The puzzle of lichen symbiosis : Pieces from Thamnolia

Symbiosis brought important evolutionary novelties to life on Earth. Lichens, the symbiotic entities formed by fungi, photosynthetic organisms and bacteria, represent an example of a successful adaptation in surviving hostile environments. Yet many aspects of the lichen symbiosis remain unexplored. This thesis aims at bringing insights into lichen biology and the importance of symbiosis in adaptation. I am using as model system a successful colonizer of tundra and alpine environments, the worm lichens Thamnolia, which seem to only reproduce vegetatively through symbiotic propagules. When the genetic architecture of the mating locus of the symbiotic fungal partner was analyzed with genomic and transcriptomic data, a sexual self-incompatible life style was revealed. However, a screen of the mating types ratios across natural populations detected only one of the mating types, suggesting that Thamnolia has no potential for sexual reproduction because of lack of mating partners. Genetic data based on molecular markers revealed the existence of three morphologically cryptic Thamnolia lineages. One lineage had a clear recombination structure and was found in the tundra region of Siberia, shorelines of Scandinavia, and Aleutian Islands. The other lineage was allopatric with the previous, and was highly clonal; only two haplotypes were found across the alpine region of central and southeastern Europe. However, the third lineage was sympatric with the other two, had a worldwide distribution, and although highly clonal, showed a recombinant population structure. Our data could not reveal whether the signs of recombination resulted from rare recombination events due to the extreme low frequency of the other mating type or ancestral variation before the loss of sexual reproduction. However, investigation of Thamnolia’s green algal population showed that in different localities, different algal genotypes were associated with the same fungal genotype. Furthermore, data suggest that Thamnolia carried several algal genotypes within its thalli and shared them with other distantly related but ecologically similar fungal species