Development of thecal plates and pellicle in the dinoflagellate Scrippsiella hexapraecingula (Peridiniales, Dinophyceae) elucidated by changes in stainability of the associated membranes

Abstract

The development of the pellicle and thecal plates of the dinoflagellate Scrippsiella hexapraecingula Horiguchi et Chihara was investigated with particular emphasis on cellulose synthesis. In young motile cells, incipient thecal plates appear as several groups of granular material within individual sites that are thought to be amphiesmal vesicles and develop into thin, sheet-like plates. These plates subsequently thicken with the deposition of amorphous material and cellulose microfibrils to form mature thecal plates. After ecdysis, non-motile cells form a pellicle consisting of three layers, L1-3. L1 is electron dense, while L2 and L3 are electron transparent. L1 and L2 are non-cellulosic, but L3 contains cellulose microfibrils. A new type of putative cellulose-synthesizing enzyme complex was found in non-motile cells. It consists of two rows of particles occurring on the plasmatic fracture face of the plasma membrane and begins to appear 0.5-1 h after ecdysis. The periodic acid-thiocarbohydrazide-silver proteinate test (PATAg) strongly stains the plasma membrane in both motile and non-motile cells throughout most of the life cycle of this species. The only time when it does not stain the plasma membrane is during an approximately 1-h period following ecdysis. During this period, the plasma membrane stains strongly with a silicotungstic acid-chromic acid mixture (STA-CA). STA-CA also stains the inner membranes of amphiesmal vesicles in both motile and non-motile cells. Thus, changes in the ability of PATAg and STA-CA to stain membranes during development suggest that the plasma membrane in non-motile cells is undergoing transformation after ecdysis.