Comparison of small subunit ribosomal RNA gene and internal transcribed spacer sequences among isolates of the intranuclear microsporidian Nucleospora salmonis

Nucleospora salmonis is an intranuclear microsporidian associated with a pr oliferative disorder of the lymphoid cells of captive salmonid fish in the northwestern and northeastern regions of North America, in France, and in C hile. Newer diagnostic approaches have used the polymerase chain reaction ( PCR) to detect the parasite in fish tissues. The target sequences for these assays lie in the small subunit ribosomal RNA (ssu rRNA) gene or internal transcribed spacer (ITS) as determined from N. salmonis from chinook salmon (Oncorhynchus tshawytscha) from the Pacific Northwest of North America. Th e lack of sequence data on parasites from diverse geographic origins and ho sts led us to compare several isolates of N. salmonis. There was a high deg ree of similarity in the ssu rDNA sequences (> 98%) among all the isolates of N. salmonis examined, regardless of host or geographic origin. The great est sequence differences were found between isolates from the Pacific regio ns of America. Isolates from Chile shared sequences with one or both geogra phic groups from North America. A similar distribution of sequence types wa s observed when ITS-1 sequences of selected isolates were analyzed. Sequenc e data from two N. salmonis-like isolates from marine non-salmonid fish sho wed one closely related and the second less closely related to N. salmonis isolates from salmonid fish. These results provide evidence for a homogeneo us group of aquatic members of the genus Nucleospora found among salmonid f ish (N. salmonis) that can be detected using diagnostic PCR assays with ssu rDNA target sequences. The presence of parasites related to N. salmonis am ong marine fish suggests a potentially broad host and geographic distributi on of members of the family Enterocytozoonidae.