THE ROSETTE AGENT OF CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA) IS CLOSELY-RELATED TO CHOANOFLAGELLATES, AS DETERMINED BY THE PHYLOGENETIC ANALYSES OF ITS SMALL RIBOSOMAL-SUBUNIT RNA

The rosette agent of Chinook salmon (Oncorhynchus tshawytscha), initia lly described and characterized in the mid 1980s, is the cause of a se rious infectious disease in the Pacific Northwest of North America. Pr evious work, utilizing rosette agent maintained by growth in embryo sa lmon cell-culture, has shown it to be a eukaryotic obligate intracellu lar parasite. However, its ultrastructural features do not suggest a r elationship with any specific eukaryotic group. We have utilized a mol ecular approach to further investigate the phylogeny of rosette agent previously maintained in vitro during 1990 and 1991. We have amplified the genomic DNA encoding the small subunit ribosomal RNA (16S-like rR NA), and determined the complete DNA nucleotide sequence of this gene segment. Comparison with other eukaryotic 16S-like rRNA sequences sugg ests that the rosette agent shares a unique evolutionary history with choanoflagellates.