with antibodi es against tubulins, and by transmission electron microscopy of ultrat hin sections. Amoebae in interphase usually contain two microtubule ce nters (MCs) on opposite sides of the nucleus, from which microtubules (MTs) radiate into the cytoplasm. During prophase these MTs shorten to form two asters between which the mitotic spindle develops during pro metaphase. Concomitantly, the nucleolus fragments, the numerous small chromosomes orient amphitelically in the spindle and congress to the s pindle equator, and the asters diminish further until metaphase. The s pindle is open and acentric, but with complex spindle pole bodies. Eac h sister-chromatid is attached to a single MT by a tiny, layered kinet ochore. During anaphase and telophase, asters develop anew and enlarge to become the elaborate MT cytoskeletons of the daughter cells. Anaph ase lasted 2 min on average (s.d. = 0.6 min, n = 4), during which the chromosomes moved poleward with a mean velocity of 4.0 mu m/min (s.d. 0.8 mu m/min, n= 5). The intermingling of kinetochore MTs and the nume rous non-kinetochore MTs allows for a sliding interaction between them , but depolymerisation-driven chromosome movement is also possible. Th e spindle elongated at a mean rate of 5.9 mu m/min (s.d. = 2.2 mu m/mi n, n = 5), and the mean elongation factor at a mean rate was 2.4 in li ve cells. In immunofluorescence preparations the longest spindles were 3.5 times longer than the average metaphase spindle. Spindle elongati on thus requires the growth of interzonal MTs that assemble as several bundles from an ample pool of tubulin. At the end of telophase the nu clear envelope is reconstructed from membrane vesicles and flattened c isternae that appose to the masses of decondensed chromosomes and nucl eolar material.