CENTROSOME AND MICROTUBULE DYNAMICS IN APICAL CELLS OF SPHACELARIA-RIGIDULA (PHAEOPHYCEAE) TREATED WITH NOCODAZOLE

Treatment of young thalli of Sphacelaria rigidula with 0.04 mu g of no codazole (Nz) per mi for up to 36 h affects microtubules (Mts) only sl ightly, but blocks a large number of mitotic cells in metaphase, witho ut disruption of the metaphase plate. Higher concentrations of Nz (0.1 mu g/ml) depolymerize interphase Mts. Only a few perinuclear and some short Mts resist and remain associated with the centrosomes. Fragment ed Mts or groups of Mts sometimes remain in the apical dome. After tre atment with 0.1 mu g of Nz per mi, prometaphase cells are blocked at m etaphase, while post-metaphase cells become binuclear, due to the fail ure of cytokinesis. With anticentrin immunofluorescence, a positive ce ntrin signal is always observed in the centrosome area. Centrosome dup lication is not affected by Nz, but separation is disturbed. After rec overing for 2-4 h, most of the blocked metaphases proceed normally. In such cells duplicated centrosomes are seen in different stages of sep aration. In some cells independent aster-like microtubule configuratio ns appear in the apical dome, occasionally displaying centrin at their centre. During recovery various configurations of bimitosis or multip olar mitosis were found. The multipolar spindles may share common cent rosomes. Up to four centrosomes may accompany each nucleus. In some 24 h treated cells, as well as in cells recovering for 2 h, the centrin- positive structure is rod-like, extending in opposite directions from the usual position to the poles. Electron microscopical examination of thin sections revealed that the growth pattern of the apical cells is disrupted after Nz treatment. The observations show that: (a) the Mt cytoskeleton is involved in maintaining the polarity and growth patter n of apical cells, (b) mitosis is blocked by low concentrations of Nz without significant depolymerization of Mts, (c) the centrosome cycle is independent of the nuclear cycle, (d) centrosome separation and dif ferentiation are disturbed by Nz treatment, (e) during recovery from N z treatment, centrosomal material that may have separated from the cen trosomes, as well as Mt fragments that resisted depolymerization, may operate as MI nucleation centres.