Transcriptional regulation of lipid metabolism when salmon fry switches from endogenous to exogenous feeding

The onset of first feeding is a critical period for Atlantic salmon (Salmo salar). The fish goes through a dramatic transition, from using the yolk sac as an energy and nutrient resource to rely on exogenous food. However, the digestive system of salmon is not fully developed at start feeding. This is especially true for digestion and absorption of dietary lipids. To optimize lipid composition of the initial feed for better growth and development of salmon, it is important to have a systemic understanding of lipid metabolism. The present study combines transcriptomics and lipid composition data to obtain an overview of how lipid metabolism pathways change when salmon switch from endogenous to exogenous feed. Transition to exogenous feeding had higher influence on gene expression and fatty acids composition in pyloric caeca relative to liver, suggesting metabolism is more responsive in pyloric caeca when diet switches. The pathways of phospholipid and lipoprotein synthesis were both up-regulated in pyloric caeca, while phospholipid content unchanged. Because phospholipids are a major component of intestinal lipoproteins, it suggests that a higher level of dietary phospholipids is required for optimal lipid transport at first feeding stages. De-novo cholesterol synthesis pathway was up-regulated in pyloric caeca after exogenous feeding, while a down-regulation of bile acid synthesis pathway was found in liver. This suggests a higher requirement of cholesterol in salmon fry after switching to exogenous feeding. The present study has provided new insights on the systemic changes of lipid synthesis and transport pathways in salmon fry when switching from endogenous to exogenous feeding.