Go West, Young Woman

2019-03-08 05:08:09

By Nell Boyce in Washington DC FROM Marco Polo to Alexander the Great and Genghis Khan, our image of explorers has traditionally been one of brave men striking out into the wilderness. But a new study of DNA comes to the surprising conclusion that women have historically travelled farther and more frequently than men, and may be more responsible for spreading genes around the globe. Researchers can study the migration of past generations by comparing DNA from people in different countries, taking natural mutation rates into account. To simplify matters, many geneticists have concentrated on mitochondrial DNA (mDNA), which is only passed down from the mother. But over the past few years they have also started to look at the Y chromosome, which only comes from the father (“All About Adam”, 11 July, p 34). Mark Seielstad of the Harvard School of Public Health in Boston, along with Eric Minch and Luca Cavalli-Sforza at Stanford University in California, decided to compare the worldwide diversity of the Y chromosome with that of mDNA to see whether different patterns of migration emerged for men and women. They took previously published information on variations in mDNA within populations and between continents, and compared it with Y chromosome data from men in 54 populations from Africa, Oceania, Asia, Europe and the Americas. For mDNA, there was much more similarity between populations than for Y chromosomes, suggesting that women had a much higher migration rate than men. Calculations revealed that the migration rate for women was eight times greater than for men, they say in next month’s Nature Genetics (vol 20, p 278). The reason for the large difference is unclear. Seielstad believes that marriage traditions may have played a role—men have often travelled to find wives and returned home with them. “It sounds like a sort of feminist take on things, that women migrate more, but the irony is that it’s only because the patriarchal system forced them to move,” he says. “It’s intriguing that you see this difference on the large scale,” comments Mark Stoneking, a molecular anthropologist at Pennsylvania State University in University Park. “It points to a very interesting area for future research.” He suggests that computer models might be able to pinpoint the extent to which marriage traditions could explain the global genetic similarity in mDNA. Another possible explanation for the results is polygamy. Polygamous cultures would naturally have less variation in Y chromosomes than other societies. But even in populations where polygamy is considered ideal, few men have the resources to support many wives, so Seielstad doubts whether polygamy could account for the entire eight-fold difference. Steven Strogatz of Cornell University in Ithaca, New York, points out that not many women would need to travel long distances for this effect to occur. He has shown that only a few connections between far-flung groups of people can result in apparently unrelated people having friends in common—the so-called “small-world effect” (This Week, 6 June, p 7). “It probably requires only a tiny minority of women making enormous journeys to globalise their mDNA—that’s the essence of the small-world effect,” says Strogatz. “But surely a few men have made some long travels too,