A NOVEL CLADE OF PROTISTAN PARASITES NEAR THE ANIMAL-FUNGAL DIVERGENCE

Sequences of nuclear-encoded small-subunit rRNA genes have been determ ined for representatives of the enigmatic genera Dermocystidium, Ichth yophonus, and Psorospermium, protistan parasites of fish and crustacea ns. The small-subunit rRNA genes from these parasites and from the ''r osette agent'' (also a parasite of fish) together form a novel, statis tically supported clade. Phylogenetic analyses demonstrate this clade to diverge near the animal-fungal dichotomy, although more precise res olution is problematic In the most parsimonious and maximally likely p hylogenetic frameworks inferred from the most stably aligned sequence regions, the clade constitutes the most basal branch of the metazoa; b ut within a limited range of model parameters, and in some analyses th at incorporate less well-aligned sequence regions, an alternative topo logy in which it diverges immediately before the animal-fungal dichoto my was recovered. Mitochondrial cristae of Dermocystidium spp. are fla t, whereas those of Ichthyophonus hoferi appear tubulovesiculate. Thes e results extend our understanding of the types of organisms from whic h metazoa and fungi may have evolved.