Molecular phylogenies based on the small subunit ribosomal RNA gene (SSU or 18S ribosomal DNA (rDNA)) revealed recently the existence of a relatively large and widespread group of eukaryotes, branching at the base of the fungal tree. This group, comprising almost exclusively environmental clones, includes the endoparasitic chytrid Rozella as the unique known representative. Rozella emerged as the first fungal lineage in molecular phylogenies and as the sister group of the Microsporidia. Here we report rDNA molecular phylogenetic analyses of two endonuclear parasites of free-living naked amoebae having microsporidia-like ultrastructural features but belonging to the rozellids. Similar to microsporidia, these endoparasites form unflagellated walled spores and grow inside the host cells as unwalled nonphagotrophic meronts. Our endonuclear parasites are microsporidia-like rozellids, for which we propose the name Paramicrosporidium, appearing to be the until now lacking morphological missing link between Fungi and Microsporidia. These features contrast with the recent description of the rozellids as an intermediate wall-less lineage of organisms between protists and true Fungi. We thus reconsider the rozellid clade as the most basal fungal lineage, naming it Rozellomycota.
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We thank B. Hauröder and E.N. Schmid for assistance and previous electron microscopy data. This study was supported by internal fundings of each laboratory.
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a The presence of a fungal-like cell wall for KSL3, KAUN and KW19 (infecting Vannella), is indicated by the chitin/cellulose-binding dye calcofluor white (CFW), producing blue/apple green under UV. b After permeabilization of infected amoebae, fluorophore labelled molecular DNA beacons using specific oligonucleotide probes for KSL3 and eukaryote 18S rDNA were added. FISH identification of nuclear endoparasites KSL3 and KAUN is shown in red indicated by arrows (TIFF 458 kb)
Maximum-Likelihood tree topology of Fungi, other opisthokonts (Animalia and Choanozoa), Apusozoa and Amoebozoa, the latter used as root. The holomycotan groups Nuclearoidea and Aphelidea are evidenced by a coloured sheet to show their putative intermediate position between the rest of Choanozoa and Rozellomycota + Fungi. Full tree topology is show for the basal opisthokont radiation, major fungal groups are collapsed. Bootstrap values are after 1000 replicates (PDF 229 kb)
ITS2 secondary structures of the four rozellomycotans (upper side), and representatives of Chytridiomycota (Spizellomyces), Blastocladiomycota (Allomyces) and two holomycotan relatives (Amoeboaphelidium and Nuclearia) (lower side). Fungal ITS2 have 2-4 helices. Helix II had characteristic pyrimidine-pyrimidine (Y) bulge (red arrow). To note the similarity between KSL3 and WS-CM2, which share also some degree of identity in the phylogenetic hallmark 5′-side of the helix III, and differ from both the Rozella spp. Amoeboaphelidium (strain X-5) resembles chytrids but apparently lacks an Y bulge, while Nuclearia shows the most divergent structure (PDF 1365 kb)
(XLS 24 kb)
Length variability in ITS2 among basal holomycotan taxa (XLS 20 kb)
(DOC 29 kb)
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Corsaro, D., Walochnik, J., Venditti, D. et al. Microsporidia-like parasites of amoebae belong to the early fungal lineage Rozellomycota. Parasitol Res 113, 1909–1918 (2014). https://doi.org/10.1007/s00436-014-3838-4