The Azolla Genome and the Metagenomes of Its Obligate Endosymbionts: Unlocking the Massive Green Potential of a Little Fern

Ferns, the second largest clade of vascular plants, are the only major plant lineage for which a reference genome has not yet been sequenced. Without a fern genome, we cannot fully comprehend the processes that govern the evolution of land plant genes and genomes, and the patterns underlying major evolutionary transitions in plants will remain elusive.

Azolla, a genus of aquatic ferns, is the perfect target for starting to explore fern genomics. First, its 750Mb genome is tiny compared with other fern genomes, which are typically >10Gb. Second, because of its obligate endosymbiotic relationship with a variety of bacteria, Azolla is a natural nitrogen-fixer that bolsters rice productivity when these two are grown together. Third, Azolla lacks lignin and is capable of doubling its biomass every two days, making it a prime candidate for biofuel investment. Indeed, around 50 million years ago, there was an enormous Azolla bloom that sequestered over 10¹² tons of carbon and helped to shift the greenhouse world towards our present icehouse climate.

We aim to (1) generate a reference genome for Azolla, as well as the metagenomes of its obligate endosymbiotic bacteria; (2) resequence Azolla and its symbionts from across all Azolla species to investigate genomic co-evolution; (3) use comparative transcriptomics to infer the functionality of the symbionts and to identify candidate genes essential to this symbiosis and the nitrogen-fixation process; and (4) revisit plant genome and gene family evolution with this very first genome for the fern lineage. Here we present our preliminary results.