Molecular phylogeny and ultrastructure of Selenidium serpulae (Apicomplexa, Archigregarinia) from the calcareous tubeworm Serpula vermicularis (Annelida, Polychaeta, Sabellida)

213 Leander, B. S. (2007). Molecular phylogeny and ultrastructure of Selenidium serpulae (Apicomplexa, Archigregarinia) from the calcareous tubeworm Serpula vermicularis (Annelida, Polychaeta, Sabellida). — Zoologica Scripta , 36 , 213–227. Archigregarines are dynamic single-celled parasites that inhabit the intestinal systems of marine invertebrates, especially suspension feeding and deposit feeding polychaetes. Certain archigregarines in the genus Selenidium have retained several plesiomorphic characters, and improved knowledge of these species is expected to shed considerable light onto the earliest stages in apicomplexan evolution. Although archigregarines are related to some of the most notorious parasites known (e.g., C ryptosporidium and Plasmodium ), current knowledge of the group is meagre. In an attempt to improve our understanding of archigregarine diversity and evolution, I have characterised the general ultrastructure and molecular phylogenetic position of Selenidium serpulae (Lankester) Caullery and Mesnil. The parasites were isolated from the intestines of the calcareous tubeworm Serpula vermicularis (Polychaeta) collected in the eastern Pacific Ocean. The trophozoites (extracellular feeding stages) were spindle-shaped and capable of slow and continuous bending, and coiling, especially when dislodged from the host epithelium. The trophozoite surface was composed of 19–23 longitudinal folds, prominent transverse folds and a robust, trilayered pellicle subtended by a single row of microtubules, each surrounded by an electron transparent sheath. Putative mitochondria were observed, but they were inconspicuous and apparently highly reduced, a condition that is indicative of anaerobic metabolism. The small subunit (SSU) rDNA sequence from S. serpulae was very closely related to two (short) sequences derived from an environmental PCR survey of an oxygendepleted hydrothermal vent system in the Gulf of California, namely C1-E017 (AY046619) and C2-E016 (AY046806). This result suggested that archigregarines with a morphology and lifecycle much like that in S. serpulae are thriving in this oxygen poor ecosystem. In addition, phylogenetic analyses of a larger SSU rDNA dataset (excluding the shorter environmental sequences) indicated that the nearest sister lineage to S. serpulae was S. vivax , a bizarre tape-like gregarine found in the intestines of sipunculids. This relationship was bolstered by comparative ultrastructural data and helped to illustrate that the diversification and biogeographical distribution of archigregarine parasites is probably more extensive than usually assumed. Brian S. Leander, Departments of Botany and Zoology, University of British Columbia, #3529-6270 University Blvd., Vancouver, BC, V6T 1Z4, Canada. E-mail: [email protected] Blackwell Publishing Ltd Molecular phylogeny and ultrastructure of Selenidium serpulae (Apicomplexa, Archigregarinia) from the calcareous tubeworm Serpula vermicularis (Annelida, Polychaeta, Sabellida)