Archigregarines, an early branching lineage within Apicomplexa, are a poorly-known group of invertebrate parasites. By their phylogenetic position, archigregarines are an important lineage to understand the functional transition that occurred between free-living flagellated predators to obligatory parasites in Apicomplexa. In this study, we provide new ultrastructural data and phylogenies based on SSU rDNA sequences using the type species of archigregarines, the Selenidiidae Selenidium pendulaGiard, 1884. We describe for the first time the syzygy and early gamogony at the ultrastructural level, revealing a characteristic nuclear multiplication with centrocones, cryptomitosis, filamentous network of chromatin, a cyst wall secretion and a 9+0 flagellar axoneme of the male gamete. S. pendula belongs to a monophyletic lineage that includes several other related species, all infecting Sedentaria Polychaeta (Spionidae, Sabellaridae, Sabellidae and Cirratulidae). All of these Selenidium species exhibit similar biological characters: a cell cortex with the plasma membrane - inner membrane complex - subpellicular microtubule sets, an apical complex with the conoid, numerous rhoptries and micronemes, a myzocytosis with large food vacuoles, a nuclear multiplication during syzygy and young gamonts. Two other distantly related Selenidium-like lineages infect Terebellidae and Sipunculida, underlying the ability of archigregarines to parasite a wide range of marine hosts.

Cell Biology and Electron Microscopy Laboratory François Rabelais University 10 Boulevard Tonnellé BP 3223 37032 Tours Cedex France

Department of Botany and Zoology Faculty of Science Masaryk University Kotlářská 2 611 37 Brno Czech Republic

Laboratoire des Sciences de l'Environnement Marin Technopole Brest Iroise 29280 Plouzané France

Sorbonne Universités Université Pierre et Marie Curie Paris 6 CNRS UMR 7144 Station Biologique de Roscoff Place Georges Teissier CS90074 29688 Roscoff cedex France

Unité Molécules de Communication et Adaptation des Microorganismes Muséum National Histoire Naturelle Sorbonne Universités CNRS CP 52 57 Rue Cuvier 75005 Paris France

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