PIRSONIA, PHAGOTROPHIC NANOFLAGELLATES INCERTAE SEDIS, FEEDING ON MARINE DIATOMS - ATTACHMENT, FINE-STRUCTURE AND TAXONOMY

After being chemotactically attracted by its host, marine diatoms of t he genus Coscinodiscus, the Pirsonia flagellates are guided by the top ography of the host cell surface to the rimoportulae through which the trophosome, a basally formed pseudopodium, penetrates into the diatom . Cytochalasin D does not disturb the chemotaxis of the flagellates bu t inhibits their attachment. Pirsonia diadema and P. guinardiae, feedi ng on Coscinodiscus and on Guinardia flaccida, respectively, are very similar in their fine structure. The anterior flagellum has tubular ha irs which seem to belong to the tripartite, stramenopile type. They ar e synthesized within the ER and exocytosed by the Golgi apparatus. In certain developmental stages the posterior flagellum and the cell body bear ''knotted hairs'' which differ in length. The mode of their form ation is unknown. The mitochondria have unique non-contorted tubular c ristae which are parallel to each other. The few dictyosomes have a pa rabasal position. The flagellar root apparatus consists of fibrillar b ands which connect the basal body of the anterior flagellum with the n ucleus and with the basal body of the posterior flagellum. Other fibri llar bands and a few microtubules run from the basal bodies to the cel l surface. The organization of the flagellar root apparatus thus diffe rs from that of typical stramenopites. The taxonomic position of Pirso nia is discussed.