Vondrak, J. | bioRxiv Science

bioRxiv2024

Semilichen - an unjustly neglected symbiotic system between green biofilms and true lichens

- Symbiotic systems of photosynthetic microorganisms and fungi are widespread in terrestrial biomes and lichens are probably the most advanced and complex. Conversely, the least complex systems are "green biofilms" with a completely unexplored mycobiome. We describe here a new system intermediate between green biofilms and lichens - semilichens. - Light and fluorescence microscopy, eDNA sequencing, molecular phylogeny, Chlorophyll a fluorescence and 13C labelling/metabolomics were used to study algal and fungal identity, morphology and physiology of the symbiosis. - Tight contact between algae and a single predominant fungus (mycobiont) is revealed in semilichens. The algae are from the symbiotic lineages of Trebouxiophyceae and Ulvophyceae, the fungi belong to Arthoniomycetes, Dothideomycetes, Eurotiomycetes, Lecanoromycetes and Lichinomycetes. Algae are alive and perform substantial photosynthetic activity. 13C labelled photosynthates are partially converted into specific fungal polyols (arabitol, mannitol) demonstrating the C-flow from algae to fungi. - The new symbiotic system was defined and compared with other terrestrial algal-fungal symbioses. It is characterized by minimalist environmental requirements and extremely low production of biomass. As a result, it also inhabits environments unfavourable for lichens. Our research supports the hypothesis that the long-term existence of algae and fungi in terrestrial conditions affected by frequent and repeated drying is likely dependent on their mutual coexistence.

physiology↗

bioRxiv2023

Introgressive descent and hypersexuality drive the evolution of sexual parasitism and morphological reduction in a fungal species complex.

Taxonomists consider species as discrete units of biological organization, which are subject to a continuous process of evolutionary change and are connected through their shared ancestry. However, the continuous nature of evolutionary change is difficult to reconcile with the discrete outcome of speciation, especially where species boundaries are permeable. A good example of this inconsistency is the lichen genus Pyrenodesmia, which shows a high morphologic and genetic diversity that that defies systematization by taxonomic or phylogenetic methods. Here we show that hybridization explains the presence of discordant morphs and that European species are interconnected through cross-mating in a single reproductive network, a syngameon, despite which species remain largely distinct and distinguishable. Whole genome data reflect the important role played by genome defense mechanisms in the genomic stabilization of fungal hybrids. The recurrence of Repeat Induced Point mutations (RIP) shapes genomes with islands of suppressed recombination and loss of gene content, which in turn generates a feedback loop reinforcing the lack of reproductive isolation through the loss of heterokaryon incompatibility and a tendency towards morphological reduction.

evolutionary biology↗