Molecular technologies shed light on early sexual maturation in cod

Early sexual maturation is currently one of the biggest obstacles to cod aquaculture. The risk of cod spawning in cages and thereby causing genetic pollution of wild cod stocks has made Norwegian authorities very cautious about granting permits for cod farming.

Early sexual maturation also leads to economic losses for the farmer, as the fish do not reach optimal size before puberty sets in.

"Wild cod become sexually mature from 3 years onwards, whereas their farmed counterparts reach puberty within the first 2 years of age, just before attaining the market size of 2 to 4 Kg," says researcher Ioannis Konstantinidis from Nord University's Faculty of Biosciences and Aquaculture.

Puberty initiates a shift of energy reserves towards the maturation of the gonads (eggs and sperm), which has a negative effect on growth, fillet quality and post-spawning mortality.

This limits the profitability of the industry and has been partly responsible for the collapse of cod farming in the past.

Moreover, precocious maturation has a negative impact on the environment.

"Sexually mature farmed cod can reproduce in sea cages and release fertilized eggs in the sea. As the farms are located within fjords along the Norwegian coastline, genetic pollution is unavoidable and represents a major threat for wild fish stocks”, Konstantinidis says.

Now the researchers want to understand more about what triggers the fish to become sexually mature. It is well known that sexual maturation is regulated by daylight, but little is known about how this regulation occurs at the cellular level.

In the Epicod project, researchers from Nord University and their partners are utilizing the latest advancements in molecular research to study how light affects the fish's DNA.

"Research on gene expression in single cells is a hot topic today. Many researchers use methods to observe changes in the expression of all genes at the single-cell level, what we call the transcriptome. We are taking it a step further, studying how DNA in cells first becomes accessible for gene expression," says Professor Jorge Fernandes.

In other words, the researchers will study what unlocks specific genes so that they can begin to function.

Normally, DNA in cells is tightly bound together with proteins in large complexes called chromatin. When chromatin is tightly packed, genes are inaccessible for reading. For genes to start functioning, the chromatin must open up so that the relevant part of the DNA becomes accessible for reading.

"It is important to gain knowledge about why certain genes, such as those responsible for sexual maturation, are not expressed. It is not necessarily about the function of the genes, but rather what influences them to become activated. We can study this by examining the structure of chromatin," says Fernandes.

The researchers will now study how light affects chromatin in cod cells.

"Cod reach puberty because they receive signals from the environment. Light is important here. During the summer, there is almost continuous daylight, especially in the north. But when summer is over, the difference between night and day starts to reappear. This change triggers a signal that leads to sexual maturation."

Inside the brain, cod have the pineal gland which responds to light.

"Cod perceive light through a small opening on the top of their head. It is like a third eye and provides stimuli to the pineal gland, which secretes melatonin. Melatonin affects another gland, the pituitary gland, which in turn secretes a variety of hormones into the bloodstream."

The result of this cascade of hormones is the initiation of sexual maturation.

“Our goal is to understand the biological mechanisms responsible for the early initiation of puberty in farmed Atlantic cod and identify key markers that control it. To achieve this, we will screen specific cells in two main glands responsible for light sensing (pineal) and hormonal regulation (pituitary) using state-of-the-art genetic, epigenetic and transcriptomic approaches," says Konstantinidis.

In the long term, the biological markers may be used to select fish for delayed puberty.

"This way we can minimize gene pollution and prevent mortalities and losses in body mass and fillet quality," says Konstantinidis.