Ancient magnolia DNA reveals plant's past.

Ancient magnolia DNA reveals plant's past

The climate in northern Idaho was generally pleasant in those days. So maybe it was just an autumn breeze that freed this particular magnolia leaf to carve its final path through the air, land on a lake surface, then sink about 25 feet to the cold, mucky bottom.

Compressed under the weight of accumulating silt and sequestered from the degrading forces of oxygen and heat, the leaf "just sat there," Charles Similey says. It sat for almost 20 million years -- long after the lake had disappeared -- until the University of Idaho paleobotanist and his colleagues peeled back the remaining layers of water-saturated shale, exposing the still-green, unmineralized fossil to the warm rays of a forgotten sun.

The researchers photographed their remarkable find, quickly ground it to a fine powder with some dry ice prepared for just such an occasion, then subjected the sample to sophisticated genetic testing. When it was over, they had decoded an 820-piece DNA sequence from the leaf's photosynthetic organelle, the chloroplast -- by far the olderst bit of genetic material ever analyzed, dating back to the Miocene epoch.

Their successful dissection of such ancient DNA (the previous record was from a 13,000-year-old ground sloth) promises a wealth of information, these and other researchers say. Sequence analysis of DNA from long-extinct organisms allows scientists to gauge mutation rates over millions of years. These values, which taxonomists otherwise must infer from modern specimens, are critical to understanding evolutionary trends and to determining the degree of relatedness among modern plants and animals.

In their study of the Miocene magnolia, the researchers confirmed the notion that the common photosynthetic molecule known as rubisco has changed very little over millions of years. Of the 820 DNA subunits, or base pairs, tested with the polymerase chain reaction, only 12 had mutated. That finding led the team to modify the family tree connecting modern magnolias and the tulip poplar tree. The results of their work, led by Edward M. Golenberg of the University of California, Riverside, appear in the April 12 NATURE.

Few spots in the world have experienced the right combination of conditions needed to preserve such ancient specimens, Smiley says. But the Idaho site holds remains of about 100 other plant and animal species with modern relatives, and analyses of their DNA may well provide surprises. "It's a fantastically important site" with the potential to provide "one blockbuster finding after another," he says.