In laboratory experiments in which fry were raised on freshwater food, three-spined sea sticklebacks grew properly, whereas Japanese sea sticklebacks starved. With the help of genetic analysis, the researchers discovered that the Japanese sea stickleback and also the slightly more distant relative, the blackspotted stickleback, both had just one copy of the gene, whereas three-spined sticklebacks had more. When the researchers inserted additional copies of Fads2 into the genetic material of the Japanese sea stickleback, the offspring of these fish survived on freshwater food.
“What my Japanese colleague Asano Ishikawa made to happen in the laboratory, happened to the three-spined stickleback in nature only by pure coincidence”, says Ole Seehausen, a biologist at Eawag and the University of Bern. The gene was duplicated in the genome by chance a long time ago, which then allowed the three-spined stickleback to later colonise freshwater bodies.
As time went on and the fish adapted to life in freshwater lakes and rivers, even additional copies were made, and so the oldest freshwater populations of three-spined sticklebacks that were formed long before the last ice age have been found to possess significantly more Fads2 copies than the sea populations and the many younger freshwater populations that emerged after the last ice age.
Genetic mutations are usually disadvantageous. According to the researchers it is very rarely the case, particularly with duplications, that they lead to something advantageous. For example, a gene can be copied incompletely or reintroduced into the wrong place. It is therefore not surprising that relatives of the three-spined stickleback such as the Japanese stickleback have only one copy of the Fads2 gene and therefore remained sea-bound.
The study was the result of a collaboration between three research departments: with scientists from Eawag and the University of Bern, and Japanese researchers at the National Institute of Genetics in Shizuoka under Dr Asano Ishikawa und Dr Jun Kitano.
A key metabolic gene for recurrent freshwater colonization and radiation in fishes.