A new PhD study provides insights into the diversity and evolution of the PRDM9 gene in Atlantic salmon. The findings reveal that variation in this gene underpin different patterns of recombination that promotes genetic diversity between generations.
When sexually reproductive organisms produce cells that will become sperm or eggs, two matching chromosomes, one inherited from each parent, line up in pairs and physically exchange different segments of DNA. This ‘shuffling’ process, termed meiotic recombination, creates new combinations of genes that will be passed on to the next generation. Without such mixing, genetic variation would be reduced, limiting the ability of populations to adapt and evolve over time.
In many vertebrates, PRDM9 is known to play a key role in determining where in the genome recombination occurs. However, most research on this gene has so far been conducted on mammals such as mice and humans. The function and evolution of PRDM9 in Atlantic salmon is less well understood.
In his PhD thesis, Øyvind Sætren Gulbrandsen aimed to understand how variation in PRDM9 influences recombination patterns in Atlantic salmon, and what this can tell us about the mechanisms driving the evolution of the gene.
To answer these questions, Gulbrandsen applied a method based on long-read sequencing of the PRDM9 gene, which makes it possible to map genetic variation across individuals on a much larger scale than earlier methods. This approach was used to sequence thousands of individuals from both wild and farmed salmon populations, resulting in the largest dataset of its kind.
The results show that different versions of PRDM9 (i.e. with subtle differences in the PRDM9 genetic sequence) give rise to different recombination patterns in the genome.
“This leads to significant differences between individuals in terms of which combinations of genetic material are passed on to the next generation”, explains Gulbrandsen.
The analyses have also revealed that North American and European salmon populations have developed different PRDM9 variants and recombination patterns. At the same time, Gulbrandsen found that a European PRDM9 variant has recently spread to North America, considerably reshaping how their genomes recombine. This has important consequences for introgression of genetic variants between the two populations, as Gulbrandsen explains:
“Our comparisons between North America and Europe suggest that such genetic differences in PRDM9 do not prevent successful reproduction between salmon from the two continents, despite the fact that PRDM9 in other species has been linked to the development of reproductive barriers.”
The findings of Gulbrandsen’s PhD project provide important novel insights into how PRDM9 functions in fish. Furthermore, his work implies that the some of the most widely used models scientists have used so far to study PRDM9 evolution do not fully explain the patterns observed in nature. Gulbrandsen concludes:
“How different populations are structured and connected, and the migration of individuals between populations, may play an important role in the evolution of PRDM9, and these factors should be taken into account in future models.”
Øyvind Sætren Gulbrandsen will defend his doctoral thesis, "Function, diversity and evolution of meiotic recombination localization factor PRDM9 in atlantic salmon", at NMBU on 15 June 2026.
Gulbrandsen’s main supervisor has been Professor Sigbjørn Lien, NMBU.
Co-supervisors: Professor Matthew Peter Kent, NMBU; Marie Baudement; and Researcher Nicola Barson, NMBU.