A reference metagenome sequence of the lichen Cladonia rangiformis

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, aiming to illuminate the genomic complexity and evolutionary interactions within lichen symbioses.

Using long-read sequences from an entire symbiotic complex, plus short-read libraries from 28 additional diverse European lichen samples, we were able to separate genome sequences of 20 individual species. We constructed chromosome-scale assemblies of the C. rangiformis fungus and its trebouxioid green algal photobiont Asterochloris mediterranea. 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 strong candidates for genes that were horizontally transferred from bacteria to both alga and fungus. Furthermore, we isolated 18 near-complete bacterial genomes, of which 13 are enriched in the lichen compared to surrounding soil. Analysis of gene content in fungus, algae, and bacteria identified 22 distinct biosynthetic gene cluster categories for known secondary metabolites.

Our findings 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. This study provides the first chromosome-scale genomic framework for a lichen holobiont, offering a foundational resource for future research into metagenomics, symbiosis, and microbial ecology in lichens.