Introduction
Given both the economic and societal impact of the Atlantic Salmon (Salmo salar) farming industry in Norway, increased knowledge around heritable traits, which may translate into welfare gains in both wild and farmed salmon, is of high demand. This may include traits associated with disease resistance, growth and sea-water adaption. G enetic potential may reside in landlocked populations which have undergone different selection pressures compared to their anadromous counterparts for the last 10,000 year s, due to their lack of sea water transfer, lack of marine pathogen pressures and different feed availability . We have in previous studies identified genes and genomic regions diverging between anadromous and landlocked Atlantic salmon populations from a widespread geographical distribution. H owever, we do not yet understand the biological significance of these diverged genomic regions, spanning about 250 genes distributed on several chromosomes. Hence, the objective of this study was to increase the biological understanding of the genetic differences between anadromous and landlocked populations. From the 250 genes present in diverging regions, we picked 22 candidate genes previously associated with either disease resistance, growth and sea-water adaption in other species . Gene expression was assayed in several tissues before and after the predicted smoltification season in a common garden smoltification experiment including one-year old anadromous farmed salmon (Mowi) and landlocked salmon (Gullspång).
Results
From the 22 genes we identified differential expression in 4 genes; ncor1 , encoding a thyroid hormone mediator in the gill was shown to be down- regulated in Gullspång post smoltification, while remaining fixed in the Mowi counterpart. spcs3 encodes a protein involved in microsomal peptide signalling in the h ead kidney and was shown to be up- regulated in Mowi , while remaining fixed in Gullspång . csf2rb2 encodes an activator in the JAK-STAT pathway and was down- regulated in Gullspång while being fixed in Mowi . bcl2l13 , linked to adipocyte biogenesis, was significantly higher Gullspång than it was in Mowi . To further investigate whether the identified genomic regions contain genetic variation relevant for understanding and improving traits in farmed salmon, we are currently screening farmed salmon strains for potential landlocked alleles using a genotype-by-sequencing (GT-seq) approach . The plan is to perform genotyping of 28 regions in the salmon genome in a vast material of farmed fish in which traits of interest have been quantified, such as growth and sea-water adaption. This study provides a basis for further functional studies of candidate genes through the creation of knockout models and subsequent functional studies including growth, disease and feeding trials.