Cultural habits can have a distinct effect on a population's genetic diversity, according to a new study among minority peoples in the mountains of Thailand. Whether men usually move in with their wives' families after marrying or vice versa influences the amount of genetic variation in some of the DNA inherited from each parent.
In most places, sons inherit a Y chromosome from their fathers that tends to be fairly similar to other Y chromosomes in that population. Much more diverse are the DNA in mitochondria, cellular energy plants that children inherit only from their mother. One explanation is that in most cultures, women tend to move from their own population to their mates' after marriage, thus constantly refreshing the mitochondrial gene pool, while men tend to stay put in their birthplace. But the same pattern could emerge if only few males in a population reproduced, reducing the number of Y chromosomes that get passed on. Until now, though, no one had data supporting either explanation.
It so happens that the hill tribes of northern Thailand include several "patrilocal" groups, in which the women move to their husbands' birthplace, as well as "matrilocal" ones, in which men leave home to join their wives. All the groups have similar agricultural lifestyles, live in similar climates, and speak related languages. Molecular anthropologist Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, decided to take advantage of this arrangement to address the question of genetic diversity.
Stoneking's team analyzed DNA from men and women of three matrilocal and three patrilocal tribes, focusing on certain highly variable DNA sequences. In the September issue of Nature Genetics, they report that in patrilocal groups, diversity within populations was greater for mitochondrial DNA than for the Y chromosome, and the differences between groups were bigger for the Y chromosome--the situation seen almost universally. But in matrilocal groups, both patterns were completely reversed. "This is exactly what you would expect if patrilocality is responsible for the normal pattern," says Stoneking. "This is the first real test of that hypothesis, and so far it holds it up."
Still, the study has its limits, cautions geneticist David Goldstein of University College London. "It's hard to extrapolate very far from this, because it's a small, localized study," he says. "But it makes an interesting broader point that we may see culture having effects on patterns of genetic variation," he adds. "That may be emerging as a rule."