Ultrastructure and molecular diagnosis of Spironucleus salmonis (Diplomonadida) from rainbow trout Oncorhynchus mykiss in Germany.

Diplomonad flagellates infect a wide range of fish hosts in aquaculture and in the wild in North America, Asia and Europe. Intestinal diplomonad infection in juvenile farmed trout can be associated with morbidity and mortality, and in Germany, diplomonads in trout are commonly reported, and yet are poorly characterised. We therefore undertook a comprehensive study of diplomonads from German rainbow trout Oncorhynchus mykiss, using scanning and transmission electron microscopy, and sequencing of the small subunit (ssu) rRNA gene. The diplomonad was identified as Spironucleus salmonis, formerly reported from Germany as Hexamita salmonis. Our new surface morphology studies showed that the cell surface was unadorned and a caudal projection was present. Transmission electron microscopy facilitated new observations of functional morphology, including vacuoles discharging from the body surface, and multi-lobed apices of the nuclei. We suggest the lobes form, via hydrostatic pressure on the nucleoplasm, in response to the beat of the anterior-medial flagella. The lobes serve to intertwine the nuclei, providing stability in the region of the cell exposed to internal mechanical stress. The ssu rRNA gene sequence clearly distinguished S. salmonis from S. barkhanus, S. salmonicida, and S. vortens from fish, and can be used for identification purposes. A 1405 bp sequence of the ssu rRNA gene from S. salmonis was obtained and included in a phylogenetic analysis of a selection of closely related diplomonads, showing that S. salmonis was recovered as sister taxon to S. vortens.

Ultrastructure of Spironucleus salmonis n. comb. (formerly Octomitus salmonis sensu Moore 1922, Davis 1926, and Hexamita salmonis sensu Ferguson 1979), with a guide to spironucleus species.

Diplomonad flagellates can be associated with significant morbidity and mortality in fishes, particularly in farmed salmonids. Diagnosis using only light microscopy is limited, and species cannot be confirmed. We therefore undertook a comprehensive transmission electron microscopy study of 20 trophozoites from the intestine of farmed juvenile rainbow trout Oncorhynchus mykiss from Northern Ireland (reported as Hexamita salmonis by Ferguson in 1979). Re-assignment to the genus Spironucleus was determined based on the anteriorly tapering and intertwined elongate nuclei, anterior-medial kinetosomes, and the presence of a flagellar pocket. At the species level we observed a tri-radiate pattern of microtubules in the opening of the asymetrical striated lamina, comprising 3 to 5 microtubules following the edge of the flagellar pocket, 3 radiating away from the opening of the striated lamina, and 4 to 5 curving over the striated lamina. We observed electron-dense plaques adjacent to the kinetosomes, electron-dense bodies, numerous free ribosomes, aggregations of glycogen, bowl-shaped membranous structures, rough endoplasmic reticulum, and a novel distinctive pyriform sac of densely packed free ribosomes at the posterior of the cell (8-shaped in transverse section, and delineated by furrowed endoplasmic reticulum). We now propose to rename H. salmonis sensu Ferguson (1979) Spironucleus salmonis. Our review of the morphology of Octomitus salmonis illustrated by Moore (1922a,b) and Davis (1926), has shown that this organism also belongs to the genus Spironucleus. We synonymise H. salmonis sensu Ferguson (1979) with O. salmonis sensu Moore (1922) and Davis (1926), and rename them S. salmonis. An expanded diagnostic guide, including new cytoplasmic elements, is presented for the 4 species of Spironucleus from fishes (S. barkhanus, S. salmonis, S. torosa and S. vortens). We recommend that type descriptions of diplomonads be based on organisms taken directly from the host rather than from in vitro culture in order to reliably include the cytoplasmic organelles.