Ultrastructure ofTessellaria volvocina(Synurophyceae)

Abstract

The ultrastructure of Tessellaria volvocina is described by means of transmission electron microscopy. The large colonies consist of a single peripheral layer of cells with long tails that extend to the centre. The scales making up the colonial investment cover the cell anterior only. No scales are present within the colony. The cell organelles are arranged in a typically synurophycean manner, with the exception of the chloroplasts which lie horizontally within the cell as opposed to vertically in Synura and Mallomonas. The flagellar root system of T. volvocina is simpler than that described for other synurophytes, comprising only one microtubular root which arises from the rhizoplast and encircles the apical flagellar pit. This root is associated with a band of dense material and acts as a microtubule-organizing centre, originating cytoskeletal microtubules in two planes. Tessellaria volvocina possesses several ultrastructural characters typical of the Synurophyceae but differs in its flagellar root system, photoreceptor system, apparent absence of flagellar scales, biradially symmetrical plate-scales, non-imbricating scale case, and site of scale formation. We conclude that Tessellaria should be placed within the Synurophyceae as a specialized member of the class, rather than in a class of its own.