Chardonnay, Zinfandel, or Rosé? Such variety in fine wines would be unthinkable without red and white grapes. New research shows that the evolution of grape color comes down to chance mutations that first turned black grapes white, then white grapes red.
Grape skins get their color from a buildup of a red pigment called anthocyanin. Previously, Shozo Kobayashi of the National Institute of Fruit Tree Science in Tsukuba, Japan, identified a specific gene that controls the production of this pigment in black-skinned grapes. Now, in the 14 May issue of Science, Kobayashi and his colleagues report that the same gene has been mutated in white grapes by a bit of pushy DNA, resulting in loss of skin color.
Using a slew of molecular techniques, the team identified the color-stealing culprit in white grapes as a mobile fragment of DNA, called a retrotransposon, which inserts itself into the gene controlling pigment production. The white grapes they studied (Vitis vinifera) had this piece of DNA stuck into both copies of the gene, leaving the grapes no way of making color. In two red-skinned varieties, however, they found only one version of the mutated gene; the other version worked just fine.
The results lead the researchers to hypothesize that the hopscotching piece of DNA first turned off the pigment pathway in a black-skinned ancestor, turning it red. This variety probably crossed with itself to produce an entirely white-skinned grape. Kobayashi says the mutant gene is present in a wide variety of white grapes today, suggesting that they all share the same heritage.
The researchers also found bits of the retrotransposon in red grapes. That suggests that a second mutation cut the insert out and restored coloration. If this is true, then the red grapes examined in the study are actually derived from white ancestors, as opposed to the other way around, as most researchers had thought.
"The striking thing is that different whites may have all derived from one vine, the granddaddy of all the whites we know," says Judith Strommer, a molecular biologist at the University of Guelph in Ontario, Canada. She'd like to see more molecular evidence from white grapes worldwide, showing the same mutation in the same gene, before she's completely convinced.