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Lack of sex bad for grapes' health

For the last 8,000 years, the wine grape has had very little sex. This unnatural abstinence threatens to sap the grape's genetic health and the future pleasure of millions of oenophiles.

The lack of sex has been discovered by Sean Myles, a geneticist at Cornell University. He developed a gene chip that tests for the genetic variation commonly found in grapes. He then scanned the genomes of the thousand or so grape varieties in the Department of Agriculture's extensive collection.

Much to his surprise he found that 75 percent of the varieties were as closely related as parent and child or brother and sister. "Previously people thought there were several different families of grape," Myles said. "Now we've found that all those families are interconnected and in essence there's just one large family."

Thus merlot is intimately related to cabernet franc, which is a parent of cabernet sauvignon, whose other parent is sauvignon blanc, the daughter of traminer, which is also a progenitor of pinot noir, a parent of chardonnay.

This web of interrelatedness is evidence that the grape has undergone very little breeding since it was first domesticated, Myles and his co-authors report in the Proceedings of the National Academy of Sciences.

The reason is obvious in retrospect. Vines can be propagated by breaking off a shoot and sticking it in the ground, or onto existing rootstock. The method gives uniform crops, and most growers have evidently used it for thousands of years.

The result is that cultivated grapes remain closely related to wild grapes, apart from a few improvements in berry size and sugar content, and a bunch of new colors favored by plant breeders.

Cultivated grapes have almost as much genetic diversity as wild grapes. But because there has been very little sexual reproduction over the last eight millenniums, this diversity has not been shuffled nearly enough. The purpose of sex, though this is perhaps not widely appreciated, is recombination, the creation of novel genomes by taking some components from the father's and some from the mother's DNA. The new combinations of genes provide variation for evolution to work on, and in particular they let slow-growing things like plants and animals keep one step ahead of the microbes that prey on them.

The grapevine fell extinct through much of Europe in the phylloxera epidemic of the 19th century. The French wine industry recovered from this disaster only by grafting French scions, as the grape's shoots are called, onto sturdy American rootstock resistant to the phylloxera aphid.

Despite that close call, grape growers did not rush to breed disease resistance into their vines. One obstacle is that wine drinkers are attached to particular varieties, and if you cross a chardonnay grape with some other variety, it cannot be called chardonnay. In many wine-growing regions there are regulations that let only a specific variety be grown, lest the quality of the region's wine be degraded. More than 90 percent of French vineyards are now planted with clones — genetically identical plants — certified to possess the standard qualities of the variety.


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