Comparison of anadromous and landlocked Atlantic salmon genomes reveals signatures of parallel and relaxed selection across the Northern Hemisphere

Most Atlantic salmon (Salmo salarL.) populations follow an anadromous life cycle, spending early life in freshwater, migrating to the sea for feeding, and returning to rivers to spawn. At the end of the last ice age similar to 10,000 years ago, several populations of Atlantic salmon became landlocked. Comparing their genomes to their anadromous counterparts can help identify genetic variation related to either freshwater residency or anadromy. The objective of this study was to identify consistently divergent loci between anadromous and landlocked Atlantic salmon strains throughout their geographical distribution, with the long-term aim of identifying traits relevant for salmon aquaculture, including fresh and seawater growth, omega-3 metabolism, smoltification, and disease resistance. We used a Pool-seq approach (n = 10-40 individuals per population) to sequence the genomes of twelve anadromous and six landlocked Atlantic salmon populations covering a large part of the Northern Hemisphere and conducted a genomewide association study to identify genomic regions having been under different selection pressure in landlocked and anadromous strains. A total of 28 genomic regions were identified and includedcadm1on Chr 13 andppargc1aon Chr 18. Seven of the regions additionally displayed consistently reduced heterozygosity in fish obtained from landlocked populations, including the genes gpr132, cdca4, and sertad2 on Chr 15. We also found 16 regions, includingigf1on Chr 17, which consistently display reduced heterozygosity in the anadromous populations compared to the freshwater populations, indicating relaxed selection on traits associated with anadromy in landlocked salmon. In conclusion, we have identified 37 regions which may harbor genetic variation relevant for improving fish welfare and quality in the salmon farming industry and for understanding life-history traits in fish.

Automated summary

This study identified consistently divergent loci between anadromous and landlocked Atlantic salmon strains by sequencing their genomes, targeting traits relevant for salmon aquaculture and life-history characteristics in fish. A total of 37 genomic regions were identified, which may harbor genetic variation useful for improving fish welfare and quality in the salmon farming industry.