Hepatic Expression of Growth Hormone Receptor (GHrec) and Insulin-like Growth Factor-I (IGF-I) Genes and Cellular Location of IGF-I mRNA in Diploid and Triploid Atlantic Salmon (Salmo salar) Undergoing Parr–Smolt Transformation

The induction of triploidy, a strategy to mitigate unwanted pre-harvest sexual maturation and a genetic containment measure for escaped farmed Atlantic salmon (<i>Salmo salar</i>), may give rise to challenges because of the distinct environmental and dietary requirements of sterile triploid fish. Smoltification is a critical phase in the life cycle of Atlantic salmon, so knowledge about parr-smolt transformation in triploids is important for the salmon farming industry. This study covered an investigation of hepatic expression of growth hormone receptor (<i>GHrec</i>) and insulin-like growth factor-I (<i>IGF-I</i>) genes, both of which are intimately involved in the regulation of osmoregulation and growth. Additionally, hepatic presence and location of <i>IGF-I</i> mRNA were examined using RNAscope^®, an advanced in situ hybridization technique. Triplicate groups of juvenile diploid and triploid salmon were reared at low temperature (10 °C) and fed either a standard diet or one enriched with hydrolyzed fish proteins from the start of feeding onwards. Liver samples were collected from three fish per tank each month from October to December (2454-3044 degree-days post-start feeding), the period encompassing smoltification, and hepatic expression of <i>IGF-I</i> and <i>GHrec</i> genes was quantified by real-time PCR. The results indicated that neither ploidy nor diet significantly influenced <i>IGF-I</i> or <i>GHrec</i> gene expression, suggesting that, under our conditions, triploidy and diet did not adversely affect this molecular pathway linked to growth and osmoregulation. <i>IGF-I</i> gene expression exhibited significant temporal variation, correlating with the progression of smoltification, while <i>GHrec</i> gene expression showed a similar, albeit non-significant, trend. Triploids exhibited <i>IGF-I</i> and <i>GHrec</i> gene expression patterns comparable to diploids, and both the temporal changes and lack of difference between triploids and diploids were mirrored in the quantification of <i>IGF-I</i> mRNA within the liver cells. The potential applicability to a commercial aquaculture setting requires further investigation.