Fish Migration Supergene Makes Cod Venture to the North
Atlantic cod along the coast of Norway can live two very different lives. Some of them stay close to the coast, some of them migrate north to the Barents Sea. Their genomes are extremely similar except from one region. This difference may explain why some Atlantic cod migrate to the Barents Sea.
The migrating cod, also referred to as skrei, return south every winter to spawn, mainly around the Lofoten archipelago. The skrei is the basis of stockfish and clipfish, for centuries an important Norwegian export industry. It has been known that cod along the Atlantic coast of Norway behave diffierently from those living in the Batents Sea. A new study identifies a supergene that explains the different behaviours of the Atlantic cod. “This supergene may explain why some cod swim north to the Barents Sea and why others stay close to coast,” says Michael Matschiner, Associate Professor at the Natural History Museum in Oslo.
A supergene is a region on the chromosome that consists of multiple genes that are tightly linked. A supergene is never broken up and rearranged. “It is one set of genes that act together and have a joint consequence on morphology or behaviour,” Matschiner says to Titan.uio.no. A supergene can according to Wikipedia, contain just a few, functionally related genes. In other cases, a supergene can contain hundreds of genes.
There are three other supergenes as well in Atlantic cod, but they don’t seem to be important for migratory behaviour. “The three other supergenes detected are not important for the migratory behaviour,” says Sissel Jentoft, researcher at Centre for Ecological and Evolutionary Synthesis (CEES). “They are thought to be linked to environmental conditions and local adaptation, including ocean temperature and salinity. For instance, some of these supergene variants are found to be more prominent in its southern distribution,” Jentoft says.
Stable Over Millions of Years
The term supergene dates back almost 100 years but it is just recently that scientists have been able to identify and characterize them in detail. “In the past we were not able to observe these supergenes. This is because the genome sequencing was not able to reconstruct the larger structures of the genome. With new sequencing technologies it is possible to do that. I think we are going to find these supergenes in many more organisms,” Matschiner says. One of the reasons it has been difficult to study and identify supergenes, is that the individual genes can be the more or less the same. “All genes are present in the genome, but large blocks of individual genes are flipped — and thus, found in the opposite direction — as for the supergene found in the migratory cod vs the non-migratory coastal cod,” Jentoft says.
Their study also shines new light on the age and longevity of supergenes. “It has long been the theory that the supergenes erode over time, which, in the case of cod would mean that the migrating and the stationary cod would become the same again. Our study shows that those supergenes appear to be stable millions of years,“ Matschiner says. “We don't see any evidence that this is going to change so I think the skrei is going to remain there, unless we fish it all of course,” he says.
This novel distinction between the two behaviour types will be crucial to take into consideration for policy makers. “A better understanding of the evolution and dynamics of these supergenes, and how they are impacted by the ongoing ocean warming and increased human impact, are essential for future management of these important cod stocks,” Jentoft says.
Why Not Two Species?
Whether they migrate or not, Atlantic cod is one species and they all share the vast majority of their genome. “There is really no difference at all between the migratory and the stationary type except for this one region of the genome. It is basically half a chromosome that separates them,” Matschiner says.
So, why don’t they drift apart and become two separate species? “Their habitats clearly overlap and they regularly have offspring together. If there wasn't this kind of overlap they might have gone in two different directions and become two different species,” Matschiner says.
Scientific article: Michael Matschiner, Julia Maria Isis Barth, Ole Kristian Tørresen, Bastiaan Star, Helle Tessand Baalsrud, Marine Servane Ono Brieuc, Christophe Pampoulie, Ian Bradbury, Kjetill Sigurd Jakobsen and Sissel Jentoft: Supergene origin and maintenance in Atlantic cod, Nature Ecology & Evolution, February 2022.