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What makes the differences between organisms? This is one of the major questions in evolutionary biology, which seeks to trace changes in a genome during evolution, based on the molecular differences between organisms. The detailed understanding of the mechanisms underlying the plant traits of the most recent common ancestor of algae and terrestrial plants that allowed them to colonize the terrestrial environment remains an open question.

The genome of Chara: towards terrestrial plants

One group of algae in particular is known for its proximity to terrestrial plants: the charophytes. Although the genome of one of Charophyte species (Klebsormidium nitens) was already sequenced, the research community dreamed of having a second genome of a species that shared more common history with terrestrial plants. It is now done thanks to the Chara braunii genome that has just been sequenced.

« One of the remarkable things about this genome is that there are a number of genes thought to be specific to terrestrial plants. » explains Pierre-Marc Delaux « we can thus propose that several characters that we thought were specific to terrestrial plants were in fact present in the genome of their aquatic common ancestor » he adds.

The authors finally note innovations in Chara braunii that are absent in terrestrial plants. They showed that two gene families were particularly abundant in Chara braunii: peroxidases, necessary for the control of the homeostasis of reactive oxygen species ; as well as genes allowing the recognition of surrounding microbes. These gene multiplications by diversification events could explain the establishment of new associations with a broad spectrum of microbes.

Fern genomes shed light on the evolution of terrestrial plants and cyanobacterial symbiosis

Ferns are the closest sister group to all seed plants, and are one of the only two groups of terrestrial plants among the seven existing large groups for which  genomes were not available. Indeed, fern genomes are usually colossal, with up to 250 chromosomes in some species, making their sequencing extremely complex. However, two fern species with less complex genomes have just been sequenced: Azolla filiculoides and Salvinia cucullata (Salviniales).

Among the genes identified in these genomes,  the international collaboration led by Fay-Wei Li (BTI, Cornell University, USA) identified a fern-specific gene that appears to have been derived from bacteria by horizontal gene transfer. What makes Azolla remarkable compared to other ferns, is its ability to form a unique symbiosis with nitrogen-fixing cyanobacteria, with a clear pattern of co-speciation between the two symbiotic partners. In addition, the Azolla genome lacks the genes common to the arbuscular mycorrhizal and root-nodule symbioses studied classically in TULIP’s labs: the symbiosis with a cyanobacterium is therefore independent of these genes.

These two articles on the study of symbiotic genes and their macroevolution fully enters into TULIP’s major research themes and, as Pierre-Marc Delaux concludes: « They are a great opportunity to take into account a dimension of diversity of life that is at the heart of TULIP concerns, but so far understudied: the diversity between species. »