A Novel Clade of Protistan Parasites near the Animal-Fungal Divergence

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 93; no. 21; pp. 11907 - 11912
• Main Authors: Ragan, Mark A., Goggin, C. Louise, Cawthorn, Richard J., Cerenius, Lage, Angela V. C. Jamieson, Plourde, Susan M., Rand, Thomas G., Söderhäll, Kenneth, Gutell, Robin R.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 15.10.1996; National Acad Sciences; National Academy of Sciences
• Subjects: Animals; Base Sequence; Biologi; Biology; Brackish; CONFIDENCE; Conserved Sequence; Crustacea - parasitology; Cysts; Dermocystidium; Eukaryota - classification; Eukaryota - genetics; Eukaryota - isolation & purification; EUKARYOTES; EVOLUTIONARY; Freshwater; Fungi; Fungi - classification; Fungi - genetics; Genes, Protozoan; Gills - parasitology; Ichthyophonus; ICHTHYOPHONUS HOFERI; Marine; Mesomycetozoea; metazoa; Microbiology; Microscopy, Electron; Mitochondria - ultrastructure; Molecular Sequence Data; NATURAL SCIENCES; NATURVETENSKAP; Nucleic Acid Conformation; Parasites; Parsimony; PERSPECTIVE; PHYLOGENETIC RELATIONSHIPS; Phylogenetics; Phylogeny; Psorospermium; Ribonucleic acid; Ribosomal DNA; RIBOSOMAL RNA; RNA; RNA, Ribosomal - chemistry; RNA, Ribosomal - genetics; ROSETTE AGENT; Salmon - parasitology; SALMON ONCORHYNCHUS TSHAWYTSCHA; SEQUENCES; Topology; Trout - parasitology; Zoology
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.93.21.11907

Sequences of nuclear-encoded small-subunit rRNA genes have been determined for representatives of the enigmatic genera Dermocystidium, Ichthyophonus, and Psorospermium, protistan parasites of fish and crustaceans. The small-subunit rRNA genes from these parasites and from the ``rosette agent'' (also a parasite of fish) together form a novel, statistically supported clade. Phylogenetic analyses demonstrate this clade to diverge near the animal-fungal dichotomy, although more precise resolution is problematic. In the most parsimonious and maximally likely phylogenetic frameworks inferred from the most stably aligned sequence regions, the clade constitutes the most basal branch of the metazoa; but within a limited range of model parameters, and in some analyses that incorporate less well-aligned sequence regions, an alternative topology in which it diverges immediately before the animal-fungal dichotomy was recovered. Mitochondrial cristae of Dermocystidium spp. are flat, whereas those of Ichthyophonus hoferi appear tubulovesiculate. These results extend our understanding of the types of organisms from which metazoa and fungi may have evolved.