𝘓𝘪𝘤𝘩𝘦𝘯𝘰𝘮𝘱𝘩𝘢𝘭𝘪𝘢: Trials of a Basidiolichen

Fruiting bodies of (A) Lichenomphalia hudsoniana (H. S. Jenn.) Redhead, Lutzoni, Moncalvo & Vilgalys, (B) L. umbellifera (L.) Redhead, Lutzoni, Moncalvo & Vilgalys, and (C) L. velutina (Quél.) Redhead, Lutzoni, Moncalvo & Vilgalys. Figures from Liu et al. (2018).

 Lichens are very weird. Undeniably ubiquitous, they encrust just about every surface that will let them (Asplund & Wardle (2016) claim as much as 8% of the Earth's surface is covered in lichens!). So common are these things that they remain chronically overlooked and understudied. Lichens are the quintessential symbiotic relationship, a composite organism comprising unicellular algae (or sometimes cyanobacteria), referred to as the "photobiont," living intertwined with the hyphal strands of a fungus. The relationship is nearly inseparable and lichens are truly their own type of organism distinct from their component parts. Taxonomically, lichens are named according to the fungal component and they most certainly do not form a monophyletic group. Many different lineages of fungi have "lichenized" throughout Earth's history, lending to the abundance and diversity of these composites. However, most lichens are derived from ascomycete fungi. Lichenization is much less common among the Basidiomycota, the large phylum of fungi that includes the most familiar mushroom-forming lineages. Liu et al. (2018) refer to 172 described basidiolichens, which themselves have evolved convergently across five orders. 

Figures showing the types of thalli exhibited by Lichenomphalia from Liu et al. (2018). (D) Globular "Botrydina"-type. (E) Sqamulose "Coriscium"-type.

The subjects of this post are Lichenomphalia, fifteen (likely more) species of basidiolichens in the family Hygrophoraceae. These lichens are widely distributed in temperate to arctic and alpine regions of the Northern Hemisphere as well as disjunct populations in Australia and Chile. The lichen identity of these fungi is a relatively recent revelation; the typical mushroom fruiting bodies and asexual thalli of these species were described as separate, unrelated organisms dating back to Linnaeus (1753).  Two of these former genera, "Botrydina" and "Coriscium," are still used to refer to different types of thalli produced by these lichens. "Botrydina" are globular, while "Coriscium" are "squamulose" (referring to the flat, scaly texture of many common lichens). 

Ecologically speaking, Lichenomphalia are fairly typical basidiomycetes, growing in damp, mossy places and often leaching nutrients from decaying wood. They are also common in harsh environments, such as the windy tundras of Greenland and Nunavut. The photobionts of Lichenomphalia are members of the globally distributed algae genus Coccomyxa, which contribute to the vibrantly green thallus of some species. These lichens were formerly placed in the tricholomatacean genus Omphalina; their status as a monophyletic radiation of lichenized agarics was determined via a phylogenetic analysis by Redhead et al. (2002), establishing the genus Lichenomphalia. Lichen and fungi in general are among the most common organisms on Earth with untold importance to global ecosystems, yet taking the time to research any single lineage only reveals how little we actually understand about our fungal neighbors. 

Lichenomphalia hudsoniana (H. S. Jenn.) Redhead, Lutzoni, Moncalvo & Vilgalys. Notice the contrast between the yellow fruiting body and green thallus, it's no surprise they were considered separate organisms for hundreds of years. Photo by Troy McMullin from here.

REFERENCES

Liu, D., Goffinet, B., Wang, X. Y., Hur, J. S., Shi, H. X., Zhang, Y. Y., ... & Wang, L. S. (2018). Another lineage of basidiolichen in China, the genera Dictyonema and Lichenomphalia (Agaricales: Hygrophoraceae). Mycosystema, 37(7), 849-864.

Moncalvo, J. M., Vilgalys, R., Redhead, S. A., Johnson, J. E., James, T. Y., Aime, M. C., ... & Miller Jr, O. K. (2002). One hundred and seventeen clades of euagarics. Molecular phylogenetics and evolution, 23(3), 357-400.