Mitosis in Barbulanympha. II. Dynamics of a two-stage anaphase, nuclear morphogenesis, and cytokinesis.

Anaphase in Barbulanympha proceeds in two discrete steps. In anaphase-A, chromosomal spindle fibers shorten and chromosomes move to the stationary centrosomes. In anaphase-B, the central spindle elongates and ("telophasic") bouquets of chromosomes, with kinetochores still connected by the shortened chromosomal fibers to the centrosomes, are moved far apart. The length, width, and birefringence of the central spindle remain unchanged throughout anaphase-A. In anaphase-B, the central spindle elongates up to fivefold. During elongation, the peripheral fibers of the central spindle splay, first anteriorly and then laterally. The remaining central spindle progressively becomes thinner and the retardation decreases; however, the coefficient of birefringence stays approximately constant. The nuclear envelope persists throughout mitosis in Barbulanympha and the nucleus undergoes an intricate morphological change. In prophase, the nucleus engulfs the spindle; in early anaphase-A, the nuclear envelope forms a seam anterior to the spindle, the nucleus thus transforms into a complete sleeve surrounding the central spindle. In late anaphase-A, the middle of the seam opens up in a cleft as the lips part; in anaphase-B, the cleft expands posteriorly, progressively exposing the central spindle. Finally, the cleft partitions the nucleus into two. The nuclear envelope shows an apparent elasticity and two-dimensional fluidity. Localized, transient deformations of the nuclear envelope indicate poleward and counter-poleward forces acting on the kinetochores embedded in the envelope. These forces appear responsible for nuclear morphogenesis as well as anaphase chromosome movement. At the end of anaphase-B, the two rostrate Barbulanympha may swim apart of be poked apart into two daughter cells by another organism cohabiting the host's hindgut.