Phylogenetic analysis of Rhinosporidium seeberi's 18S small-subunit ribosomal DNA groups this pathogen among members of the protoctistan Mesomycetozoa clade

For the past 100 years the phylogenetic affinities of Rhinosporidium seeber i have been controversial. Eased on its morphological features, it has been classified as a protozoan or as a member of the kingdom Fungi, We have amp lified and sequenced nearly a full-length 18S small-subunit (SSU) ribosomal DNA (rDNA) sequence from R. seeberi, Using phylogenetic analysis, by parsi mony and distance methods, of R. seeberi's 18S SSU rDNA and that of other e ukaryotes, rue found that this enigmatic pathogen of humans and animals clu sters with a novel group of fish parasites referred to as the DRIP clade (D ermocystidium, rossete agent, Ichthyophonas, and Psorospermium), near the a nimal-fungal divergence. Our phylogenetic analyses also indicate that R see beri is the sister taxon of the two Dermocystidium species used in this stu dy. This molecular affinity is remarkable since members of the genus Dermoc ystidium form spherical structures in infected hosts, produce endospores, h ave not been cultured, and possess mitochondria with flat cristae, With the addition of R. seeberi to this clade, the acronym DRIP is no longer approp riate. We propose to name this monophyletic clade Mesomycetozoa to reflect the group's phylogenetic association within the Eucarya.