AN EXTENDED CORONA ATTACHED TO METAPHASE KINETOCHORES OF THE GREEN-ALGA OEDOGONIUM

Mitotic cells of the green alga Oedogonium were treated with the anti- microtubule agent oryzalin (1.0-0.1 muM) for 5 to 10 min. Within 5 min treatment of living cells, metaphase spindles became spherical with d isorganized chromosomes, and anaphase spindles collapsed. At lower con centrations, the effects were slower, and partial recovery was observe d about 10 to 20 min after the drug was washed out. Following breakdow n of the spindle, considerable disorganized activity detected by time- lapse continued within the nucleus, isolated from the cytoplasm by its intact nuclear membrane. Under the electron microscope, spindle micro tubules (MTs) were absent in oryzalin-treated cells. Paired metaphase kinetochores displayed an array of fine filamentous material extended, usually straight, about 3 mum into the nucleoplasm. In cells recoveri ng from oryzalin treatment, MTs became associated with kinetochores in the usual manner. However, this filamentous array, the ''extended cor ona'' (EC), was almost undetectable, even when the MTs were short and poorly organized. The EC is appreciably larger by metaphase than the c orona of prophase chromosomes and so it may assemble during early mito sis. Fine filaments interspersed with kinetochore MTs have been descri bed in carefully fixed cells of this alga (M. J. Schibler, J. D. Picke tt-Heaps, Eur. J. Cell Biol. 22, 687-698 (1980)). The EC apparently re presents a less organized form of this material remaining after its sc affold of MTs has been removed. These fibers appear involved in MT cap ture upon spindle recovery from anti-MT drugs. They could function dur ing prometaphase and even anaphase movement along spindle MTs.