Phylogenetic position and ultrastructure of two Dermocystidium species (Ichthyosporea) from the common perch (Perca fluviatilis)

Sequences of small-subunit rRNA genes were determined for Dermocystidium percae and a new Dermocystidium species established as D. fennicum sp. n. from perch in Finland. On the basis of alignment and phylogenetic analysis both species were placed in the Dermocystidium-Rhinosporidium clade within Ichthyosporea, D. fennicum as a specific sister taxon to D. salmonis, and D. percae in a clade different from D. fennicum. The ultrastructures of both species well agree with the characteristics approved within Ichthyosporea: walled spores produce uniflagellate zoospores lacking a collar or cortical alveoli. The two Dermocystidium species resemble Rhinosporidium seeberi (as described by light microscope), a member of the nearest relative genus, but differ in that in R. seeberi plasmodia have thousands of nuclei discernible, endospores are discharged through a pore in the wall of the sporangium, and zoospores have not been revealed. The plasmodial stages of both Dermocystidium species have a most unusual behaviour of nuclei, although we do not actually know how the nuclei transform during the development. Early stages have an ordinary nucleus with double, fenestrated envelope. In middle-aged plasmodia ordinary nuclei seem to be totally absent or are only seldom discernible until prior to sporogony, when rather numerous nuclei again reappear. Meanwhile single-membrane vacuoles with coarsely granular content, or complicated membranous systems were discernible. Ordinary nuclei may be re-formed within these vacuoles or systems. In D. percae small canaliculi and in D. fennicum minute vesicles may aid the nucleus-cytoplasm interchange of matter before formation of double-membrane-enveloped nuclei. Dermocystidium represents a unique case when a stage of the life cycle of an eukaryote lacks a typical nucleus.