Despite a huge evolutionary gulf between humans and the three-spined stickleback fish, both species have a genetic commonness which helps them acquire the skin pigmentation that may enable them to thrive in their new environments.
Howard Hughes Medical Institute investigator David Kingsley corroborates this finding by saying that humans experienced a significant change in the colour of their skin when they might have begun to migrate out of Africa about 100,000 years ago.
Similarly, when the last Ice Age ended about 10,000 years ago, marine ancestors of ocean-dwelling stickleback fish experienced dramatic changes in skin colouring as they colonized newly formed lakes and streams.
The change in skin colour in both species occurred in order that they could adapt to their new environments, and it was facilitated by a gene called Kit ligand (Kitlg), says a report published in the journal Cell.
Kingsley said that giant glaciers melted during when the last Ice Age ended, and created thousands of lakes and streams in North America, Europe, and Asia. He added that the stickleback's marine ancestors that colonized these waters subsequently adapted to life in freshwater, and experienced change in their colour.
"This created a multitude of little evolutionary experiments, in which these isolated populations of fish adapted to the new food sources, predators, water colour, and water temperature that they found in these new environments," Kingsley said.
He said that new colorations helped the fish camouflage themselves, distinguish species, and attract mates in their new environments.
Kinglsey said that humans also underwent pigmentation changes as they had adapted to life in new environments. The ecological reasons for such changes might be quite different from the forces driving the evolution of pigmentation in sticklebacks, he said.
When human populations migrated out of Africa into northern climates, the need for darker pigmentation necessary to protect against the intense tropical sun diminished. The researchers said that the skin that was more transparent to sunlight provided humans with a better ability to produce sufficient vitamin D in their new climate.
With a view to understanding the genetic basis of skin pigmentation changes in fish, the researchers crossed stickleback species that had different pigmentation patterns, used genetic markers, and the recently completed sequence map of the fish's genome to search for the mechanism regulating stickleback pigmentation.
Upon searching for chromosome segments in the offspring that were always associated with inheritance of dark or light gills and skin, the researcehrs found that a gene called Kit ligand (Kitlg) was associated with pigmentation inheritance.
Kingsley said that Kitlg was an excellent candidate for regulating pigmentation because mutant forms of the corresponding gene in mice produce changes in fur colour.
He said that light-coloured fish had regulatory mutations that reduced expression of the Kitlg gene in gills and skin, but that did not reduce the gene's function in other tissues.
"By altering expression of this gene in one particular place in the body, the fish can fine tune the level of expression of that factor in some tissues but not others. That lets evolution produce a big local effect on a trait like color while preserving the other functions of the gene," said Kingsley.
When the researchers tested whether the different human versions of the Kitlg gene were associated with changes in skin colour, they found that people with two copies of the African form of the Kitlg gene had darker skin colour than people with one or two copies of the new Kitlg variant that is common in Europe and Asia.
"Although multiple chromosomal regions contribute to the complex trait of pigmentation in both fish and humans, we have identified one gene that plays a central role in colour changes in both species," said Kingsley.
"Since fish and humans look so different, people are often surprised that common mechanisms may extend across both organisms. But there are real parallels between the evolutionary history of sticklebacks and humans. Sticklebacks migrated out of the ocean into new environments about ten thousand years ago. And they breed about once every one or two years, giving them five thousand to ten thousand generations to adapt to new environments," he added.
The researchers are now exploring the genetic basis of other evolved traits in the stickleback that could find a parallel in humans.
"And given the degree to which evolutionary mechanisms appear to be shared between populations and organisms, we're optimistic about finding the particular genes that underlie other recent adaptations to changing environments in both fish and humans," Kingsley said.