Mixia osmundae: transfer from the Ascomycota to the Basidiomycota based on evidence from molecules and morphology

Abstract

To determine phylogenetic placement of Mixia osmundae (T. Nishida) Kramer (Mixiaceae, Protomycetales), we sequenced the nuclear small subunit ribosomal RNA (18S rRNA) gene from M. osmundae IFO-32408 and compared it with that from 4 archiascomycetes (Ascomycota) and 24 basidiomycetes. Our molecular phylogeny indicates that M. osmundae and the basidiomycetes Rhodosporidium toruloides, Leucosporidium scottii, Sporobolomyces roseus, Sporidiobolus johnsonii, Cronartium ribicola, Peridermium harknessii, and Erythrobasidium hasegawianum group together in 100% of bootstrap replicates. The M. osmundae spores on the host fern Osmunda japonica have been regarded as ascospores (i.e., endogenously produced within an ascus), but our light microscopic, SEM, and TEM observations for fresh materials of M. osmundae on O. japonica in Japan clearly demonstrated that these are produced exogenously, blastically, and simultaneously from the sporogenous cell. Evidence from both molecular and morphological characters suggests that M. osmundae is not a member of the ascomycetes and is not related to either the Taphrinales or Protomycetales. Obviously Mixia osmundae is a member of the basidiomycetes and placed within the simple septate basidiomycete lineage. Key words: fungal evolution, basidiomycete phylogeny, Mixia osmundae, Taphrinales, Protomycetales, 18S rRNA.