The flagellar apparatus and temporary centriole-associated microtubule systems at the interphase-mitosis transition in the green alga Gloeomonas kupfferi: an example of the spatio-temporal flexibility of microtubule-organizing centres

The interphase-mitosis transition in the green flagellate Gloeomonas kupfferi (Chlamydomonadales) was analysed using indirect immunofluorescence microscopy and transmission electron microscopy of serial sections, with emphasis on the organization of the microtubular cytoskeleton. At interphase, cortical microtubules originate from different parts of the flagellar apparatus, including a connecting fibre, two basal bodies and flagellar roots. The interphasemitosis transition is characterized by: (1) loss of motility and reduction of the flagellar apparatus, resulting in two remaining, widely separated, plasma membrane-associated basal body pairs; (2) migration of the nucleus towards the anterior (basal body-containing) side of the cell, presumably resulting from contraction of a cytoskeletal system recently discovered in related flagellates; (3) depolymerization of the cortical cytoskeleton; (4) transformation of the basal body pairs and associated pericentricular material into centrosomes that are involved in spindle formation; (5) dissociation of the centrosomes from the spindle microtubules at metaphase. The present study strongly suggests that microtubule-organizing centres in green flagellates are flexible entities that may be associated with diverse components of the flagellar apparatus at interphase. At the interphase-mitosis transition, centrosomes are activated to form astral microtubule systems. The dissociation of centrosomes from the metaphase spindle suggests that microtubule initiation and microtubule (re)organization are two different processes. These findings are discussed with respect to current ideas on the control of microtubule synthesis and organization in animal cells.