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 th e few pathogenic fungal infections of marine fish. The result of an at tack is severe epizootics in herring stocks with drastic reduction in the population as a consequence. The exact phylogenetic position of th e genus Ichthyophonus is not known. In the present study, a combinatio n of molecular data, ultrastructure and biochemical characters were ut ilized to investigate the phylogeny of I. hoferi. The genomic DNA enco ding the small subunit ribosomal RNA (18S rRNA) was amplified and sequ enced. Comparisons with other eukaryotic 18S rRNA sequences indicate t hat 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 roset te agent. The clade encompassing I. hoferi and the rosette organism is the sister group to the choanoflagellate clade in the neighbor-joinin g tree, while in the parsimony tree the I. hoferi/rosette clade is equ ally distant to both the choanoflagellate and animal clades. Transmiss ion 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 thr ough-out the cell, and mitochondria with tubular cristae. The cell wal l of I. hoferi contains chitin.