Hidden complexity of lichen symbiosisTuovinen, Veera;
Lichens are tremendously diverse physical outcomes of symbiotic relationships involving fungi, algae and bacteria. This thesis aims to give insight into the functionality, composition and reproduction of lichens from the fungal perspective. When previous results from a barcoding study were re-evaluated, no support for a free-living life phase of Cladonia mycobionts could be found. Genomic and transcriptomic data were used to identify the fungal partners in thalli, and a fluorescent in situ hybridization (FISH) method for the simultaneous visualization of the different fungi was developed. This approach led to the discovery of previously unknown basidiomycetes, Cyphobasidium spp., which are widespread components of the cortex of lichens in Parmeliaceae. In some cases, the abundance of Cyphobasidium correlates with previously unexplained phenotypic variation of the lichens. In the case example Bryoria capillaris, Cyphobasidium yeasts are the dominant cells in the cortex and hence the fungus that meets the eye when looking at the lichen. With the help of hologenomic data and FISH, it could also be shown that Tremella lethariae, a lichenicolous heterobasidiomycete known to induce galls on Letharia, is dimorphic and frequently occupies the cortex of asymptomatic Letharia thalli in its anamorphic state. Finally, genomic and transcriptomic data was used to investigate the mating system of the genus Letharia, which includes species with different reproductive strategies. All studied Letharia species have a heterothallic mating system, meaning they need to find a compatible mate in order to sexually reproduce. Thus, the variation of reproduction strategies within the genus cannot be explained only by the mating system of the Letharia mycobiont. Our data on the mating-type ratios indicates that no potential for sexual reproduction exists for the red-listed L. vulpina in Sweden, as only one mating type is found in the populations. The roles the secondary fungi have in the symbiosis is not yet fully understood. However, the results indicate that we should not automatically assume that some of the organisms are negligible for the function of the holobiont, or that all functions assigned to fungal origin are conducted by the primary mycobiont. Altogether, the findings presented in this thesis support the view of lichens as a community of bionts with complex interactions and calls for a more holistic approach for the study of the symbiosis.
mycobiont; lichenicolous; FISH; Cyphobasidium; Tremella; mutualistic; parasitic
Published inActa Universitatis Agriculturae Sueciae 2017, number: 2017:25
ISBN: 978-91-576-8823-1, eISBN: 978-91-576-8824-8
Publisher: Department of Ecology, Swedish University of Agricultural Sciences