Analysis of the distribution of spindle microtubules in the diatom Fragilaria.

Abstract

The spindle of the colonial diatom Fragilaria [F. capucina f. mesolepta] contains 2 distinct sets of spindle microtubules (MT): MT comprising the central spindle, which is composed of 2 half-spindles interdigitated to form a region of overlap; and MT which radiate laterally from the poles. The central spindles from 28 cells are reconstructed by tracking each MT of the central spindle through consecutive serial sections. Because the colonies of Fragilaria are flat ribbons of contiguous cells (clones), it is possible, by using single ribbons of cells, to compare reconstructed spindles at different mitotic stages with minimal intercellular variability. From these reconstructions the changes in distribution of MT along the spindle during mitosis; the change in the total number of MT during mitosis; the length of each MT (measured by the number of sections each traverses) at different mitotic stages; the frequency of different classes of MTs (i.e., free, continuous, etc); the spatial arrangement of MT from opposite poles in the overlap; and the approximate number of MT, separate from the central spindle, which radiate from each spindle pole were determined. From longitudinal sections of the central spindle, the lengths of the whole spindle, half-spindle and overlap were measured from 80 cells at different mitotic stages. Numerous sources of error may create inaccuracies in these measurements. These problems are discussed. The central spindle at prophase consists predominantly of continuous MT (pole to pole). Between late prophase and prometaphase, spindle length increases, and the spindle is transformed into 2 half-spindles (mainly polar MT) interdigitated to form the overlap. At late anaphase-telophase, the overlap decreases concurrent with spindle elongation. The MT of the central spindle slide past one another at both late prophase and late anaphase. These changes in MT distribution have the effect of elongating the spindle and are not involved in the poleward movement of the chromosomes. Some aspects of tracking spindle MT, the interaction of MT in the overlap, formation of the prophase spindle and an interpretation of rearrangements of MT, are discussed.