Macronuclear division with and without microtubules in Tetrahymena.

The effects of cold shocks (0 degrees C) and colchicine (4 mg/ml) on macronuclear microtubules and macronuclear division have been investigated in Tetrahymena. Macronuclear division was affected by both treatments, but loss of microtubules occurred only in the presence of colchicine. When colchicine was applied immediately prior to macronuclear elongation, the nuclei underwent a partial elongation without microtubules, but were unable to constrict and separate into daughter nuclei. Such nuclei were cut in two by the advancing cytoplasmic fission furrow. When colchicine was applied to macronuclei in the fully elongated state, that maintained their elongation in the absence of microtubules, and were subsequently separated into daughter nuclei by the cytoplasmic furrow. It is suggested that macronuclear microtubules, most probably the membrane-associated microtubules, are required for the terminal stages of nuclear elongation and separation. However, the considerable macronuclear elongation which takes place in the absence of microtubules serves to focus attention on molecular mechanisms of force production which must reside elsewhere in the macronucleus.