Dynamic changes in microtubule organization during division of the primitive dinoflagellate Oxyrrhis marina

The marine dinoflagellate Oxyrrhis marina has three major microtubular syst ems: the flagellar apparatus made of one transverse and one longitudinal fl agella and their appendages, cortical microtubules, and intranuclear microt ubules. We investigated the dynamic changes of these microtubular systems d uring cell division by transmission and scanning electron microscopy, and c onfocal fluorescent laser microscopy. During prophase, basal bodies, both f lagella and their appendages were duplicated. In the round nucleus situated in the cell centre, intranuclear microtubules appeared radiating toward th e centre of the nucleus from densities located in some nuclear pores. Durin g metaphase, both daughter flagellar apparatus separated and moved apart al ong the main cell axis. Microtubules of ventral cortex were also duplicated and moved with the flagellar apparatus. The nucleus flattened in the longi tudinal direction and became discoid-shaped close to the equatorial plane. Many bundles of microtubules ran parallel to the short axis of the nucleus (cell long axis), between which chromosomes were arranged in the same direc tion. During ana-telophase, the nucleus elongated along the longitudinal ax is and took a dumbbell shape. At this stage a contractile ring containing a ctin was clearly observed in the equatorial cortex. The cortical microtubul e network seemed to be cut into two halves at the position of the actin bun dle. Shortly after, the nucleus divided into two nuclei, then the cell body was constricted at its equator and divided into one anterior and one poste rior halves which were soon rebuilt to produce two cells with two full sets of cortical microtubules. From our observations, several mechanisms for th e duplication of the microtubule networks during mitosis in O. marina are d iscussed. (C) 2000 Editions scientifiques et medicales Elsevier SAS.