P-268 - A reference metagenome sequence of the lichen Cladonia rangiformis reveals a complex symbiotic micro-environment

Lichens are an ancient symbiosis comprising the thalli of lichen-forming fungi, their photoautotrophic partners and their microbiome. So far, they were poorly studied at the genome sequence level. Here, we present a reference metagenome for the holobiont of Cladonia rangiformis. Using long read sequences from an entire symbiotic complex, plus short read libraries from 28 diverse European lichen samples, we separated genome sequences of 20 individual species. We constructed chromosome-scale assemblies of the C. rangiformis fungus and its trebouxioid green algal photobiont Asterochloris mediterranea, aswell as 18 near complete bacterial genomes. The genome of the fungus comprises ~22% transposable elements and is highly compartmentalized into genic regions and large TE-derived segments which show extensive signatures of repeat-induced point mutations (RIP). We found that A. mediterranea centromeres are predominantly derived from two interacting retrotransposon families. We also identified genes that were horizontally transferred from bacteria to both alga and fungus. By Alphafold guided 3D structural analysis of secreted proteins we identified multiple large effector families potentially involved in the complex interplay of the symbiotic species.
Our study revealed that the thalli of C. rangiformis have a highly complex microbiome, comprising a mix of species that may include opportunists, ecologically obligate symbionts and possibly even lichen-beneficial bacteria.