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 ultraplank tonic species of Pyramimonas, has been examined in detail. This specie s differs from other species of Pyramimonas in the structure of the fl agellar apparatus, the scaly periplast, the pyrenoid, the eyespot, loc ation 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 s ystem is present but a rhizoplast was not found. A very short transiti onal 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 re lative to the rest of the cell. Thus the eyespot in the various specie s is located differently relative to the flagellar apparatus and to ot her organelles such as the nucleus (in Pyramimonas there is no connect ion between the flagellar apparatus and the eyespot via a flagellar ro ot). Considerable experimentation has apparently taken place with rega rd to the orientation of the photoreceptor apparatus during the evolut ionary history of the prasinophytes. In more advanced green algae this variability does not occur. In Pyramimonas four different flagellar a pparatus/eyespot orientations have been detected, which has led us to a modified classification of the genus at the subgeneric level. New su bgenera are proposed for P. virginica (subgenus Hexactis) and P. longi cauda (subgenus Macrura).