MICROTUBULE ORGANIZATION DURING THE CELL-CYCLE OF THE PRIMITIVE EUKARYOTE DINOFLAGELLATE CRYPTHECODINIUM-COHNII

The complete microtubular system of the dinoflagellate Crypthecodinium cohnii Biecheler is described, as seen by confocal laser scanning flu orescence microscopy and labelling with anti-beta-tubulin antibody. Th is technique allowed us to observe the organization of the subcortical and internal cytoskeletons and the mitotic microtubular system, and t heir changes during the cell cycle. These observations are compared wi th those made in cryosections by light microscopy and in fast-freeze-f ixed, cryosubstituted cells by electron microscopy. We show the organi zation of the cortical microtubules, and in particular of the thick mi crotubular bundles arranged as a three-pronged fork from which they se em to emanate. This fork emerges from a peculiar cytoplasmic zone at t he pole of the cell and is in contact with the region of the kinetosom es, at the cingulum. During the G1 phase, only a single, radial microt ubular bundle (a ''desmose'') is observable in the inner part of the c ytoplasm. One of its ends is near the flagellar bases and the other en d is close to the nucleus in the centrosome region. During the S phase , the flagella drop off, the cell encysts and the kinetosomes duplicat e. In mitosis, the cortical microtubules and the intracytoplasmic micr otubular bundles do not depolymerize. The microtubular fork, desmose a nd centrosome double and migrate, while the divided kinetosomes stay i n the same place. Later, the centrosomes organize the extranuclear spi ndle, which is connected to the kinetosome region by the microtubular desmose. The convergent end of the three-pronged fork seems to be in c ontact with the centrosome region. In early and mid-prophase, thick mi crotubular bundles pass through the nucleus in cytoplasmic channels an d converge towards the two poles. Asters were never seen at the spindl e poles. The channels and microtubular bundles in the spindle double i n number during late prophase and lengthen in early anaphase. The spin dle bundles diverge in late anaphase, extend to very near the plasma m embrane and depolymerize during telophase. The cleavage furrow in whic h tubulin and actin are characterized appears in anaphase, formed by i nvagination of plasma membrane in the kinetosome region. The structure and rearrangements of the Crypthecodinium cohnii microtubular system are compared with those of other dinoflagellates and protists and of h igher eukaryotes.