Tuesday, January 25, 2011

Of Sex and Gropes and Tango Wrath

collage by marguerita

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.

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 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.


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