Since the late 1800s, mycologists have been detect- ing fungi above and beyond the assumed single fun- gus in lichen thalli [ 1–6 ]. Over the last century, these fungi have been accorded roles ranging from com- mensalists to pathogens. Recently, Cyphobasidiales yeasts were shown to be ubiquitous in the cortex layer of many macrolichens [ 7 ], but for most species, little is known of their cellular distribution and con- stancy beyond visible fruiting structures. Here, we demonstrate the occurrence of an additional and distantly related basidiomycete, Tremella , in 95% of studied thalli in a global sample of one of the most intensively studied groups of lichens, the wolf lichens (genus Letharia ). Tremella species are reported from a wide range of lichen genera [ 8 ], but until now, their biology was deduced from fruiting bodies (basidiomata) formed on lichen thalli. Based on this, they have been thought to be uncommon to rare, to occur exclusively in a hyphal form, and to be parasitic on the dominant fungal partner [ 9, 10 ]. We show that, in wolf lichens, Tremella occurs as yeast cells also in thalli that lack basidiomata and infer that this is its dominant stage in nature. We further show that the hyphal stage, when present in Letharia ,is in close contact with algal cells, challenging the assumption that lichen-associated Tremella species are uniformly mycoparasites. Our results suggest that extent of occurrence and cellular interactions of known fungi within lichens have historically been underestimated and raise new questions about their function in specific lichen symbioses.
