Cytoskeletal dynamics of Apedinella radians (Pedinellophyceae): II. Cell division and the maintenance of cell polarity and symmetry

The dynamics of the cytoskeletal proteins centrin, actin, and tubulin were followed during cell division in the unicellular phytoflagellate Apedinella radians (Pedinellophyceae). Three centrin, or centrin-like, components appear to coordinate independent developmental events during cell division. The first component, basal body centrin, maintains a physical link between basal bodies and the anterior nuclear membrane. Basal body centrin divides in two at metaphase, and each portion segregates with two basal bodies at anaphase. As the positioning of basal bodies defines the anterior region of the cell, basal body centrin appears to play a role in maintaining cell polarity throughout the cell cycle. The second centrin component consists of an array of filamentous bundles arranged as a six-pointed star. During cell division, the star undergoes a conformational change resulting in two distinct centrin triangles, one distributed to each daughter cell, suggesting that centrin filamentous bundles are involved in maintaining cell (radial) symmetry. The third centrin component is transient and associates with the spindle poles, emerging prior to mitosis and remaining until late anaphase/early telophase. Spindle pole centrin establishes temporary horizontal bipolarity, thereby establishing the spindle axis. Unlike centrin filamentous bundles, actin filamentous bundles depolymerize prior to mitosis, indicating they do not influence cell symmetry during cell division. Mitosis is described for the first time in a pedinellid and features a closed spindle, the absence of rhizoplasts and a persistent spindle.