Morphology and phylogeny of Sainouron acronematica sp n. and the ultrastructural unity of Cercozoa

Sainouron are soil zooflagellates of obscure taxonomy. We studied the ultrastructure of S. acronematica sp. n. and sequenced its extremely divergent 18S rDNA and that of Cholamonas cyrtodiopsidis (here grouped as new family Sainouridae) to clarify their phylogeny. Ultrastructurally similar, they weakly group together, deeply within Monadofilosa. Sainouron has three cytoplasmic microtubules; all organelles specifically link to them or the nucleus. Mature centrioles have fibrous rhizoplasts. The posterior centriole bearing the motile cilium (with cortical filaments) has a transitional hub-lattice; a dense spiral fibre links its thicker rhizoplast and triplets; its ciliary root has two microtubules: mt1, underlying the plasma membrane, initiates at the spiral fibre; mt2, laterally attached to mt1 and nucleus, initiates in the amorphous centrosomal region. The anterior younger cilium, an immotile stub with submembrane skeleton as in Cholamonas, lacks axoneme, microtubular root, rhizoplasts and spiral fibre, but becomes the posterior one every cell cycle. The nuclear envelope donates coated vesicles directly to the Golgi, which makes kinetocyst-type extrusomes, concentrated at the cell anterior for extrusion into phagosomes. Ciliary transition region proximal hub-lattices (postulated to contain centrin) and distal nonagonal fibres are cercozoan synapomorphies, found with slight structural variation in all flagellate Cercozoa, but not in outgroups. (C) 2008 Elsevier GmbH. All rights reserved.