Phylogenetic relationships of the intercellular fish pathogenIchthyophonus hoferi and fungi, choanoflagellates and the rosette agent

Ichthyophonus hoferiPlehn and Mulsow, 1911 is thought to be one of the few pathogenic fungal infections of marine fish. The result of an attack is severe epizooticsin herring stocks with drastic reduction in the population as a consequence. The exact phylogenetic position of the genusIchthyophonus is not known. In the present study, a combination of molecular data, ultrastructure and biochemical characters were utilized to investigate the phylogeny ofI.hoferi. The genomic DNA encoding the small subunit ribosomal RNA (18S rRNA) was amplified and sequenced. Comparisons with other eukaryotic 18S rRNA sequences indicate thatI. hoferi is not a member of the Fungi. In both the parsimony and the neighborjoining trees,I. hoferi is the sister taxon to the rosette agent. The cladeencompassingI.hoferi and the rosette organism is the sister groupto the chanoflagellate cladein the neighbor-joiningtree, while in the parsimony tree theI. hoferi/rosette cladeis equally distant to both the choanoflagellateand animal clades. Transmission electron microscopy showed thatI. hoferi has a defined cell wall, an endoplasm that consists of a fine granulated matrix with numerous ribosomes, several nuclei, vacuoles of varying density distributed throughout the cell, and mitochondria with tubular cristae. The cell wall ofI. hoferi contains chitin.