One should not question the origin of a brave man or a good wine, admonishes an old German proverb. But researchers at the University of California, Davis, couldn't resist doing the latter -- using modern DNA fingerprinting techniques to reveal the parentage of the highly esteemed Cabernet Sauvignon wine grape.
Their conclusive discovery that the Cabernet Sauvignon vine is the offspring of the Cabernet franc and the Sauvignon blanc varieties solves a centuries-old mystery that has baffled scientists and wine aficionados alike. The findings are reported in the May issue of the scientific journal Nature Genetics, accompanied by a news and review article by James Luby, a grape geneticist at the University of Minnesota, explaining the historical significance of the research.
"This truly was a serendipitous discovery that resulted from our ongoing efforts to genetically identify some of the commercially important wine grapes in California," explained UC Davis Professor Carole P. Meredith, an international authority on genetic manipulation and analysis of grapevines. Meredith conducted the study with John Bowers, a doctoral candidate in genetics.
Wine enthusiasts use only superlatives to describe Cabernet Sauvignon, a dark cherry-red wine derived from the grapevine of the same name. It has been referred to as the patriarch of the red wines. Now the second most widely cultivated wine grapevine in the world, Cabernet Sauvignon is known to have been growing in vineyards of the famed Bordeaux region of France as early as the 17th century. But where it came from, no one really knew.
Meredith and Bowers have been working for several years on genetically characterizing grapevines. (Establishing wine grape vineyards is a costly undertaking, and vineyard owners can't risk mistakenly planting the wrong variety.)
Grapevine varieties can be characterized according to physical features of their leaves, fruit and tendrils, but these traits can easily be influenced by environmental conditions. Furthermore, grapevines usually are propagated from dormant pieces of stem, which lack the fruit and foliage necessary for visual identification.
During the past decade, a method of genetic identification has been developed that has become known as DNA "fingerprinting." This technique relies on characteristic patterns in a plant's or animal's genetic material, or DNA, much as traditional fingerprinting relies on characteristic patterns in the whorls of the fingerprint. This new technology has proven so accurate that it is now commonly used to establish the lineage of racehorses, to settle paternity suits and even to provide evidence in murder cases.
Meredith and Bowers developed genetic fingerprints for 51 wine grape varieties, including Cabernet Sauvignon. They crushed young grapevine leaves from each of the varieties, extracted DNA from the leaves and examined 30 distinct DNA sites known as "microsatellite" markers that differed from the surrounding DNA in chemical makeup.
They then used statistical analysis to determine the likelihood of the Cabernet Sauvignon microsatellite fingerprint deriving from the fingerprints of any two of the other varieties. These analytical methods conclusively pointed to Cabernet franc, itself a distinguished red wine grapevine, and Sauvignon blanc, which yields a white wine grape, as the genetic parents of Sauvignon Cabernet.
"Although the similarity in names might suggest that Cabernet Sauvignon is connected to these other two varieties, the relationship came as quite a surprise," Meredith said. "A close link between Cabernet Sauvignon and Cabernet franc has been suspected, but no one had any idea that Sauvignon blanc was related."
She noted that the use in both names of the word "sauvignon" (derived from the French word for "wild") reflects the resemblance of the varieties to wild vines rather than any suspicion that a genetic relationship existed.
Meredith and Bowers suspect that Cabernet Sauvignon is the product of a spontaneous crossing between vines in adjacent vineyards. Prior to the 17th century there were no reports of grape breeders deliberately making plant hybrids, and there would have been no practical reason to make such a cross.
"We are delighted that microsatellite analysis has unraveled this mystery in viticulture heritage," Meredith said. "As we continue to apply this technique, we look forward to providing valuable genetic characterization of wine grapevines that will be useful to grape growers and wine makers.
Funding for this study was provided by the Fruit Tree, Nut Tree and Grapevine Improvement Advisory Board of the California Department of Food and Agriculture, and the American Vineyard Foundation.