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Of Sheep and Grapes: DNA Fingerprinting Tracks Ancestry

Since the mid-1980s, variations on the DNA fingerprinting theme have left their marks on diverse fields. By comparing distinctive DNA sequences, aberrations, or numbers of repeats among individuals, investigators have matched suspects to crime scenes and parents to offspring; assembled body parts in the aftermath of disasters; identified kidnap victims, Thomas Jefferson's son, and the remains of Tsar Nicholas' family, who were executed in 1918; and exonerated 64 prisoners, including nine on dea

By | September 27, 1999

Since the mid-1980s, variations on the DNA fingerprinting theme have left their marks on diverse fields. By comparing distinctive DNA sequences, aberrations, or numbers of repeats among individuals, investigators have matched suspects to crime scenes and parents to offspring; assembled body parts in the aftermath of disasters; identified kidnap victims, Thomas Jefferson's son, and the remains of Tsar Nicholas' family, who were executed in 1918; and exonerated 64 prisoners, including nine on death row. Two current applications of DNA sequence comparisons are strikingly similar in their goals--identifying founding parents--but with very different subjects: sheep and grapes.

Gary Sojka calls himself "the oldest postdoc in academia." The 58-year-old former president of Bucknell University and dean of arts and sciences at Indiana University is a microbial biochemist with an impressive publication record. He is presently a professor of biology at Bucknell.

In 1996, Sojka switched gears in a major way--he became a sheep breeder. Now, with his undergraduate assistants, he is using molecular techniques to decipher just what happened to a rare breed from Tunisia that came to the United States in colonial times.

After discussions with breeders, Sojka decided on Tunis sheep because he felt a historical bond to the beasts. "The original broad-tailed sheep first came into the port of Philadelphia from Tunisia," he relates. A judge in the area crossed the sheep with unknown others, and got his friends George Washington and Thomas Jefferson to raise them too. Some colonists took the animals to Arlington, where they were called Arlington sheep. The sheep were bred in the mid-Atlantic states and in the South until the Civil War, when starving confederates ate nearly all of them, but a flock survived in the swamps of the Carolinas. "Someone took them to Indiana, and the flock was reestablished. So I have a direct genetic connection to this particular breed--Philadelphia and Indiana," Sojka adds.

Sojka and his wife, Sandy, who compiles most of the pedigrees and weighs the lambs, started out with five female yearlings: Maude, Ethel, Natalie, Daphne, and Rosie. Then he borrowed "the celebrated late Ivan" from friends in Maryland. Ivan was assigned stud duty, the event scheduled for a time when the resultant lambing would come just after Sojka finished grading his spring semester final exams. (Unlike other sheep, the Tunis variety are nonseasonal breeders--fertile every 17 days.) To monitor exactly who was inseminated, the Sojkas boned up on paraphernalia to track breeding. They outfitted Ivan with a contraption called a "marking harness" that held red crayonlike devices that would supposedly leave a neat imprint on the lucky ladies. Sojka compares the experience of harnessing Ivan to the anticipation of parents decking out their son in a tuxedo for the prom.

Like kids on Christmas Eve, the Sojkas had to force themselves to stay away from the barn on the big night. But the next morning, they were rather shocked to find Ivan stumbling about, caught in his harness. Yet every female was festooned with red markings. "It seems Ivan was not given to monogamy," Sojka reports.

And come springtime, Ethel begat Elvira and Elspeth, and Rosie begat Raquel. Some months later, after subsequent breeding, Raquel produced Rosalind, and then the fecund Rosie added twins Rodney and Ricardo. The flock continued to grow by leaps and bounds, despite Ivan's unfortunate demise at the hooves of a bully ram when he returned home.

Sojka's sheep project isn't just a hobby, though. He's intrigued by the sharp population bottleneck that occurred between 1860 and 1975. He and undergrads Michael Wolujewicz and Michael Hemphill are using genetic markers (as opposed to the kind on the mating harness) to compare the Tunis sheep to other breeds, to ultimately identify the parental line bred to the original immigrants from Tunisia. They are using the "variable number of tandem repeat" (VNTR) approach. So far, the team has perfected drawing blood ("without banging the sheep up too much") and DNA extraction, and they are working on amplifying particular genes. Meanwhile, classic pedigrees are used to track who mated with whom.

