During the on-growth phase in sea cages, Atlantic salmon are subjected to a large variability in dissolv ed oxygen (DO) concentration. Low DO concentrations are known to limit food utilization , growth, production performance and welfare, and may contribute to some of the mortalities observed at sea. The environment (e.g. temperature, oxygen, photoperiod) during an organism's embryonic development can shape its phenotype at later life stages, through modifications in the epigenetic machinery. The aim of the present study was to determine the effects of different DO levels during early ontogeny on the transcriptomic and methylomic profiles in later life stages.
Until start feeding, salmon eggs and larvae were subjected to 30%, 60% and 100% DO levels. Development was monitored based on embryonic stages. Whole embryos and l arvae were sampled at consecutive time points to investigate differential gene expression and DNA methylation using qPCR and RNA-sequencing, and pyrosequencing and Reduced Representative Bisulfite Sequencing (RRBS), respectively.
Growth was affected especially in the 30% DO group, such that the occurrence of s tart feeding was delayed ca. 8 days compared with the 60% and 100% DO groups. The 60% and 100% showed no clear differences with regard to development and growth, suggesting that 60% DO may not visibly affect larval development. However, on the molecular level clear differences were observed between all groups . Preliminary data shows that b etween the three groups, 69 genes were differentially expressed , and had biological functions relating to, for example, oxygen transport (e.g. haemoglobin subunits), and major energetic metabolic processes (e.g. cytochrome c oxidase, p450 subunits).
Further, t he expression levels of prolyl hydrogenase-domain (PHD ) genes, which are essential regulators in maintaining oxygen homeostasis , were significantly increased with decreasing oxygen levels . Moreover, t wo putative CpG sites in the promoter of PHD 3 were found differentially methylated and showed a negative correlation with its expression. It is hypothesized that through the inhibitory effects of these PHD genes on hypoxia inducible factor (HIF) the potentially harmful effects of hypoxia may be reduced during early development.