The paradoxical gene of evolution 🧬

The recombination between parental chromosomes, a characteristic of sexual reproduction, plays a crucial role in evolution. In mammals, a protein called PRDM9 is essential for this process. In a paper published in PLOS Biology, scientists show that this protein also exists in salmonids and functions similarly. These findings, which prove the ancient origin of PRDM9, open new perspectives on genome evolution across the ages.

The crucial role of genetic recombination

Sexual reproduction ensures genetic mixing between parents through a process called meiosis. During this cell division, chromosomes exchange DNA segments. The recombination between parental chromosomes and their distribution in reproductive cells has long-term consequences for genome evolution. In particular, recombination increases the efficiency of selection and facilitates adaptation.


However, the sites where these exchanges occur are not chosen randomly. Two major distribution patterns have been identified. The first, observed in many species (plants, numerous metazoans), relies on accessible chromatin zones, which are largely the regulatory regions of gene expression. The second is determined by a specific protein called PRDM9, which binds to DNA at specific sequences to induce recombination there.

PRDM9: a gene with a paradoxical role

This PRDM9-dependent mechanism has been described only in certain mammals and exhibits paradoxical properties. Although it determines recombination sites, these sites undergo erosion over generations. This phenomenon leads to an endless evolutionary race known as the "Red Queen model," which involves the selection of new PRDM9 alleles recognizing new sites in the genome, which are in turn eroded in a recurrent process.

This genetic conflict caused by PRDM9 is all the more surprising since many species can recombine correctly without this gene (first pattern). Moreover, the phylogenetic study of PRDM9 shows that this gene has been lost several times independently during evolution, possibly in relation to the site erosion phenomenon.

All this raises the question of its actual role in living organisms. It was therefore important, as a first step, to determine whether PRDM9's function was specific to mammals.

A surprising discovery in salmonids

To answer this question, scientists, in a paper published in the journal PLOS Biology, analyzed the phylogeny of PRDM9 in vertebrates and identified that a homolog of this gene was present in salmonids, while it is absent in closely related fish. They developed a set of molecular and population genetics approaches that allowed them to observe that PRDM9 does indeed play a role in determining recombination sites there.

It exhibits high allelic diversity in its DNA-binding domain. Furthermore, they identified molecular markers characteristic of PRDM9 activity, such as chemical modifications of histones, proteins associated with DNA.


By studying different species and populations of salmon (Atlantic, Pacific, Baltic), they demonstrated a divergence in recombination sites between the most distant populations, which is precisely expected if PRDM9 is active. Also consistent with this determining role of PRDM9, the scientists were able to identify in salmon DNA sequences specifically enriched at recombination sites and detect an erosion of these sequences over evolution.

These observations confirm that the PRDM9 gene, far from being limited to mammals, was already active several hundred million years ago.

Perspectives: tracing the origins of PRDM9

Thus, the function of PRDM9 for locating recombination sites is not specific to mammals and instead appeared much earlier than anticipated, with the associated genetic conflicts persisting over several hundred million years. It will be important to trace further back in evolution to identify the emergence of PRDM9 and understand the selective force(s) that contribute to its maintenance.