Cell and nuclear division in a freshwater choanoflagellate, Monosiga ovata Kent

Monosiga ovata undergoes a longitudinal cell division not constrained in any way by a cell covering. The first stage in cell division involves the withdrawal of the single anterior flagellum. The flagellar axoneme, from the distal end of the transition region to the tip, is withdrawn deep into the cell where it is depolymerised. Replication of the two parent basal bodies follows to produce two pairs of basal bodies. The transition fibres and the immediate ring of radiating cytoskeletal microtubules around the basal body of the previously emergent flagellum remain intact throughout division. The basal body of the other future emergent flagellum quickly acquires transition fibres and a ring of radiating cytoskeletal microtubules. During prophase the collar becomes ovoid in cross section and the number of tentacles increases from about 26 to 40. Within the nucleus aggregates of heterochromatin and groups of microtubules become apparent. During metaphase the nucleus becomes elongated, spindle microtubules appear and densely staining chromatin becomes aligned at the median metaphase plate. The nuclear envelope remains intact throughout prophase and early metaphase and there is no indication that the polar basal bodies make contact with the spindle microtubules within the nucleus. The antiparallel polar basal bodies begin to regenerate flagella. At this stage the two flagellar basal bodies are at the extreme ends of the elongated collar and the two emergent flagella project through the sides of the collar. As metaphase proceeds, fenestrae appear in the nuclear envelope at the poles. Eventually the nuclear envelope disintegrates and nuclear division continues through anaphase and telophase. As cell division occurs the flagellar bases move towards the central anterior region of the two daughter cells. The number of collar tentacles continues to increase to approximately 50. As cytokinesis progresses the two daughter cells become more distinct and eventually separate each with a fully developed flagellum and a surrounding collar.