Format: ORAL

Francesco Dal Grande1,2,3, Camille Puginier4, Cyril Libourel4, Juergen Otte1, Pavel kaloud5, Mireille Haon4,6, Sacha Grisel4,6, Malte Petersen7, Jean-Guy Berrin4,6, Pierre-Marc Delaux4, Jean Keller4

1 Senckenberg Biodiversity and Climate Research Centre (SBiK-F), Frankfurt am Main, Germany 2 LOEWE Centre for Translational Biodiversity Genomics (TBG), Frankfurt am Main, Germany 3 University of Padova, Padua, Italy 4 Université de Toulouse, Toulouse, France 5 Charles University, Praha 2, Czech Republic 6 Aix Marseille Université, Marseille, France 7 University of Bonn, Bonn, Germany

Lichens are textbook examples of ecologically successful symbioses formed between thousands of fungal species and a handful of green algae. Their biology, ecology and physiology have been studied for centuries, with disproportionate focus on the fungal side of the symbiosis. By contrast, the evolutionary origins and molecular mechanisms of the symbiosis on the side of the green algal partner remain elusive. In this study we explored the evolutionary history and the molecular innovations in green algae at the origin of the ability to establish symbiotic association with fungi to form lichens. For this, we de novo sequenced the genomes or transcriptomes of 12 lichen-forming and closely related non-lichen-forming algae, filling key sampling gaps in the algal tree of life. We then combined this dataset with 129 previously sequenced green algae and performed comparative phylogenomic analyses to find genes discriminating lichen-forming and non-lichen-forming algae. In addition to gene-family expansions, we show that one carbohydrate-active enzyme, the glycosyl hydrolase 8 (GH8), was acquired in lichenizing Trebouxiophyceae by horizontal gene transfer from bacteria. Our results suggest that the ability to degrade the partners cell wall is a key acquisition for the evolution of symbiotic lifestyle in these algae. Our study unravels an important evolutionary event fundamental to the establishment of symbiotic lifestyle in lichenized algae and paves the way for future functional studies in this understudied yet ecologically important group of organisms.