‘Jumping’ genes let fish move to fresh water

While many of us get our omega-3 fatty acids from fish oil, we tend to overlook the fact that fish need them just as much as we do.

Now research published in the journal Science shows just how useful they can be: “jumping” genes associated with omega-3s may actually be key to the evolutionary spread and diversification of fish species.

Jumping genes are sequences of DNA that can copy and paste themselves into different parts of a genome, and they may be a prime engine of evolutionary diversity.

One of the most important omega-3s is docosahexaenoic acid (DHA), an essential compound for animal health. DHA is abundant in marine ecosystems but is scarce in freshwater environments. In saltwater, various algae that comprise the diet of fish produce DHA, but it can also be manufactured in small amounts by fish themselves.

Given how necessary DHA is, how then do marine fish species colonise freshwater ecosystems – something that has happened repeatedly over evolutionary history?

Lead author Asano Ishikawa of the National Institute of Genetics and the Graduate University for Advanced Studies (SOKENDAI), in Shizuoka, Japan and a team of international scientists set out to discover the answer.

The team noted that the marine fish known as the three-spined stickleback (Gasterosteus aculeatus) had successfully colonised freshwater habitats many times and in many continents over its evolutionary past.

By contrast, the closely related Japan Sea stickleback (G. nipponicus) has failed to colonise such environments at all. Ishikawa and colleagues wondered why.

They first demonstrated that when faced with a low DHA diet, three-spined stickleback can survive at much higher rates than its relative. This they traced to a metabolic gene called fatty acid desaturase 2 (Fads2) which is crucial to the synthesis of the omega-3.

What they then discovered is that Pacific Ocean populations of three-spined stickleback have multiple copies of Fads2, whereas Japan Sea sticklebacks do not. This seemed to indicate that the higher number of Fads2 was leading to higher levels of DHA manufacture.

To confirm this, the team “made transgenic Japan Sea stickleback overexpressing Fads2” which turned out to have much higher survival rates in low-DHA environments than their non-engineered counterparts.

Taken together, they write, these “data suggest that the lower Fads2 copy number may be a constraint to colonisation of DHA-deficient freshwater niches by Japan Sea stickleback.”

Intriguingly, jumping genes, known more properly as transposons, might be the cause of the repetition of Fads2 in the genome of the three-spined stickleback and thus their ability to adapt to freshwater habitats.

Transposons, say Jesse N. Weber and Wenfei Tong of the University of Alaska, in the US, “are repetitive sequences that can insert themselves, and any DNA in between them, into other parts of the genome”.

In the same issue of Science, the pair write that Ishikawa and colleagues have discovered that “transposons are responsible for the multiple independent duplications of Fads2 in different freshwater stickleback populations”.

Beyond this, the team also identified multiple copies of Fads2 in ray-finned fish (of the class Actinopterygii) that have freshwater populations. This suggests a wider and more important role for Fads2 in the process of evolution. 

This metabolic gene then, might be one of the key elements that has facilitated the adaptive radiation of fish species from marine to freshwater habitats.