Science 353, 488–492 (2016)

Lichens are not single organisms but an intimate symbiosis of fungi, most often an ascomycete, and a photosynthetic alga or cyanobacterial species. Very occasionally the symbiosis will include a second photosynthetic partner (photobiont) but only one fungus (mycobiont) per lichen seemed an inviolable rule. Spribille et al. have rewritten this dogma by identifying lichen involving two mycobionts and demonstrating that such trinogamy is very common.

The closely related lichens Bryoria fremontii and B. tortuosa are distinguishable because the latter produces vulpinic acid, giving its thalli a characteristic yellowish colour. Previous genetic studies had failed to identify any differences between the symbionts constituting the two lichens and similarly Spribille et al. saw no discernable difference in the paired transcriptomes of 15 field-collected individuals. However, along with ascomycete and viridiplantae transcripts, they identified several hundred mRNAs of basidiomycete origin, present in both lichens but more abundant in the vulpinic-acid-producing B. tortuosa.

Having realized a basidiomycete was involved with the Bryoria lichens, the researchers proceeded to find basidiomycete mRNA in many other lichens; of 52 genera from six continents so far. These lichen-associated yeasts are part of the genus Cyphobasidium. As final confirmation, Spribille et al. used fluorescence in situ hybridization (FISH) to label the Cyphobasidium cells and locate them within the cortex of the lichens.