Amoebidium parasiticum is a protozoan, not a Trichomycete

Classification of the Amoebidiales (Trichomycetes, Zygomycota) within the Fungi is problematical because their cell walls apparently lack chitin and they produce amoeboid cells during their life cycle. A nearly full length fragment of the nuclear small subunit (SSU) rRNA of Amoebidium parasiticum was amplified by the polymerase chain reaction (PCR) and sequenced to examine its phylogenetic relationships. Results of a BlastN search of GenBank revealed that the A. parasiticum SSU rRNA sequence was most closely related to that of Ichthyophonus hoferi, an ichthyosporean in the Protozoa near the animal-fungal divergence. Maximum parsimony analysis of ichthyosporean and fungal SSU sequences, using sequences of choanoflagellates to root the 18S rDNA gene trees, resolved A. parasiticum as a strongly supported sister of I. hoferi within the Ichthyophonida clade of the protozoan class Ichthyosporea. In contrast to other members of this class, which are mostly obligate or facultative parasites of various animals, A. parasiticum and other members of the Amoebidiales are only known to be arthropodophilous symbionts. The results also provide the first evidence that mitochondrial cristae types exhibit homoplastic distributions within the Ichthyosporea.