The three-spined stickleback has found success in a number of ecosystems across North America, Europe, and Asia in salt and freshwater alike. Yet the three-spined stickleback’s close relative, the Japan Sea stickleback, cannot claim the same success.
Three-spine Stickleback (Gasterosteus aculeatus), found in marine and freshwater environments in North America, Europe, and Asia.
Ryan Hagerty, US Fish and Wildlife
Found only in saltwater environments along the coast of Japan and the Korean Peninsula, less the occasional freshwater spawning adventure inland, the Japan Sea stickleback’s lack of success in freshwater ecosystems has been a scientific mystery – until now.
According to recent work from Japan’s National Institute of Genetics and the Graduate University of Advanced Studies, stickleback ecosystem expansion may be directly related to what the fish can – or cannot – eat.
Led by Dr. Asano Ishikawa, researchers first suspected a food connection to the Japan Sea stickleback’s limits due to the surprisingly different food options available in salt and freshwater habitats.
“One of the under-appreciated constraints for freshwater colonization by marine animals is the poor nutritional quality of food in freshwater ecosystems,” said Jun Kitano, a co-author of this study and professor at the National Institute of Genetics. “Generally, the food chain in marine environments is rich in the omega-3 fatty acid DHA, which is essential for animal development and health. However, freshwater ecosystems contain very little DHA.”
Examples of foods high in omega-3 fatty acids, including salmon and avocados.
Getty
Indeed, the fatty acid DHA is also one of the omega-3’s we seek in our diets.
The scarcity of fatty acids in freshwater may explain why the Japan Sea stickleback has been unable to colonize freshwater like their three-spined stickleback relatives.
After digging around the three-spined stickleback’s genome, researchers discovered extra copies of a gene that makes the omega-3 fatty acid, DHA. The Japan Sea stickleback, meanwhile, only has one copy of this gene. More gene copies, especially if the ‘extra’ genes are in a different location in the genome altogether, could enhance the fish’s fatty acid-producing abilities.
The three-spine’s extra copies likely emerged thanks to special ‘jumping genes’ known as transposons. Transposons randomly copy-and-paste themselves and nearby genes, such as the gene for fatty-acid breakdown, to new locations in the genome.
At one point, the three-spined stickleback’s fatty acid-eating gene was not just copied by transposons, but added to an entirely new chromosome.
Cartoon of a transposon, or ‘jumping gene’, which likely caused the gene duplication in the three-spined stickleback.
Jim Stallard, Memorial Sloan Kettering Cancer Center
“It’s unclear when the genetic advantage appeared,” said Kitano. “It may be that ancient extinct freshwater species possessed additional [gene] copies somewhere in the genome or adapted to DHA-deprived diets through other mutations.”
The extra gene copy likely increased the fish’s ability to produce DHA from other fatty acid building blocks, perhaps making the three-spined stickleback less reliant on the consumption of DHA from the sea and opening up the world of omega-3-lacking freshwater habitats.
To see test whether the same benefits could created in the Japan Sea stickleback, researchers over-expressed the fish’s DHA-producing gene, essentially mimicking the effect of having an extra gene copy like that of the fish’s three-spined relative.
The extra proteins did, in fact, boost the Japan Sea stickleback’s production of DHA and survival in fatty-acid limited freshwater environments, closely matching the survivability of the three-spined relative.
So, for the three-spined stickleback, a cosmopolitan marine and freshwater life may have been made possible by a random copy-and-pasting of a gene which enhanced the fish’s ability to produce it’s own omega-3’s, while leaving the Japan Sea stickleback stuck in the sea.