THE ULTRASTRUCTURE OF THE FLAGELLAR ROOT-SYSTEM OF ISOCHRYSIS-GALBANA (PRYMNESIOPHYTA)

The fine structure of the flagellar root system of Isochrysis galbana Parke (Prymnesiophyta) has been investigated and a reconstruction is presented. Associated with the left basal body and arising close to the base of the haptonema there is a root composed of a ribbon of approximately five microtubules running peripherally towards the ventral side of the cell. Near the basal body it is also associated with two additional bundles of microtubules which diverge from it and from each other passing deep into the cell. On the right-hand side of the right basal body there is a root consisting of four microtubules (2+2) which runs peripherally in a dorsal direction. Another root, consisting of a single microtubule only, originates close to the distal connecting band between the basal bodies, and passes around the ventral side of the left basal body before following a course deep into the cell. In addition, several fibrous bands connect the bases of the flagella and haptonema. The general arrangement of the root system of I. galbana is comparable with many other members of the Prymnesiophyta, but attention is drawn to a number of important differences, particularly between Isochrysis and some other members of the order Isochrysidales. The results are discussed in the context of the evolution, phylogeny and taxonomy of the Prymnesiophyta.