B. Spanggaard et al., PHYLOGENETIC-RELATIONSHIPS OF THE INTERCELLULAR FISH PATHOGEN ICHTHYOPHONUS HOFERI AND FUNGI, CHOANOFLAGELLATES AND THE ROSETTE AGENT, Marine Biology, 126(1), 1996, pp. 109-115
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.