🧬 The Odyssey of a Gene Across Genomes

In the race of evolution, innovating is certainly necessary, but recycling existing genes is sometimes more efficient. Indeed, organisms that are very distant from each other can exchange a gene through horizontal gene transfer (HGT), a process that transcends barriers between species.

In an article published in Nature Communications, scientists reveal the remarkable evolutionary history of the NAS gene, horizontally transferred multiple times within the tree of life.

The Small Story of a Major Transition

About 500 million years ago, a lineage of aquatic plants undertook one of the major events of evolution: the colonization of land. This decisive transition was made in collaboration with microorganisms, thanks to:
- mycorrhizae, symbiotic associations that facilitate nutrient absorption,
- and horizontal gene transfers (HGT), through which genes from microorganisms were transmitted to the ancestor of plants, bringing crucial new adaptive functions.

From this lineage emerged two major groups:
- the bryophytes (mosses, liverworts, hornworts),

- and the vascular plants (lycophytes, ferns, conifers, flowering plants).

Often described as "primitive" survivors due to their simple morphology, bryophytes were considered poor in genetic innovations. New genomic data challenges this view.

The study by Dong et al., published in Nature Genetics, has provided a vast resource: the sequencing of about a hundred bryophyte species, covering a large part of their evolutionary diversity.

This resource shows that bryophytes:
- possess an extremely well-stocked genetic toolkit,
- have more gene families than vascular plants,
- and show numerous genetic acquisitions throughout their evolutionary history.

Thus, behind their apparent simplicity lies a remarkable genomic richness, revealing an intense evolutionary dynamic.

The NAS, a Traveling Gene!

In an article published in the journal Nature Communications, scientists focused on the gene encoding the NAS enzyme. It catalyzes the synthesis of nicotianamine, a tripeptide that binds and facilitates the transport of metals such as iron, copper, zinc, or manganese.

Absent in animals, with the exception of one insect, nicotianamine is found in vascular plants, where it mainly participates in the long-distance transport of metals via the vessels, as well as unexpectedly in the non-vascular model moss Physcomitrium patens, or in some microorganisms.

This fragmented distribution led scientists to reconstruct the history of the NAS gene within the tree of life.

The verdict is clear: the evolution of the NAS gene does not correspond to the evolutionary history of life. Why? Because several horizontal gene transfers occurred at different periods, from distinct microbial donors.

A particularly striking example: the NAS gene from the "ancestral" moss Physcomitrium is not the ancestor of the NAS genes of "modern" vascular plants; it is in fact about 150 million years younger!

The Importance of Analysis Depth for a New Vision of Land Plant Evolution

Thanks to the depth of analysis made possible by the new genomes published by their collaborators, the scientists demonstrated that:
- Bryophytes acquired NAS from four different microbial donors.
- A gene acquired by HGT can be replaced by a second HGT several hundred million years later, a phenomenon never demonstrated before.
- An NAS gene was even transferred from a seed plant to a fungus as well as to an insect, the whitefly.

This work reveals an evolutionary scenario where:
- horizontal gene transfers play a much more central role than previously thought,
- these transfers are probably much more frequent than estimated until now,
- and they can serve not only to introduce new functions, but also to replace an existing gene.