What Makes a White Grape White? Scientists Think They've Found the Color Key

Scientists have long observed that, in the wild, all grapes are dark-skinned. Few and far between in nature, white grapes represent a genetic mutation of grapes' typical color, but no one understood just how they developed.

Now a team of Japanese researchers, led by Shozo Kobayashi from Japan's National Institute of Fruit Tree Science in Tsukuba, has discovered a gene mutation that may influence grape color.

The so-called "black" or "red" grapes owe their color to the presence of a group of plant pigments called anthocyanins found in their skins. A specific gene in the grape controls the synthesis of these pigments.

In an article in the May 14 issue of Science magazine, Japanese researchers announced their discovery that a sort of renegade sequence of DNA, called a retrotransposon, is responsible for suppressing the production of anthocyanins in grapes. According to Kobayashi's research, the retrotransposon inserts itself into the gene responsible for pigment synthesis.

If one of the pair of genes responsible for color is affected, the grape color will be "red" instead of the darker "black." But if both genes are affected by the mutation, the grape will be "white" in color. Kobayashi wrote in the study that the mutant gene identified by his group is present in "most, if not all, white grape cultivars in the world."

Kobayashi theorized that the mutation first occurred spontaneously, before vines were cultivated, in a dark-skinned grape variety along the eastern shores of the Black Sea, and that a spontaneous crossing of two vines with the mutation produced a white-skinned grape.

Renowned grapevine geneticist Carole Meredith, professor emeritus at University of California, Davis, said that, although she thinks Kobayashi's work is fascinating and well done, she disagrees with his theory that the mutation is responsible for most or all white grape varieties.

"I don't think all white cultivars in the world have descended from a single common progenitor cultivar," Meredith said. "It's much more likely that white cultivars have arisen independently in different places and at different times and that several different mutations are individually responsible for white fruit color in different groups of cultivars."

She said further research with a wider range of white grapes varieties is needed. Kobayashi's study looked at eight varieties, including Muscat of Alexandria and Italia, a table grape.