Lichens look like plants, usually plastered flat against substrates or stringing off into the air as spanish moss does. Lichens are something more than a plant, they are a superorganism, sensu latu. A superorganism is a unique system where individuals’ behaviors and processes are so coordinated that the system takes on the qualities of an individual itself. The classic example is a eusocial ant colony, where individuals perform distinct functions, insufficient to support a single ant, but in concert perform all functions necessary to sustain life. Such is the case with lichens.
Biologists heretofore saw lichens as two species symbiotic systems. Symbiotic species interactions occur when two or more species have complementary needs and processes, that, when combined, help both to live and reproduce. In lichens, those species were thought to be a fungus and a photosynthesizing algae and/or bacteria. The fungus, the structure visible to the naked eye provides a shelter to the photosynthesizer while the photosynthesizing organism provides food in the form of sugar to the fungus–something to eat and somewhere to live.
Work by Toby Spribille, et al, published in Science last July, identified a third component to the lichen system: yeast (yeasts are also of the fungi kingdom). The investigators selected the lichens B. fremontii and B. tortuosa for analysis, noting that the latter produces vulpinic acid and appears yellow, rather than brown in the case of the former. Examining messenger RNA from the two species did not reveal differences to explain the distinct phenotypes. Further, the lack of difference in level of gene expression reinforced earlier suspicions that the two species are indeed one and the same, however when examining relative gene expression across all fungi, certain basidiomycota gene transcripts were found to be present in both lichen phenotypes, with higher expression in the vulpinic acid phenotype (B. tortuosa). The implication of this finding being that the the Bryoria lichens have another endosymbiont, and that it might be driving the phenotypic difference.
Spribille and colleagues then looked for and observed the presence of ribosomal RNA markers in other species of lichens collected from Montana, adjacent to where the Bryoria had been collected. Moreover, they found unique sequences of basidiomycetes associated each species of lichen, raising the possibility of intimate coevolution between the classical fungus-algae/bacteria and the yeast partners. Adding yet another layer of interest to the research, the identified basidiomycetes sequence, Cyphobasidium, was a sister to Cystobasidium minutum, the only known member of Basidiomycetes (its region of the fungi kingdom) associating with lichens, and pathologically at that. The fossil record and molecular clock put the split in lineages at 200 million years ago (for comparisons, the human and chimpanzee lineages share a most recent common ancestor from about 5 million years ago). How the most recent common ancestor to Cyphobasidium (identified within lichens) and Cystobasidium (identified living in growths on lichens) made its living, and how the diverging lineages got to make their livelihoods so differently, is a mystery.
Neither able to see microscopically nor culture the basidiomycetes, the researchers used FISH (fluorescent in situ hybridization) to attach markers to the basidiomycetes which could be visualized, thereby confirming its presence in both B. fremontii and B. tortuosa, enhanced presence in B. tortuosa (consistent with gene expression results), and allowing researchers to see localization to the lichen cortex. That these yeasts localize to the lichen cortex and that in vitro lichen models have used only classically identified fungi and algal/bacterial constituents without these yeasts, may be the reason that such cultured lichens have underdeveloped cortexes.
The virtually universal presence of the yeast Cyphobasidiales in Bryoria and that unique species of yeasts are found in different species of lichens radically changes our understanding of a long known symbiotic system. It reminds us that ecosystems are complex, and the genes necessary to support life are varied. In the symbiotic partnership within lichens, we see the presence of vulpinic acid associated with Cyphopasidiales, rendering B. tortuosa toxic. Whether it is produced by the yeast, its production controlled by the yeast, or related in some other way, the third partner to the lichen seems to exchange defense for shelter. Whereas lichens have a morphology distinct from that of their symbiotic partners, they are able to support the partners in environments where they could not survive individually. The discoveries by Spribille, et al, will certainly reshape our thinking about the simplicity, or better put complexity, of symbiotic relationships.
© Peter Roehrich, 2016