Biological processes are adapted to and driven by rhythmicity in the natural environment and periodicity of light is one of the most influential factors. In the aquaculture of Atlantic salmon, artificial light conditions are widely used, even though the life history transitions of salmon are closely linked to the seasonally changing light environment. The onset of circadian rhythmicity
might indicate the time point of functional necessity for aligning processes to the environment, and here, the circadian clock mechanism has been studied in the developing brain of Atlantic salmon. By a transcriptomic approach, the endogenous feeding alevin, independent on the environment for nutritional supply, has been compared to the exogenous feeding fry, dependent on the light period for detecting and catching prey. The results revealed that circadian rhythmicity becomes established during the first feeding period in the salmon brain, when the main components of the molecular clock mechanism start to cycle.
RNA sequencing has been done to analyze 4 8 h circadian sampling series of 104 Atlantic salmon brains , in the endogenous feeding alevins and exogenous feeding fry, to study the onset of circadian rhythmicity.
The results show that many of the circadian clock genes have started to cycle with a period of 24 h in the exogenous feeding salmon fry , while few clock genes were cyclic in the exogenous feeding yolk sac alevin. Fig 1 illustrate s the expression profile of clock genes, revealing a significantly higher and cyclic expression in the fry , compared to the non-cyclic alevin. F ew genes were differentially expressed between timepoints in the circadian sampling series before feeding, but in the exogenous feeding salmon fry, several hundred were differentially expressed . In the KEGG pathway circadian rhythm many genes were cyclic in the exogenous feeding fry and analyzes of the KEGG pathway cell cycle indicated a clock-controlled cell cycle at exogenous feeding.
Taken together, the life history transition related to onset of exogenous feeding is linked to the establishment of a persistent circadian rhythmicity in the salmon brain, which needs to be synchronized to light-dark cycles to enable the fry to search and capture feed.