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Diseases of Aquatic Organisms

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DAO 96:229-237 (2011)  -  DOI:

Geographical variation in spore morphology, gene sequences, and host specificity of Myxobolus ­arcticus (Myxozoa) infecting salmonid nerve tissues

Shigehiko Urawa1,*, Mark A. Freeman2, Stewart C. Johnson3, Simon R. M. Jones3, Hiroshi Yokoyama

1Hokkaido National Fisheries Research Institute, Fisheries Research Agency, Toyohira-ku, Sapporo 062-0922, Japan
2Institute of Ocean and Earth Sciences & Institute of Biological Sciences, University of Malaya, Kuala Lumpur 50603, Malaysia
3Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo, British Columbia V9T 6N7, Canada
4Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo, Tokyo 113-8657, Japan

ABSTRACT: Myxobolus arcticus Pugachev and Khokhlov, 1979 is a freshwater myxosporean parasite infecting the nerve tissues of salmonid fishes throughout the Pacific region of Far East Asia and North America. The principal fish host is sockeye salmon Oncorhynchus nerka in North America and masu salmon O. masou in Japan. Actinospores of M. arcticus were isolated from the lumbriculid oligochaetes Lumbriculus variegatus and Stylodrilus heringianus in Japan and Canada, respectively. Morphological comparisons indicated that Japanese actinospores from L. variegatus have significantly shorter caudal projections than Canadian isolates from S. hering­ianus, whereas the corresponding myxospores are indistinguishable. Transmission experiments showed that sockeye salmon were rarely susceptible to the Japanese actinospores, while masu salmon are highly susceptible to this parasite. Sequences of 4560 base pairs of the ribosomal RNA (rRNA) gene, including small subunit (SSU) and internal transcribed spacer (ITS) regions, from Japanese and Canadian isolates had a high similarity over 99.9%, suggesting that they may be conspecific. However, the biological data indicate that they are at least distinct strains. M. arcticus may be geographically isolated due to the specific homing migration of the anadromous fish hosts and has specialized its morphology and host selection for its local environment in the ongoing process of differentiation, potentially leading to speciation.

KEY WORDS: Myxobolus arcticus · Actinospore · Morphology · SSU rRNA · ITS rRNA · Susceptibility · Geographical variation · Salmonid

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Cite this article as: Urawa S, Freeman MA, Johnson SC, Jones SRM, Yokoyama H (2011) Geographical variation in spore morphology, gene sequences, and host specificity of Myxobolus ­arcticus (Myxozoa) infecting salmonid nerve tissues. Dis Aquat Org 96:229-237.

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