Cod provided new knowledge on important supergenes

Supergenes help many species adapt to life in different environments. They are found, for example, in cod, where stocks can live at different depths, and in different temperatures and salinities. Humans have supergenes too – but not much is known about them yet.

The species with the best studied supergene is, not unexpectedly, the banana fly. It is easy to carry in the laboratory, and therefore popular in genetic research.

The fact that cod have super genes was discovered a few years ago, when different cod stocks were studied to understand how to protect them.

– We found that certain parts of the genome differed very clearly between different stocks, and that several genes that were very close to each other differed by exactly the same amount,” says Carl André, Professor of Marine Ecology at the University of Gothenburg.

– Supergenes hold together several different genes, perhaps 100 genes, which are inherited as a common package from parents to codlings, he continues.

Sitting back-to-front

The genes in the package are therefore not mixed with genes from the other parent. Characteristics that are favourable, for example for cod in the Baltic or the North Sea, are constantly passed on intact.

Now a study, in which Carl André participated, has shown how supergenes arise.

– A chromosome has broken off during a mutation and turned the wrong way, so that a piece of the chromosome is backwards. The genes are there, but they come in the wrong order, so to speak,” he says.

Cod has 22 chromosomes, just like humans. Supergenes have been found on four different chromosomes in different cod stocks.

– In the Baltic Sea, almost all cod have a reverse chromosome package to cope with low salinity, says Carl André.

Half have supergene

In the North Sea, it is more important to cope with different temperatures. About half of the cod have super genes with inverted chromosome packages. The other half do not. One group benefits when the water is cold, the other when it’s warmer.

The Barents Sea cod of Svalbard, known as skrei, has a gene package that helps it find its way around. The Skrei swims up to 100 miles to the Norwegian coast to breed.

– Intact genes are a key to adapting to different conditions. In recent years, we’ve realised that this is really important for a lot of species,” says André.

In the case of cod, we humans can put our knowledge of supergenes to practical use. A fishing port in Lofoten, Norway, has a DNA laboratory where skrei can be separated from the catch of local, stationary cod stocks.

Local cod stocks

The supergenes have also helped marine scientists to identify local cod stocks in Bohuslän, including in Gullmarsfjorden and off the islands of Tjörn and Orust.

– Previously, these rather small cod were thought to be North Sea cod on a temporary visit,” says Carl André.

What about humans’ own supergenes and chromosome packages?

– In medicine, attempts have been made to see if disease susceptibility can be grouped into supergenes. But it hasn’t been investigated that much yet,” says Carl André.