The liver, an important immunological organ in the gut-liver axis, has also recently been linked to smoltification during seawater transfer (SWT) (1 ). Previous studies have shown liver plasticity when fish were exposed to a different functional ingredient (2) and variation in genotype-specific transcriptomic profiles in liver in responses to dietary interventions (3 ). The main objective was to evaluate the effect of functional diets containing D. hansenii yeast-based products (LAN4 and LAN6) on hepatic transcriptomic profiles in two genetic groups of Atlantic salmon during SWT. The study also investigated whether these functional diets could facilitate the parr-smolt transformation and SWT and whether genetic background affects the hepatic transcriptome and growth performance in Atlantic salmon.
Materials and methods
F ish from two genetic lines ( Family A and Family B, AquaGen, Norway) were fed standard diets prior the experiment , vaccinated and transferred to 18 experimental tanks ( 60 fish per tank) for a total of 7 weeks in fresh water (FW) and then moved to SW for 6 weeks (20 fish per tank) . During the whole experimental period, f ish were fed a control diet (CD) or two experimental diets containing 0.1% of either component LAN4 or LAN6 (D. hansenii yeast-based products) . Dead fish with skin ulcers were detected during a natural outbreak in the SW phase , and samples were collected for pathogen detection. At the end of each phase, fish were sampled for various tissues including liver samples for RNA-seq analysis, and comparison between groups (Figure 1) was performed using KEGG enrichment analysis in ShinyGO v.077.
Results and discussion
Fish were healthy during the 7-week FW phase with no mortalities. Seven fish died during the 5th week of the SW phase due to skin ulcers caused by Moritella viscosa. All groups tested positive for the bacterium in gill samples, but none of the fish had clinical signs of winter ulcers at the end of the SW phase. No significant differences were found in growth parameters between dietary groups within the same family, but significant differences were observed between families in both FW and SW phases, with Family A exhibiting better growth than Family B, in b oth water phases.
KEGG pathway analyses revealed unique pathways linked to distinct effects of water phase, diet, and family in SWT.
Table 1. Unique up-and down-regulated pathways in Family A and B
in LAN 4 group (FW vs SW).
In the comparison between FW and SW, Family A showed unique significantly upregulated pathways related to lipid family changes and anabolic processes when fed LAN4 diet during SWT, while Family B had unique downregulated pathways related to immune signaling and activation when fed the same diet. (Table 1, Figure 2)
In the LAN6 group, Family A had unique upregulated pathways related to protein production and cellular growth proliferation . Family B had no unique upregulated pathways in the LAN6 group, but unique downregulated pathways related to primary bile acid biosynthesis, retinol metabolism, lysosome, fatty acid, and sphingolipid metabolism. These results suggest that LAN4 and LAN6 ( D. hansenii yeast-based products) may have distinct modes of action in salmon liver , resulting in differences in their transcriptomes. These differences may be influenced by the fish’s genetic background and environmental conditions .
(1) Shwe, A. et al., 2022 , https://doi.org/10.1016/j.fsi.2011.02.023 (3) Morais, S. et al., 2012 , Effects of genotype and dietary fish oil replacement with vegetable oil on the intestinal transcriptome and proteome of Atlantic salmon (Salmo salar), BMC Genomics 13(1) .