Introduction
Fillet color is one important trait defining flesh quality in Atlantic salmon. Dietary supplementation of carotenoids, mainly astaxanthin, is a common strategy to increase fillet pigmentation and its retention in tissues appear to be influenced by nutritional factors. Recent studies have shown a link between omega-3 levels in the diet, particularly EPA and DHA, and fillet color in Atlantic salmon (Bou et al., 2017, Lutfi et al., 2022). Nevertheless, the specific regulation of factors determining the deposition of astaxanthin in different tissues and the fillet color is still not clearly elucidated. The present study aims to evaluate the potential relationship between omega-3 fatty acids and fillet color by looking into transcriptional and metabolic regulations in the three main tissues involved in astaxanthin metabolism (muscle, liver, and intestine).
Materials and methods
Atlantic salmon from Mowi Genetics were distributed across six sea cages (5x5m2) located at the Mowi’s research station in Averøy (Norway) and fed 2 different diets based on a common low marine base formulation (Skretting ARC) coated with either a high vegetable oil/low fish oil mix (Control diet, CT) or an omega-3 LC-PUFA-rich microalgal oil from Veramaris® (Omega3, Ω3). At the final sampling, all fish from each sea cage were randomly taken and 10 fish from each dietary group were selected for High (5 fish) and Low (5 fish) fillet color based on VIS/NIR spectroscopy. The main purpose of this selection strategy was to create high contrast among the different conditions (CT-High, CT-Low, Ω3-High and Ω3-Low) in order to establish potential correlations between results obtained by transcriptomics, metabolomics and chemical analyses. Samples of muscle, liver, and intestine were taken for different analyses. The present study was conducted within the AquaIMPACT project (H2020 BG2018-818367) and analyses were co-financed by Nofima’s internal projects.
Results
The microarray results showed that the “pigment selection” had the most significant impact, particularly in the liver (Fig.1A). The PCA plots display a clustering of samples with similar gene expression profiles in the same area. Scores of samples belonging to either high pigment or low pigment, were located together respectively and slightly influenced by the dietary treatments, particularly within the low pigment group (in green). In this regard, 23.94% (muscle), 28.17% (liver) and 24.94% (intestine) of the variation was explained by the first principal component (x-axis), which distributed the samples along the axis from high pigment, in the left quadrants, to low pigment, in the right quadrants (Fig. 1B-D). This distribution is again particularly evident in the liver. Moreover, the liver showed the largest number of unique differentially expressed genes (315) compared to intestine (139) and muscle (144) (Fig. 1E). However, no differential regulation of the common genes (9) was observed in the different tissues. Further analyses comparing common genes in one-by-one tissues (i.e. intestine vs. muscle) will be conducted.
Discussion and conclusions
Overall, our results showed a clear effect of the fillet pigment selection on the transcriptional regulation of all tissues and particularly in the liver (also with the highest number of differentially expressed genes) highlighting the potential involvement of this tissue in regulating astaxanthin deposition and fillet color. Moreover, diets also had a significant differential effect within the high and specially the low pigment group supporting the idea that dietary omega-3 fatty acids also influence fillet color. Additional integrative analyses including transcriptomics, metabolomics and fatty acid composition analyses are currently being conducted.
Bou, M. et al., 2017. Low levels of very-long-chain n-3 PUFA in Atlantic salmon (Salmo salar) diet reduce fish robustness under challenging conditions in sea cages. J Nutr Sci 6, e32.
Lutfi, E. et al., 2022. Increasing dietary levels of the omega-3 long-chain polyunsaturated fatty acids, EPA and DHA, improves the growth, welfare, robustness, and fillet quality of Atlantic salmon in sea cages. Br J Nutr, 1-48.