Different Patterns of Frequency, Lichen Specificity and Thallus Location Between the Yeast and Filamentous Phases of Two Lichen-Inhabiting Basidiomycetes.

Many fungi have a dimorphic life cycle, alternating between unicellular yeast and multicellular filamentous phases. Although dimorphism is assumed for many lichen-associated basidiomycetes, the existence of a yeast stage has rarely been confirmed. Using taxon-specific PCR and FISH-CLSM, we studied Tremella hypogymniae and T. tubulosae Tremellomycetes), two presumably dimorphic species previously known only from their filamentous phase in galls on the lichens Hypogymnia physodes and H. tubulosa, respectively. We investigated their presence and frequency, lichen ranges and within-thallus distribution of life-cycle stages. We also explored the co-occurrence of both species with Cystobasidiomycetes-one of the most widespread lichen-associated yeast lineages-in the same lichen thalli. The filamentous phase of Tremella hypogymniae and T. tubulosae was confined to a single lichen species each, whereas the yeast phase occurred in several closely related lichens. Both phases co-occurred with various Cystobasidiomycete lineages. Filamentous structures were restricted to galls, whereas gall-free thalli contained Tremella yeasts in the cortex, soredia and medulla, and pseudohyphae in the cortex. The presence of yeasts in soredia suggests co-dispersal with other lichen symbionts. These findings reveal narrow specificity in the filamentous phase but broader associations in the yeast phase, pointing to complex interactions within the lichen symbiosis.