FINE-STRUCTURAL STUDIES ON AN ULTRAPLANKTONIC SPECIES OF PYRAMIMONAS, P-VIRGINICA (PRASINOPHYCEAE), WITH A DISCUSSION OF SUBGENERA WITHIN THE GENUS PYRAMIMONAS

The ultrastructure of Pyramimonas virginica, the only known ultraplanktonic species of Pyramimonas, has been examined in detail. This species differs from other species of Pyramimonas in the structure of the flagellar apparatus, the scaly periplast, the pyrenoid, the eyespot, location of the eyespot in the cell, and mitosis. The flagellar apparatus is simple and shows signs of reduction. A 2-2-2-2 microtubular root system is present but a rhizoplast was not found. A very short transitional helix (perhaps two rings) is present in the flagellar transition region, together with a short one-piece stellate structure. Numbering of individual flagella in different species of Pyramimonas has shown a surprising variation in the orientation of the flagellar apparatus relative to the rest of the cell. Thus the eyespot in the various species is located differently relative to the flagellar apparatus and to other organelles such as the nucleus (in Pyramimonas there is no connection between the flagellar apparatus and the eyespot via a flagellar root). Considerable experimentation has apparently taken place with regard to the orientation of the photoreceptor apparatus during the evolutionary history of the prasinophytes. In more advanced green algae this variability does not occur. In Pyramimonas four different flagellar apparatus/eyespot orientations have been detected, which has led us to a modified classification of the genus at the subgeneric level. New subgenera are proposed for P. virginica (subgenus Hexactis) and P. longicauda (subgenus Macrura).