Tagirdzhanova, G.; Raistrick, J.; Talbot, N. J.

Lichens are symbiotic assemblies consisting of multiple organisms, chiefly a fungus and a photosynthetic microorganism or the photobiont. Among diverse photobionts, the most prevalent is the chlorophyte alga Trebouxia. We produced a chromosome-level assembly of Trebouxia sp. \'A48\', a photobiont of Xanthoria parietina. The genome was assembled into 20 contigs, of which 16 had telomeric repeats on both ends and likely represent complete chromosomes. We compared the genome to genomes of other Trebouxia species, and analyzed it to investigate the biology of the species, including its lifecycle and adaptations to lichen lifestyle. Trebouxia sp. \'A48\' genome is haploid and encodes genes involved in sexual reproduction and meiosis. The predicted secretome is enriched in hydrolases and redox enzymes and contains carbohydrate-binding proteins potentially involved in cell-to-cell recognition and adhesion. We identified genes potentially involved in carbon concentrating and confirmed two instances of ancient horizontal gene transfer from fungi. The genome and the strain of Trebouxia sp. \'A48\' are available for the community for research on algal evolution and lichen symbiosis.