Lack of Sex Among Grapes Tangles a Family Vine
By NICHOLAS WADE
Published: January 24, 2011
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.
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Get Science News From The New York Times »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,” Dr. 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, Dr. 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.
The consequence of this genetic conservatism is that a host of pests have caught up with the grape, obliging growers to protect their vines with a deluge of insecticides, fungicides and other powerful chemicals.
This situation cannot be sustained indefinitely, in Dr. Myles’s view. “Someday, regulatory agencies are going to say ‘No more,’ ” he said. “Europeans are gearing up for the day, which will come earlier there than in the U.S., for laws that reduce the amount of spray you can put on grapes.”
At that point growers will have three options. One is to add genes for pest resistance, risking consumer resistance to genetically modified crops. A second is to go organic, which may be difficult for a plant as vulnerable as the grape. A third is to breed sturdier varieties.
Breeding new grapes takes time and money. The grower has to plant a thousand seedlings, wait three years for them to mature, and then select the few progeny that have the desired traits. But a new kind of plant breeding now offers hopes of an efficient shortcut.
The new method depends on gene chips, like the one developed by Dr. Myles, that test young plants for the desired combination of traits. The breeder can thus discard 90 percent of seedlings from a cross, without waiting three years while they grow to maturity.
The new method, called marker-assisted breeding, or genomic selection, is already being used in breeding corn. “We can predict flowering within a couple of days by looking at the DNA,” said Edward S. Buckler, a leading corn geneticist at the Agriculture Department’s research lab at Cornell.
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Dr. Buckler said he felt the government’s large collections of crop plants could be used much more efficiently by analyzing the genomes of each species. He recruited Dr. Myles to work on the grape genome.
In major crops like corn, rice and wheat, “everyone is shifting to these new technologies,” Dr. Buckler said. He expects grape growers to follow the trend. Wine drinkers’ insistence on their favorite varieties need not necessarily be a problem, because with enough genetic markers the breeder could identify and maintain the genes responsible for the taste of varieties like chardonnay or merlot. Genomically selected grape varieties may be ready for market in about a decade, said Dr. Buckler, who is a co-author on Dr. Myles’s report.
M. Andrew Walker, an expert grape breeder at the University of California, Davis, said that there are “ample pest- and disease-resistance genes” in the grapevine genus, which has about 60 species, but few in Vitis vinifera, the particular species to which wine and table grapes belong. He agreed that it will be necessary to introduce many of these genes from other Vitis species into vinifera. “Consumers and wine promoters will have to move beyond dependence on traditional vinifera varieties,” Dr. Walker said.
So far Dr. Myles has only 6,000 useful genetic markers on his grape gene chip, and needs a larger chip to identify all the traits of interest to breeders. He started his scientific career working on human genetics at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. On a bicycle tour of German vineyards he decided the grape’s genome might hold as many surprises as the human one. The pursuit fit in well with another aspect of his life — his wife is a winemaker in Nova Scotia.
Canada might seem too far north for vineyards to thrive, but the growing season is like that of Champagne in France, Dr. Myles said. “For high-acid grapes that don’t fully ripen, which is the Champagne strategy, you can make fantastic sparkling wines in Nova Scotia and lots of good whites.”
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