Paleoscatology: prying DNA from dated dung.

Taking their clue from confectioners, molecular biologists have discovered a chemical trick for extracting ancient DNA from desiccated dung left by ice-age animals. The new procedure could help in a variety of biological endeavors, from studies of extinct saber-tooth cats to analyses of endangered living populations.

Past attempts to pull DNA from old feces have proved disappointing because investigators did not realize that the DNA was hidden inside caramelized sugar compounds, says Hendrik N. Poinar, a researcher at the Max-Planck-Institute for Evolutionary Anthropology in Munich. Poinar and his colleagues discovered this molecular sugarcoating after they initially failed to find any DNA in a sample of 20,000-year-old dung from an extinct ground sloth.

The researchers suspected that the DNA had degraded while the dung sat through the millennia in Gypsum Cave in Nevada. When they performed chemical tests, however, they found an abundance of long sugar compounds formed by a chemical transformation called the Maillard reaction--the same browning process that gives chocolate, bread crusts, and meat their distinctive, intense tastes. Confectioners have researched the Maillard reaction extensively, as have clinicians working with diabetic patients who have a problem with sugars condensing in their vascular system, says Poinar.

The researchers hypothesized that the long sugar chains had bound up and preserved much of the DNA in the dung. To liberate the genetic information, they added N-phenacylthiazolium bromide, a compound that cleaves the sugar links made by the Maillard reaction. They then isolated fragments of preserved DNA and copied them using the polymerase chain reaction technique, according to their report in the July 17 Science.

"Now we know that the Maillard reaction is beneficial not only to Nestle but also to the fossil researchers," says Poinar.

The DNA recovered confirmed that the dung belonged to the extinct ground sloth, whose bones lie nearby in the cave. More importantly, it also revealed aspects of the sloth's diet. Poinar's group found eight types of plant DNA, the majority of which came from wild capers, mustard plants, and yucca. All these still grow in the vicinity of the cave, about 30 kilometers east of Las Vegas. The researchers also turned up DNA from a wild grape that requires surface springs and doesn't grow near the cave today. During the last ice age, the nearest springs would have been 10 to 20 km away, indicating that the sloth ranged far while foraging for food.

The new technique holds promise for other researchers who study DNA from dung, says Robert K. Wayne, a biologist at the University of California, Los Angeles. In many cases, biologists find it easier to obtain dung than to collect blood samples from wild animals. Endangered animals, he notes, are often hard to track and experience stress when captured. Researchers, however, have had trouble isolating DNA from feces of some species, and the new methods may improve results, Wayne says.

Poinar has tested his technique on dung as ancient as 40,000 years old. The process will only work with unfossilized material that has been preserved by desiccation, he says. Older coprolites, such as the recently discovered tyrannosaur dung (SN: 6/20/98, p. 391), have turned to stone and would not contain any DNA, he says.