# 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

## TL;DR

This study found that triploid and normal Atlantic salmon have similar liver gene activity during a key life stage, suggesting they adapt to seawater in the same way.

## Contribution

The study provides new insights into the molecular mechanisms of smoltification in triploid Atlantic salmon.

## Key findings

- Triploid and diploid salmon showed similar gene expression patterns for IGF-I and GHrec during smoltification.
- Dietary hydrolyzed fish proteins did not affect gene expression in either ploidy group.
- IGF-I gene expression varied over time, aligning with the progression of smoltification.

## Abstract

Farmed Atlantic salmon are made sterile to prevent them from breeding with wild fish if they escape. These sterile fish, called triploids, may have different needs than normal salmon. One of the most delicate stages in a salmon’s life is the transition from freshwater to seawater, known as smoltification. If this process is not well timed, fish may struggle with growth, health, and survival after they are moved to the sea. In this study, we examined whether triploids and normal salmon differ in how their livers respond during this transition. We also tested whether a diet containing hydrolyzed fish proteins influences this response. We measured two genes in the liver that help control growth and help the fish adapt to salty water, and we also looked at where signals from one of these genes appear inside liver cells. We found no major differences between triploids and normal salmon, and the diet had no effect. Instead, both groups showed clear natural changes as smoltification progressed. These findings suggest that triploid salmon cope with this life stage in the same way as normal fish and that these genes could help identify the best moment to move fish to the sea.

The induction of triploidy, a strategy to mitigate unwanted pre-harvest sexual maturation and a genetic containment measure for escaped farmed Atlantic salmon (Salmo salar), 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 (GHrec) and insulin-like growth factor-I (IGF-I) genes, both of which are intimately involved in the regulation of osmoregulation and growth. Additionally, hepatic presence and location of IGF-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 IGF-I and GHrec genes was quantified by real-time PCR. The results indicated that neither ploidy nor diet significantly influenced IGF-I or GHrec gene expression, suggesting that, under our conditions, triploidy and diet did not adversely affect this molecular pathway linked to growth and osmoregulation. IGF-I gene expression exhibited significant temporal variation, correlating with the progression of smoltification, while GHrec gene expression showed a similar, albeit non-significant, trend. Triploids exhibited IGF-I and GHrec 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 IGF-I mRNA within the liver cells. The potential applicability to a commercial aquaculture setting requires further investigation.