Pop's Surprising Influence on Mom's DNA

23 December 1999 7:00 pm

Women have struggled to gain equality in society, but biologists have long thought that females wield absolute power in a sphere far from the public eye: in the mitochondria, cellular organelles whose DNA is thought to pass intact from mother to child with no paternal influence. A study in tomorrow's Science, however, finds signs of mixing between maternal and paternal mitochondrial DNA (mtDNA) in humans and chimpanzees. Because biologists have used mtDNA as a tool to trace human ancestry and relationships, the finding has implications for everything from the identification of bodies to the existence of a "mitochondrial Eve" 200,000 years ago.

Researchers have assumed that mtDNA passes only through the mother, in part because experiments have shown that eggs destroy sperm after fertilization, and that mitochondrial traits, including a variety of inherited disorders, seem to come only from mothers. But some mtDNA sequences didn't fit neatly into a tree of maternal descent (Science, 5 March, p. 1435), so Philip Awadalla of the University of Edinburgh and Adam Eyre-Walker and John Maynard Smith of the University of Sussex in Brighton decided to look for signs of mixing between paternal and maternal mtDNA.

Such mixing, which commonly happens to the chromosomal DNA during germ cell formation, is called recombination. It takes place when a piece of DNA inherited from one parent crosses over and pairs up with a strand inherited from the other. This generates a novel DNA molecule combining features donated from both parents. To probe whether recombination occurs in the mitochondrial genome, the researchers analyzed DNA variations. In four out of five human data sets and one chimp set, nonrandom mutations at distant sites were less likely to be linked than nearby mutations--implying recombination between maternal and paternal DNA, says Eyre-Walker.

Many researchers aren't ready to accept these data as ironclad evidence of recombination for mtDNA. Still, the study "is pretty compelling and I can't think of good alternative explanations," says Richard Hudson, a population geneticist at the University of Chicago. Anthropologists agree that if the study holds up, it could trigger a major shake-up in their field. "There is a cottage industry of making [mitochondrial] gene trees in anthropology, and then interpreting them," says Henry Harpending, an anthropologist at the University of Utah in Salt Lake City. "This paper will invalidate most of that."

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