A gene for all sizes

Breakthrough study pinpoints gene responsible for determining the age at maturity and size of Atlantic salmon returning to rivers, and how it operates differently in the two sexes. Results will have benefits for salmon management and conservation.
Published: 11 November 2015

​​​​​​​​​​​​​​​​​​Influential discovery 

The breakthrough research ​shows how the gene, which surprisingly also plays a role in regulating the timing of puberty in humans, strongly affects variation in the age at which an Atlantic salmon becomes reproductively mature (its age at maturity) and therefore its size. The gene, known as VGLL3, does this​ through governing the build-up of body fat in the fish, a trait which goes on to play a critical role in its maturation. ​​​

Atlantic salmon exhibit diverse life histories, more so in males than females. Whilst females’ size is proportionate to number of eggs and reproductive capacity, for males it’s much more complex, with diverse characteristics such as age at maturity often being contingent on factors like the conditions of a salmon’s homing river. 

Despite the knowledge that the size of salmon heading back to spawn in river systems from saltwater hinges on how many years it has been at sea, the genetic foundation for this had not been known, leading to problems in managing the impact of fishing. 

War of the sexes

The Norwegian-Finnish-Scottish ​research group also touched on a previously unobserved genetic determination through studying the gene’s effects on age at maturity in male and female salmon. A conflict comes about as both sexes share a common set of genetic material (the genome) and, because one of the fish’s traits is so closely controlled by one single gene, the males and the females try to pull this trait in different directions to achieve their respective favourable outcomes. This evolutionary battle can result in a stalemate, with neither sex achieving the trait in its optimal form. 

A way of decoupling the sexes to allow them to operate independently is therefore required. The answer lies in the way the gene is regulated differently in males and females. 

“One possibility is through sex-dependent dominance that has been proposed, but never seen before,” explained researcher Nicola Barson of the Norwegian University of Life Sciences, “because until now it has been very difficult to identify genes experiencing conflict, where selection acts in opposite directions in the two sexes – that’s what we actually managed to map here.”

“We found two variants of VGLL3: one causing salmon to mature earlier and one later. What is interesting is that when a salmon has both variants, there is a difference between sexes. Males with both mature earlier and smaller whilst females with both mature later and larger. This effect allows them to approach the ideal reproductive age for their sex and so reduces the conflict between the sexes.”

"Because of this difference in dominance, natural selection favours both variants being retained in the population. So this effect is important as it means we don’t lose variation,” she added.

“The way the two versions of the gene behave in the different sexes is important because it means the sexes can be different despite the trait being largely controlled by this single gene.”

A plus for management

Now that the scientists have a handle on the genetic undertow of age at maturity and size, the question it begs is how this new information could be practically applied. 

For co-researcher Jaakko Erkinaro, Research Professor at the Natural Resources Institute Finland and member of ICES Working Group on North Atlantic Salmon (WGNAS), the age at maturity issue is one at the very heart of management of the species.

“The age of returners is a fundamental issue from management and biological perspective – because that’s the size of the fish. There’s a huge difference between those that return after one year and after three or four years.”

“There are some badly depleted salmon populations, like those that are entirely in artificial conditions like hatcheries, especially in the Baltic Sea,” continued Erkinaro, one of several ICES scientists involved in the process. ​“Our findings could be used in some kind of selection, so to direct future generations of these populations to more resemble what they used to be. One way to do this would be to increase the proportion and contribution of individuals that are more likely to mature later.”

“That doesn’t mean producing large salmon, but somehow rebuilding the variation and life histories of these suffering populations based on genetics that we know now better than ever before.”

As well as WGNAS, the findings could also be relevant for other ICES expert groups such as the Assessment Working Group on Baltic Salmon and Trout (WGBAST​), the Working Group on Effectiveness of Recovery Actions for Atlantic Salmon (WGERAAS), and on the Science Requirements to Support Conservation, Restoration, and Management of Diadromous Species (WGRECORDS).

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Atlantic salmon leaping in freshwater

​Leaping A​tlantic salmon; photo - David Hay, Marine Scotland Image Bank

​The research was undertaken by projects funded by the Academy of Finland and the Research Council of Norway. The consortium comprised researchers from the two interviewees’ organizations as well as from the University of Turku, the Norwegian Institute of Food, Fisheries and Aquaculture Research, the University of Edinburgh, AquaGen​, and Radgivende Biologer​​
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A gene for all sizes

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