Developmental studies on the loricate choanoflagellate Stephanoeca diplocostata ellis : VIII. Nuclear division and cytokinesis

Abstract

Summary In S. diplocostata the end of interphase coincides with the completion of costal strip production. The protoplast enlarges and the endomembrane system, consisting of the Golgi apparatus and ER, is active in producing fibrillar material which is stored in vesicles at the anterior end of the protoplast. Immediately prior to division the flagellum is withdrawn into the protoplast to one side of the flagellar base. The axonematal microtubules are apparently severed in the transition region and the axoneme is withdrawn deep into the protoplast where it becomes surrounded by the endomembrane system and is depolymerised. A second flagellar base appears beside the base of the resorbed flagellum. Both flagellar bases have a proximal ring of radial cytoskeletal microtubules. At the beginning of nuclear division the anterior portion of the nuclear envelope disperses and, as one of the flagellar bases moves away from the anterior end, the spindle microtubules appear. In a median longitudinal section of the nucleus during metaphase the symmetrical arrangement of spindle microtubules between the two polar flagellar bases is observed. Following nuclear division the two flagellar bases migrate, one to the lower end of the parent protoplast and the other, which was at the anterior end, moves towards the equator. The proximal ring of cytoskeletal microtubules around each flagellar base is prominent at this stage and it seems likely that the movement of the collar tentacles towards the equator is coordinated by these microtubules. During telophase organelles are shared out between the developing daughter protoplasts. As the protoplast divides to produce two discrete daughter protoplasts the two flagellar bases move to a central equatorial position and are inverted with respect to each other. The vesicles containing fibrillar material remain within the anterior (juvenile) protoplast. Their ultimate disappearance coincides with lorica assembly and it is probable that the fibrillar material is used in the formation of the lorica investment.