Freshwater find: Genetic advantage allows some marine fish to colonize freshwater habitats

Freshwater find: Genetic advantage allows some marine fish to colonize freshwater habitats
A Japan Sea male stickleback. They cannot colonize freshwater because they have a poor ability to synthesize DHA. Credit: Yasuyuki Hata

How did some marine fish manage to make their way from the salty sea to the newly available freshwater niches after the last ice age and eventually differentiate from their marine brethren?

It comes down to genetics and diet, according to a team of scientists at the Research Organization of Information and Systems (ROIS) in Japan. The study is published on May 30 in the journal Science.

"One of the underappreciated constraints for freshwater colonization by is the poor nutritional quality of food in ," said Jun Kitano, the study's author and professor in the Ecological Genetics Laboratory at the National Institute of Genetics, part of ROIS, "Generally, the food chain in is rich in the omega-3 fatty acid DHA, which is essential for animal development and health. However, freshwater ecosystems contain very little DHA."

Kitano, along with lead author Asano Ishikawa, and his team compared the genomes of freshwater stickleback to marine stickleback, a small with three spines on the back.

They found that freshwater three-spined stickleback has multiple copies of a gene called Fads2, which helps synthesize DHA. With more copies of the gene, the fish can synthesize more DHA, compared to their marine cousins who have an abundance of DHA available in their diet.

Freshwater find: Genetic advantage allows some marine fish to colonize freshwater habitats
A freshwater three-spined stickeback from Gifu, Japan. They have higher DHA synthetic ability than the Japan Sea stickleback. Credit: Yasuyuki Hata

To further test this theory, the researchers developed a genetic mutant of the marine stickleback that over-expresses Fads2. It synthesized more DHA and demonstrated a longer lifespan than control stickleback, providing further evidence that the genetic ability to make use of available DHA dictates where fish colonize.

"It's unclear when the genetic advantage appeared," Kitano said. "It may be that ancient extinct freshwater species possessed additional Fads2 copies somewhere in the genome or adapted to DHA-deprived diets through other mutations."

According to Kitano, it could also be that multiple copies of the gene allowed some versions to acquire new functions, such as a way to better synthesize DHA.

++**Freshwater find: Genetic advantage allows some marine fish to colonize freshwater habitats
Marine (right) and freshwater sticklebacks (left). Credit: Asano Ishikawa and Jun Kitano

The Fads2 gene may allow certain selections in species beyond the stickleback, as it also appears at higher rates in fishes that colonized freshwater. A previous study also showed signatures of natural selection in humans that colonize polar regions.

"It certainly appears to be an important gene beyond fish, in a wide variety of organisms, including humans," Kitano said.


Explore further

How one gene in a tiny fish may alter an aquatic ecosystem

More information: A. Ishikawa el al., "A key metabolic gene for recurrent freshwater colonization and radiation in fishes," Science (2019). science.sciencemag.org/cgi/doi … 1126/science.aau5656

J.N. Weber el al., "Jumping gene gave fish a freshwater start," Science (2019). science.sciencemag.org/cgi/doi … 1126/science.aax7936

Journal information: Science

Provided by Research Organization of Information and Systems
Citation: Freshwater find: Genetic advantage allows some marine fish to colonize freshwater habitats (2019, May 30) retrieved 27 June 2019 from https://phys.org/news/2019-05-freshwater-genetic-advantage-marine-fish.html
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