Somatic nuclear division in the sporidia of Ustilago violacea. IV. Microtubules and the spindle-pole body

Abstract

In unbudded cells of the anther smut fungus Ustilago violacea there is a dome-shaped spindle-pole body (SPB) consisting of a core 0.1 μm in diameter surrounded by a ribosome-free region 0.3–0.4 μm in diameter lying in a pocket of the nuclear membrane. After budding the nucleus moves towards the bud and begins to rotate rapidly. At about this stage the SPB divides into two parallel bars each about 0.1–0.15 μm in diameter and 0.3 μm long, separated by a distance of about 0.3 μm. Microtubules associated with the nuclear membrane but not with the SPB are present at the time of nuclear rotation. These microtubules disappear when rotation stops. Microtubules attached to the SPB are found during migration of the chromatinic portion of the nucleus into the bud cell. These microtubules disappear when migration stops and the nuclear membrane begins to break down. The twin SPB bars appear to move into the nucleus through a break in the membrane and begin to move apart forming a spindle about 1 μm long. Chromosomal microtubules (one per kinetochore) were found in several serial sections, and in addition there appeared to be several continuous microtubules present. The separation of the two chromatinic masses appeared to result from elongation of the continuous microtubules to about 3 μm long. Cytoplasmic microtubules and spindle microtubules were both found attached to the SPB as it elongated and one nucleus returned to the mother cell.The paper concludes with a discussion of the SPB as a multifunctional control center affecting nuclear migration, spindle formation, membrane breakdown and synthesis, karyogamy, conjugation, budding, chromosomal movement, replication, and disjunction.