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Scientists Describe Molecular Mechanisms in Evolution of Pinot Noir Clones

NEW YORK (GenomeWeb) – French scientists have pinned down the mechanics of genetic mutations that cause different colors in Pinot gris and Pinot blanc, offshoots of the famous winemaking grape Pinot noir.

Led by Frédérique Pelsy of the University of Strasbourg, the scientists used next-generation sequencing to compare large portions of the genome in the three Pinot grapes, and in a study published Thursday in PLOS Genetics she and her co-authors said that "Extended chromosomal structural changes, which could result from gene conversion, are responsible for the color impairing of Pinot gris and Pinot blanc clones."

Through extensive haplotype analysis of 33 different clones of the three grapes, they showed how structural changes in the chromosomes changed the color of the grapes. They also developed a model for the evolution of color in Pinot, suggesting that the light green Pinot blanc and muddled Pinot gris did not arise independently from the midnight blue Pinot noir as previously thought. Instead, they proposed that invading mutations first gave rise to the chimeric Pinot gris and only then to Pinot blanc.

In the science of viticulture, Pinot noir is famous for the ability to mutate into interesting forms, including Pinot gris and Pinot blanc. Generally, the mutations occur naturally in the somatic regenerative cells of the plant and propagate throughout it, taking over in surrounding cells. But the mutations are mostly limited to affecting a single layer of cells, the authors said. "In most cases, somatic mutations do not affect the whole plant; instead, they affect only one cell layer, leading to periclinal chimeras," they said.

Pinot grape color is due to anthocyanins, pigments that can appear blue, purple, or red, depending on the pH, and is determined by the genes VvmybA1 and VvmybA2, two genes encoding transcription factors that regulate the anthocyanin pathway.

White grapes, like Pinot blanc, can lack anthocyanin for several reasons, including insertions, deletions, and point mutations. Pinot gris arises when a mutation happens in just one of the cell layers, leading to a difference in the genotypes of the outermost skin and second layer beneath that.

A previous study had suggested that the deep purple Pinot noir gave rise to the grey-skinned and the white-skinned mutants independently, but Pelsy and her colleagues argue that because somatic mutations are limited to the layer in which they form, the Pinot blanc, which has loss of pigment in both layers, must be derived from Pinot gris. 

Remarkably, they said the somatic mutations of the inner layer of cells can invade the outer layer through a process called displacement, leading to a loss-of-color mutation to take over the entire grape.  

"Our findings shed new light on the way molecular and cellular mechanisms shape the grapevine genotypes during vegetative propagation, and enable us to propose a scheme of evolutionary mechanism of the Pinot clones," they said.