Mixia osmundae grows yeast-like in culture. Image courtesy of Sebastian Albu

Mixia osmundae (Nishida) C.L. Kramer is the sole species in one of the more enigmatic genera of Fungi.  It is an intracellular parasite of ferns in the genus Osmunda where it forms multinucleate hyphae that are rarely septate (Bauer et al. 2006).  On the surface of host epidermal cells it forms small oblong sporangia (24-60 x 9-25 µm).  Thousands of minute spores (3-4.5 x 1.5-2.5 µm) are produced on the surface of sporangia in a simultaneous fashion that is unlike any other known method of spore production in Fungi.  It is unknown if the spores are produced through sexual or asexual reproduction and the complete life cycle is not fully elucidated.  Mixia can be pure-cultured and grows in a budding, yeast-like form under these conditions.  

The phylogenetic placement of Mixia has long been problematic.  It was originally considered a member of Taphrinales of the early diverging Ascomycota (Mix 1947, Kramer 1958), but only recently has it been determined that it is a member of the Pucciniomycotina, a subphylum of Basidiomycota that includes rust fungi (Nishida et al., 1995).  Multigene molecular phylogenies support it as a phylogenetically distinct lineage among the Pucciniomycotina and it is currently classified as the sole member of the class Mixiomycetes (Aime et al. 2006).

At 13.6Mb, M. osmundae has the smallest plant pathogenic basidiomycete genome sequenced to date. The genome structure differs from almost all other sequenced basidiomycetes in having high gene density and almost no repetitive regions. Genome analysis indicates that the spores and yeast state in culture are haploid and likely produced asexually although the possession of a full set of mating and meiosis genes indicates that sexual reproduction may occur in some as yet unobserved part of the life cycle of M. osmundae. A study of carbohydrate active enzymes shows that M. osmundae possesses enzyme sets characteristic of biotrophic fungi (Toome et al. 2013).