Mitosis in Surirella is described; this organism displays a number of unusual features including an unorthodox method of chromosome attachment to the spindle, and the differentiation of an extranuclear central spindle from a large spherical organelle named the microtubule center (MC). The MC, present during interphase, breaks down at late prophase as the central spindle is formed. Later, the spindle enters the nucleus; the chromatin, in association with microtubules (MTs) from the poles, increasingly aggregates around the middle "overlap" region of the central spindle, and by metaphase completely encircles it. Throughout, MTs usually associate laterally with the chromatin. We were not able to identify kinetochore MTs with confidence at either metaphase or anaphase. Instead, at anaphase the leading point of the chromosomes is embedded in a ring of electron-dense material, named the "collar," which encircles each half spindle and extends from the chromatin to the pole. Anaphase separation of the chromosomes is achieved by at least three separate mechanisms: (a) between metaphase and late anaphase the central spindle increases in length by the addition of MT subunits; (b) at late anaphase the central spindle elongates concurrent with a reduction in the overlap; this apparently results from an MT/MT sliding mechanism; (c) each set of chromosomes moves to the poles by a thus far unknown mechanism; however, we anticipate some interaction of the collar and central spindle. At telophase, the polar complexes, (i.e., structures at the spindle pole) separate from the spindle, and later a new MC is formed near each polar complex, after which the polar complexes break down. Aspects of the complex differentiation of the MC, spindle formation, and some unusual characteristics of the diatom spindle as they relate to anaphase motion and spindle function are discussed.
Mitosis in the pinnate diatom surirella ovalis