The Tunis sheep provide a perfect adjunct to Sojka's course at Bucknell. "I teach a course in domestic plants and animals. Our students are urban kids who think food grows on trees in plastic bags. This course changes their attitudes," he says.

If disaster comes with the millennium, survivors can salvage several popular wines by saving just two parental types, thanks to the work of researchers in the United States and France. "We now know that you can conserve the entire gene pool of 16 classic varieties just by keeping the Pinot and Gouais blanc varieties," says Carole Meredith, a professor of viticulture and enology at the University of California, Davis.1

Meredith, long interested in identifying the family trees of California wines, began her work in the UC-Davis vineyards. Two years ago she expanded the scope to the French wines, collaborating with Jean-Michel Boursiquot from the Ecole Nationale Superieure Agronomique and Patrice This of the Institut National de la Recherche Agronomique, both in Montpellier, France. After scouring historical French literature and current knowledge of French wines, the team narrowed down the 2,000 varieties in the Montpellier collection to a group of 300 within which they would investigate ancestry.

Photo: Carole Meredith, University of California

Gary A Sojka, microbiologist and former Bucknell president, trims the hooves of one of his Tunis ewes.
So UC-Davis doctoral candidate John Bowers sampled leaves and extracted the DNA in France, then he returned to Meredith's lab to start the DNA fingerprinting. An initial check of 17 microsatellite sequences indicated 60 grape variants that seemed to have genetic connections to each other. Probing at 17 other sites whittled the search to 16 that were closely related--all were the direct offspring of the familiar bluish-purple Pinot grape, the basis of burgundy wine. Those 16 varieties account for nearly all of the varieties grown in northeastern France today, including Chardonnay. But the second parent was a surprise: Gouais blanc, a variety so unpopular as a grape wine that it was banned in the Middle Ages, and in modern times it has not been cultivated in either France or the United States. "We are more than 99.99 percent sure that Pinot and Gouais blanc are the original parents for these 16 varieties," says Meredith. The 16 grape variants, like all siblings, share 50 percent of their genes.

DNA fingerprinting in viticulture, as the technique does in other fields such as forensics, adds precision. Past growers distinguished grape varieties by easily seen physical features, such as leaf shape and fruit color, and ecology. "Pinot and Chardonnay have been grown in the same part of France for a very long time, and their leaves have some similarities. So if you had to guess at some relatives, you might have put these two together. When we did discover that Pinot and Chardonnay were related, it did not come as a big surprise," explains Meredith.

But meiosis has a way of mixing up traits so that the characteristics of offspring are not always what was expected, as most parents know from experience. So it was for the revelation that Gouais blanc lies behind several popular wines. The poor parent yields abundant white grapes that aren't terribly tasty, whereas the Pinot parent produces sparse fruits, the slim pickings a sign of a good wine grape.

Now that the team has uncovered the genetic relationships, they are forging ahead to investigate specific traits and the DNA sequences that specify them. By combining mutational analysis and historical records of when each variety is first mentioned, they may be able to place a time frame on the genetic events that underlie the development of the wines.

Meanwhile, the work has practical benefits. "We now know that you'd never want to cross Chardonnay and Pinot, because you would be more likely to get genetically weak progeny, similar to the inbreeding problems that occur when closely related people marry," Meredith explains. More importantly, the work shows that an unlikely parent can produce spectacular offspring.

If this recent genetic analysis of Tunis sheep and wine grape varieties is any indication, the public need not fear cloning after all. For variety--in addition to being the spice of life--has always been the basis of evolution.

Ricki Lewis (rickilewis@nasw.org) is a contributing editor for The Scientist.

  • J. Bowers et al., "Historical genetics: the parentage of Chardonnay, gamay and other wine grapes of northeastern France," Science, 285:1562-5, Sept. 3, 1999.

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