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Diseases of Aquatic Organisms

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DAO 42:185-190 (2000)  -  doi:10.3354/dao042185

Protistan parasite QPX of hard-shell clam Mercenaria mercenaria is a member of Labyrinthulomycota

Mark A. Ragan1,2,*, Greg S. MacCallum3, Colleen A. Murphy1, Jamie J. Cannone4, Robin R. Gutell4, Sharon E. McGladdery3

1Institute for Marine Biosciences, National Research Council of Canada, 1411 Oxford St, Halifax, Nova Scotia B3H 3Z1, Canada
2Canadian Institute for Advanced Research, Program in Evolutionary Biology, 180 Dundas Street West, Suite 1400, Toronto, Ontario M5G 1Z8, Canada
3Fisheries and Oceans Canada, PO Box 5030, Moncton, New Brunswick E1C 9B6, Canada
4Institute for Cellular and Molecular Biology, University of Texas, Austin, Texas 78712-1095, USA
*Present address: Institute for Molecular Bioscience, The University of Queensland, Brisbane, Queensland 4072, Australia. E-mail:

ABSTRACT: Biomass of the protistan parasite QPX (quahaug parasite X) of hard-shell clam Mercenaria mercenaria was enriched from in vitro culture. The nuclear gene encoding the 18S RNA of the small-subunit ribosomal (ssu-rDNA) was recovered using the polymerase chain reaction (PCR) and sequenced. Phylogenetic analysis clearly showed that QPX is a member of phylum Labyrinthulomycota, within which it appears as a specific relative of Thraustochytrium pachydermum. These results confirm the provisional assignment of QPX to the Labyrinthulomycota made previously on the basis of morphological and ultrastructural characters found in some, but not all, geographic isolates.

KEY WORDS: QPX, quahaug parasite X · Mercenaria mercenaria · Hard-shell clam · Small-subunit ribosomal DNA · Labyrinthulomycota · Thraustochytrium pachydermum

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