May 4, 2012
Blondeness In Solomon Islanders Due To Genetic Variations
Excess sun exposure, a diet rich in fish, and gene inheritance from ancient explorers and traders, are all possible theories why some dark-skinned indigenous Solomon Islanders are naturally blonde, according to new research published today in the journal Science.
The study, led by Stanford University researchers, found that 5 to 10 percent of the indigenous Solomon population have a gene that is responsible for blondeness. The trait, however, is distinctly different from the gene that causes blond hair in Europeans. Their findings reveal a genetic variant which has led the islanders to have simultaneously the darkest skin pigmentation outside of Africa and the highest prevalence of blonde hair outside of Europe.
Previous studies have proven that pigmentation is largely genetic but also has evolved to adapt to the Sun´s ultraviolet rays -- with populations near the equator having the darkest skin and hair color. However, the native Solomon Islanders differ from this trend.
“This is one of the most beautiful examples to date of the mapping of a simple genetic trait in humans,” David Reich, PhD, a professor of genetics at Harvard University, who was not involved in the study, said in a Stanford press release.
The research, co-led by researchers at Stanford University and Dr. Nic Timpson from the University of Bristol, sought out to find why these islanders possess such strikingly dissimilar hair and skin patterning in the world.
For the study, the team took samples from a pool of more than 1,000 Melanesian participants, 43 of which had blonde hair and 42 of which had dark hair. They carried out genetic analyses on the samples to compare their genomes. The results showed that across the whole genome, one key gene area contained the gene variation -- TYRP1 -- responsible for cell differences that produce dark pigmentation.
TYRP1 is known to influence pigmentation in humans. But the researchers found the variant of TYRP1 that causes the blonde hair in Solomon Islanders is entirely absent from the genomes in Europeans.
“Here you go into an unstudied population with a small sample size and you can really find some cool things,” said study coauthor Carlos Bustamante, a geneticist at Stanford University´s School of Medicine. “So what about other places, like what about light pigmentation in parts of Africa? How do we not know the genetic basis of skin and hair pigmentations across the globe?”
“Naturally blonde hair is a surprisingly unusual trait in humans which is typically associated with people from Scandinavian and Northern European countries,” said Timpson. “Our findings help explain the fascinating differences in these physical characteristics, but also underline the importance of genetic mapping using isolated populations to help shed new light on the epidemiology of disease.”
Many experts believed the blonde hair of Melanesia was the result of a trait passed on by Europeans who visited the islands centuries ago.
The islanders themselves also have several possible explanations for the presence of blonde hair, said coauthor Sean Myles, PhD, a former Stanford postdoctoral scholar who is now an assistant professor at the Nova Scotia Agricultural College. The general consensus among islanders is the color comes from sun exposure or a fish-rich diet.
Myles, who spent time on the islands in 2004 during a study to investigate whether language variations correlated with genetic variations, said he was fascinated by the omnipresence of blonde hair, which was especially common among children.
“They have this very dark skin and bright blond hair. It was mind-blowing,” he said. “As a geneticist on the beach watching the kids playing, you count up the frequency of kids with blond hair, and say, 'Wow, it's 5 to 10 percent.”
A grant from the Wenner-Gren Foundation for Anthropological Research gave Myles his chance to study the hair genetics of the Solomon Islanders. Myles worked with Bustamante, of Cornell University at the time, to design the study. The back in the islands, Myles and Timpson went to villages explaining their study and asking for permission to gather data.
Once the OK was given, by a local tribal chief, the researchers began their experiments. They assessed hair and skin color, took blood pressure readings, and measured heights and weights. They also took saliva samples for DNA testing. In all, they got 1,000 samples.
In 2010, when Bustamante joined Stanford´s faculty, and with funding from the Department of Genetics, the team conducted their analyses of the samples, and ultimately discovered the gene variation that led them to their conclusions.
“Whether this genetic variation is due to evolution or a recent introgression requires further investigation, but this variant explains over 45 percent of the variance in hair color in the Solomons,” said Timpson.
The finding highlights the importance of genetic studies on isolated populations, said Bustamante. “If we´re going to be designing the next generation of medical treatments using genetic information and we don´t have a really broad spectrum of populations included, you could disproportionately benefit some populations and harm others.”
The research team is hoping to raise more money to further investigate the Solomon Islands data.
“For instance, the genetics of skin pigmentation might be different there too – not the same as in Europe or Africa or India. We just don't know,” said Bustamante.