CYLINDROCAPSA: CELL DIVISION AND PHYLOGENETIC AFFINITIES

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Abstract
SUMMARY: Each vegetative cell of Cylindrocapsa is enclosed by its own wall, and the filamentous form is maintained by a separate sheath of wall material derived from parental cells. Cell division differs from most other Ulotrichalean genera so far described. Cleavage is initiated even before prophase, usually first from that side of the cell where the interphase nucleus and centrioles lie near the wall. Centrioles replicate and the pairs then separate around the nucleus. The spindle is closed; the poles are broad and flattened with the centrioles coming to lie to one side of them. The nuclear envelope is highly perforated at each pole and spindle microtubules run into these small holes. As anaphase progresses, the ingrowing cleavage furrow impinges upon the interzonal spindle, often deforming it. No phycoplast is formed for cytokinesis. Instead, the spindle seems to collapse as the furrow pushes through it so that the daughter nuclei eventually lie on either side of it.Recent work has shown this type of cell division to occur in several other Ulotrichalean algae. A recent redefinition of the order Ulvales based on this type of cell division includes several algae formerly in the Ulotrichales. We support this redefinition and the inclusion of Cylindrocapsa in the Ulvales as the only genus in the family Cylindrocapsaceae.