Mixia osmundae

Introduction

Mixia osmundae is the only species currently known in Class Mixiomycetes. The fungus is an intracellular parasite of ferns in the genus Osmunda L. in which it causes small yellow to brown leaf spots.  In cases of heavy infection, spots may coalesce to form larger lesions and even cover whole leaflets.  Mixia is known only from Osmunda regalis L. in Japan and Taiwan, and O. cinnamomea L. in the United States (Kramer, 1958; Mix, 1947; Nishida, 1911; Sugiyama and Katumoto, 2008). Many aspects of the biology and ecology of this fungus are still unknown, since it is rarely found and not sufficiently studied.

Characteristics

When growing within a fern host, Mixia forms intercellular hyphal cells with many nuclei and few cross walls (termed septa), a condition known as coenocytic. Once the fungus reaches the surface of the host’s epidermal cells, it forms sac-like swellings which develop into non-septate, oblong to ellipsoid spore-producing (i.e., sporogenous) cells (24–60 × 9–2.5 µm). Tiny spores (3–4.5 × 1.5–2.5 µm) are formed on the surface of these sporogenous cells, creating a powdery layer on the lower side of fern leaves. It remains undetermined whether these spores are meiospores or mitospores (i.e., sexually or asexually produced) and thus many aspects of the life cycle of Mixia remain to be worked out (Bauer et al., 2006; Nishida et al., 1995). In culture, M. osmundae forms colonies composed of single yeast-like cells.  The culture at first is white to cream colored with a slightly slimy texture that then turns pinkish with age. The cells in culture reproduce by budding; no hyphal growth has ever been detected on culture media. The production of coenocytic hyphae is a very rare condition in Basidiomycota and the sporogenous cells produced by Mixia are unique in the phylum.

Classification

Mixia osmundae was first described from Japan by Nishida (1911) as an ascomycete that he named Taphrina osmundae. Later, Mix (1947) described a second species, T. higginsii, as closely related to T. osmundae, but differing in host and by slight differences in sporogenous cell sizes. In 1958 Kramer created a new genus Mixia, into which he combined both species as Mixia osmundae (after determining that they were, indeed, the same species), and placed the genus in the Protomycetaceae (Taphrinales, Ascomycota). Mixia remained classified within phylum Ascomycota for more than eighty years, primarily due to superficial similarities between the sporogenous cells of Mixia and the asci produced by some Ascomycota. However, molecular and closer morphological studies of the sporogenous cells in the 1990’s provided multiple lines of evidence that Mixia belongs to Basidiomycota (Nishida et al., 1995), and that the structure previously interpreted as an ascus was instead a unique type of sporogenous cell (Bauer et al., 2006). Currently M. osmundae represents a monotypic taxon, Class Mixiomycetes in Pucciniomycotina (Aime et al., 2006; Bauer et al., 2006; Hibbett et al., 2007; Sjamsuridzal et al., 2002).

Discussion of Phylogenetic Relationships

To date the phylogenetic position of Mixiomycetes has only been examined with ribosomal DNA sequences, which support Mixiomycetes within Pucciniomycotina. Nevertheless, the sister group to Mixiomycetes has not yet been resolved (Aime et al., 2006).

Other Names for Mixia osmundae

• Mixiomycetes
• Mixiales
• Mixia

References

Aime, M. C., P. B. Matheny, D. A. Henk, E. M. Frieders, R. H. Nilsson, M. Piepenbring, D. J. McLaughlin, L. J. Szabo, D. Begerow, J. P. Sampaio, R. Bauer, M. Wei?, F. Oberwinkler, and D. S. Hibbett. 2006. An overview of the higher-level classification of Pucciniomycotina based on combined analyses of nuclear large and small subunit rDNA sequences. Mycologia 98: 896?905.

Bauer, R., D. Begerow, J. Sampaio, M. Weiβ, F. Oberwinkler. 2006. The simple-septate basidiomycetes: a synopsis. Mycological Progress 5: 41?66.

Hibbett, D. S., M. Binder, J. F. Bischoff, M. Blackwell, P. F. Cannon, O. E. Eriksson, S. Huhndorf, T. James, P. M. Kirk, R. L?cking, T. Lumbsch, F. Lutzoni, P. B. Matheny, D. J. Mclaughlin, M. J. Powell, S. Redhead, C. L. Schoch, J. W. Spatafora, J. A. Stalpers, R. Vilgalys, M. C. Aime, A. Aptroot, R. Bauer, D. Begerow, G. L. Benny, L. A. Castlebury, P. W. Crous, Y.-C. Dai, W. Gams, D. M. Geiser, G. W. Griffith, C. Gueidan, D. L. Hawksworth, G. Hestmark, K. Hosaka, R. A. Humber, K. Hyde, J. E. Ironside, U. K?ljalg, C. P. Kurtzman, K.-H. Larsson, R. Lichtwardt, J. Longcore, J. Miądlikowska, A. Miller, J.-M. Moncalvo, S. Mozley-Standridge, F. Oberwinkler, E. Parmasto, V. Reeb, J. D. Rogers, C. Roux, L. Ryvarden, J. P. Sampaio, A. Sch??ler, J. Sugiyama, R. G. Thorn, L. Tibell, W. A. Untereiner, C. Walker, Z. Wang, A. Weir, M. Wei?, M. M. White, K. Winka, Y.-J. Yao, and N. Zhang. 2007. A higher-level phylogenetic classification of the Fungi. Mycological Research 111: 509?547.

Kramer, C.L. 1958. A new genus in the Protomycetaceae. Mycologia 50 (6): 916?926.

Mix, A.J. 1947. Taphrina osmundae Nishida and Taphrina higginsii sp. nov. Mycologia 39 (1): 71?76.

Nishida, H., K. Ando, Y. Ando, A. Hirata, and J. Sugiyama. 1995. Mixia osmundae: Transfer from the Ascomycota to the Basidiomycota based on evidence from molecules and morphology. Can. J. Bot., 73 (Suppl. 1): S660?S666.

Sjamsuridzal, W., H. Nishida, A. Yokota. 2002. Phylogenetic position of Mixia osmundae inferred from 28S rDNA comparison. Journal of General and Applied Microbiology 48: 121?123.

Sugiyama, J., K. Katumoto. 2008. Identity of the plasmodial slime mold Phytoceratiomyxa osmundae and the lectotypification of Taphrina osmundae, the basionym of Mixia osmundae. Mycoscience 49: 192?198.