Discovered “supergene” leading to sex-dependent migration patterns in rainbow trout

Why does some fish migrate into the ocean while others stay in freshwater? A study providing new insight into this phenomenon, was published this week in Nature Ecology and Evolution. The study was led by researchers at CIGENE, NMBU, in collaboration with NOAA and USDA.

Rainbow trout is a species of Pacific salmon with a similar lifestyle to the European brown trout. While some trout spend their entire lives in freshwater (rainbow trout), others migrate to sea to feed and grow before returning to spawn in freshwater (steelhead trout). The difference between these two lifestyles is so great that they were thought to represent two species for a long time. Going to sea can be highly profitable as there is more food which can be invested in producing eggs or becoming a large dominant male. However, the sea is also a dangerous place and some trout may not survive to reproduce at all. This cost-benefit balance is different between the sexes, with females gaining more from going to sea than their male counterparts. This results in more females migrating than males, but it has not been understood what mechanism underlies this difference.

In other species, such as humans, sex-specific differences are caused by genes and variation distributed along the sex chromosomes that clearly differ between males and females. However, unlike the X and Y of humans, rainbow trout sex chromosomes are not dramatically different and so are able to exchange genetic information freely. The recent study describes a large “supergene” on chromosome 5 that influences whether the trout migrates or not. The supergene is formed because two large sections of this chromosome have been flipped around, with the resulting mirror-image chromosome variants unable recombine, allowing them to become different over time.

Whilst these differences influence whether the individual trout migrates to sea or not, they are not enough on their own to explain the sex differences in seaward migration. This sex difference comes about because the variant that increases the chance of migrating to sea is dominant over the one that makes them tend to stay in females, while the reverse is true in males. This mechanism helps to resolve the conflict between male and female trout over whether to migrate or not without the involvement of the sex chromosomes.

The research team from CIGENE, NMBU


The full article can be found at the webpages of nature ecology and evolution.

This news can also be found in Norwegian at NMBU’s webpages and a summary of the research is also published at


Illustration photo: Morgan Bond


Published 27.11.2019, updated 06.12.2019

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