CELL-CYCLE AND PHENOTYPES OF BIDDULPHIOPSIS TITIANA IN CULTURE

Abstract

The cell cycle and intraclonal variation were studied in the bipolar centric diatom Biddulphiopsis titiana (Grunow) v. Stosch & Simonsen. Three differently sized subclones, small [S], large [L], and extra large [XL], and a tripolar strain were all derived from a single cell and cultivated under identical conditions. No difference in ultrastructure was apparent, except for the additional pole in the tripolar cells, coupled to a reduction in the number of rimoportulae per pole. Cell size parameters were compared using SDS-cleaned cell walls of 5 different cell-cycle stages.

In the living cell, except during cell division, plastids perform karyostrophic movement (i.e. to the nucleus) during the dark phase or whenever exposed to strong incident light. Plasmolysis can clearly be distinguished from a contraction of the protoplast in response to physiological or mechanical shock. The reactions differ in degree and speed. Preprophase is correlated with cell elongation and the formation of the second girdle band of the hypotheca. A hitherto undescribed diaphragm in the plane of the prospective division forms prior to prophasic nuclear migration, and coincides with the plane of cytokinesis. The pattern of cleavage is intermediate between that in circular, centric diatoms and that in pennate diatoms. In concert with mitosis and cleavage, a reorientation, complete reduction and then new outgrowth of cytoplasmic strands occurs. The cortical sites of anchorage of the strands are found chiefly at the rimoportulae (LPs), which are situated at the poles. Post cytokinetic shaping of the cleavage surface provides the mould for the gross morphology of the valves.