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

Abstract

For the past 100 years the phylogenetic affinities of Rhinosporidium seeberi have been controversial. Based on its morphological features, it has been classified as a protozoan or as a member of the kingdom Fungi. We have amplified and sequenced nearly a full-length 18S small-subunit (SSU) ribosomal DNA (rDNA) sequence from R. seeberi. Using phylogenetic analysis, by parsimony and distance methods, of R. seeberi's 18S SSU rDNA and that of other eukaryotes, we found that this enigmatic pathogen of humans and animals clusters with a novel group of fish parasites referred to as the DRIP clade (Dermocystidium, rossete agent, Ichthyophonus, and Psorospermium), near the animal-fungal divergence. Our phylogenetic analyses also indicate that R. seeberi is the sister taxon of the two Dermocystidium species used in this study. This molecular affinity is remarkable since members of the genus Dermocystidium form spherical structures in infected hosts, produce endospores, have not been cultured, and possess mitochondria with flat cristae. With the addition of R. seeberi to this clade, the acronym DRIP is no longer appropriate. We propose to name this monophyletic clade Mesomycetozoa to reflect the group's phylogenetic association within the Eucarya.

FIG. 1

Phylogenetic comparison of 18S SSU rDNA from R. seeberi and 23 other organisms showing that this human and animal pathogen is a member of the Mesomycetozoa clade. (a) Parsimony tree (length = 1,688 steps; consistency index = +0.53; retention index = 0.51; homoplasy index = 0.47) based on a heuristic search with 1,000 random taxon input orders. The numbers on the internal branches are percentages of trees based on 1,000 bootstrapped data sets possessing the branch. The branch lengths reflect the length in steps. An italicized scale is given above the Ochromonas danica branch. (b) Neighbor-joining distance tree with Kimura’s 3-parameter correction for multiple hits. The numbers on the internal branches are percentages of trees based on 1,000 bootstrapped data sets possessing the branch. The branch lengths reflect corrected distances. An italicized scale is given above the O. danica branch. The GenBank accession numbers of the organisms are as follows: Acanthocoepsis unguiculata, L10823; Diaphanoeca grandis, L10824; Dermocystidium sp., U21336; Dermocystidium salmonis, U21337; I. hoferi, U43712; Psorospermium haeckelli, U33180; the rosette agent, L29455; R. seeberi, AF118851; Microciona prolifera, L10825; Mnemiopsis leidyi, L10826; Blastocladiella emersonii, X54264; Chytridium confervae, M59758; Glomus intraradices, X58725; Pneumocystis carinii, X12708; Saccharomyces cerevisiae, J01353; Schizosaccharomyces pombe, X58056; Chlorella lobophora, X63504; Chlamydomonas reinhardtii, M32703; Gracilaria lemaneiformis, M54986; Perkinsus sp., L07375; Perkinsus marinus, X75762; Achlya bisexualis, M32705; O. danica, M32704; and Prorocentrum micans, M14649.

FIG. 2

(A) TEM photograph showing an intermediate sporangium of R. seeberi with a prominent cell wall (W), several nuclei (N), numerous mitochondria (arrowheads), and a laminated body (L). Bar, 1.0 μm. (B) Enlargement of two mitochondria with flat cristae within an intermediate sporangium. Bar, 0.250 μm. The features of R. seeberi’s flat mitochondrial cristae are analogous to those of its sister taxon, Dermocystidium species, reported by Ragan et al. (22).