Development of lipid metabolism in early life stages of Atlantic salmon (Salmo salar)
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- Institutt for biologi 
Development of lipid metabolism in early life stages of Atlantic salmon (Salmo salar) Larval and juvenile stages are critical periods for Atlantic salmon (Salmo salar), which can have a major impact on growth and development at later stages. A major challenge for the aquaculture industry involves improving the utilisation of dietary lipids at these early life stages, because salmon fry and juveniles have an underdeveloped system for regulating lipid digestion and metabolism compared to adults. The main objective of the thesis is to study the development of the lipid metabolic network in salmon at early stages, and the regulation of these pathways when fed contrasting diets of plant or fish oil. We achieved this by analysing transcriptomic and lipid composition changes across four tissues (stomach, pyloric caeca, hindgut and liver), two dietary treatment (plant oil and fish oil) and three developmental stages (initial feeding 0.16 g, 2.5 g fingerlings and 10 g juveniles). The present thesis provides a systemic overview of diet and life stage associated changes in pathways of phospholipid, cholesterol and long-chain poly unsaturated fatty acids (LC-PUFA) metabolism in salmon after first feeding. In papers I and II, we found that the expression of genes involved in phospholipid and lipoprotein synthesis pathways in pyloric caeca was positively correlated to the weight of salmon from initial feeding 0.16 g to juveniles 10 g. This suggests an insufficient capacity of de-novo phospholipid synthesis in pyloric caeca of salmon at early developmental stages, and the fish may require extra dietary phospholipids at early stages for optimum growth and development. As shown in paper III, the cholesterol and LC-PUFA synthesis pathways were also highly up-regulated during early developmental stages. More interestingly, the expression of genes in the cholesterol biosynthesis pathway has a more sensitive response to plant oil in salmon fingerling than juvenile stage. This suggests salmon also have a higher requirement of dietary cholesterol at early stages. In all three papers, we compared salmon specific gene duplicates (Ss4R ohnologs) to other classes of gene duplicates. The Ss4R ohnologs are on average regulated more similarly under different diets and developmental stages in liver and intestine. Nevertheless, 23% of the salmon duplicated pairs were still differentially regulated after feeding plant oil compared to fish oil, highlighting cases of regulatory, and potentially protein function divergence, of Ss4R duplicates in salmon. The present thesis provides new insights on early life stage and diet associated regulation of phospholipid, cholesterol, LC-PUFA and lipoprotein metabolism pathways in salmon. By integrating gene expression and lipid composition data, we conclude that salmon fry and juveniles have an underdeveloped system for lipid metabolism, especially on phospholipid, cholesterol and LC-PUFA biosynthesis pathways. This suggests that salmon fry and juveniles have a higher dietary requirement of phospholipid, cholesterol and LC-PUFA than adults, which should be taken into consideration when determining start-feed composition.