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Jumping genes allow fish to move from the sea 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 as much as we do.

Now research published in the magazine. Science It shows how useful they can be: the "jumping" genes badociated with omega-3s can be really key to the evolutionary spread and diversification of fish species.

Jumping genes are DNA sequences that can be copied and pasted into different parts of a genome, and can be a prime mover 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 it is scarce in freshwater environments. In salt water, several algae that are part of the diet of the fish produce DHA, but they can also be manufactured in small quantities by the fish.

Given how necessary DHA is, how do marine fish species colonize freshwater ecosystems, something that has happened repeatedly throughout evolutionary history?

The 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 observed that the marine fish known as the spiny thorn (Gasterosteus aculeatus) had successfully colonized freshwater habitats many times and on many continents throughout their evolutionary past.

On the contrary, the narrowness of the sea of ​​Japan closely related (G. nipponicus) has not been able to colonize such environments at all. Ishikawa and his colleagues wondered why.

First, they showed that when faced with a diet low in DHA, the thorny spine can survive at much higher rates than its relative. This can be traced back to a metabolic gene called fatty acid desaturase 2 (Fads2) which is crucial for the synthesis of omega-3.

What they later discovered is that the populations of the Pacific Ocean of thorny three-spined have multiple copies of Fads2, while the spiny ones from the Sea of ​​Japan do not. This seemed to indicate that the largest number of Fads2 was leading to higher levels of DHA manufacturing.

To confirm this, the team "made an overexpression of the transgenic mariner from the Sea of ​​Japan Fads2"That turned out to have much higher survival rates in low DHA environments than its non-designed counterparts.

Taken together, they write, these "data suggest that the low Fads2 "The number of copies can be a restriction for the colonization of freshwater niches deficient in DHA by the hawthorn of the Sea of ​​Japan".

Interestingly, jumping genes, better known as transposons, may be the cause of the repetition of Fads2 in the spiny genus of three spines and therefore its ability to adapt to freshwater habitats.

The transposons, say Jesse N. Weber and Wenfei Tong of the University of Alaska, in the USA. UU., "They are repetitive sequences that can be inserted, and any DNA between them, in other parts of the genome".

In the same number of Science, the couple writes that Ishikawa and his colleagues have discovered that "transposons are responsible for multiple independent duplications of Fads2 In different populations of freshwater spinners ".

Beyond this, the team also identified multiple copies of Fads2 in fish with radiated fins (of the Actinopterygii clbad) that have freshwater populations. This suggests a broader and more important role for Fads2 In the process of evolution.

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

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