Fish Pathology
Online ISSN : 1881-7335
Print ISSN : 0388-788X
ISSN-L : 0388-788X
Research Articles
Ultrastructural and Molecular Phylogenetic Identification of the Diplomonad Flagellate Spironucleus salmonis Infecting Hatchery-reared Salmonid Fishes in Hokkaido
Shinya MizunoShigehiko UrawaYoshitomo KatsumataTakumi MorishitaMasatoshi Ban
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2020 Volume 55 Issue 1 Pages 8-17


Systemic infections caused by diplomonad flagellates belonging to the genus Spironucleus commonly result in heavy mortalities in salmonids. ​Diplomonad flagellates were found in the intestinal lumen of hatchery-reared juvenile chum Oncorhynchus keta and masu salmon O. masou in Hokkaido. ​We performed ultrastructural observation and molecular phylogenetic analysis using the small subunit ribosomal RNA gene (SSUrDNA) of the flagellates to clarify their taxonomy. ​The flagellates showed elongated and tapered nuclei, and sub-apical location of the kinetosomes relative to the nuclei: which are the distinguishing characters of the genus Spironucleus among three genera of parasitic diplomonads. ​In addition, the flagellates had electron dense plaques at their posterior end, a posterior sac of dense free ribosomes, an electron dense body, and bowl-shaped membranous structures, identifying them as S. salmonis among four Spironucleus species that affect fish. ​The ultrastructure of the flagellates showed no variations between the two host species of salmon. ​All the isolated flagellate SSUrDNAs had the same 1,031 bp sequence, and no difference in the sequence. ​The SSUrDNA showed the highest homology (99%) with S. salmonis among 11 diplomonad flagellates. ​Thus, this study successfully used ultrastructure diagnosis and molecular phylogenetic analysis to confirm S. salmonis as the diplomonad flagellate that has infected hatchery-reared chum and masu salmon in Hokkaido.

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© 2020 The Japanese Society of Fish Pathology
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