Phylogenetic relationships of the intercellular fish pathogen Ichthyophonus hoferi and fungi, choanoflagellates and the rosette agent.

Ichthyophonus hoferi Plehn and Mulsow, 1911 is thought to be one of the few pathogenic fungal infections of marine fish. The result of an attack is severe epizootics in herring stocks with drastic reduction in the population as a consequence. The exact phylogenetic position of the genus Ichthyophonus is not known. In the present study, a combination of molecular data, ultrastructure and biochemical characters were utilized to investigate the phylogeny of I. hoferi. The genomic DNA encoding the small subunit ribosomal RNA (18S rRNA) was amplified and sequenced. Comparisons with other eukaryotic 18S rRNA sequences indicate that I. hoferi is not a member of the Fungi. In both the parsimony and the neighbor joining trees, I. hoferi is the sister taxon to the rosette agent. The clade encompassing I. hoferi and the rosette organism is the sister group to the choanoflagellate clade in the neighbor-joining tree, while in the parsimony tree the I. hoferi/rosette clade is equally distant to both the choanoflagellate and animal clades. Transmission electron microscopy showed that I. 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 of I. hoferi contains chitin.

Return to the John Taylor's Home Page