Clonal polymorphism mainly results from somatic mutations that occur naturally during plant growth. In grapevine, arrays of clones have been selected within varieties as a valuable source of diversity, among them clones showing berry color polymorphism. To identify mutations responsible for this color polymorphism, we studied a collection of 33 clones of Pinot noir, Pinot gris, and Pinot blanc. Haplotypes of the L2 cell layer of nine clones were resolved by genotyping self-progenies with molecular markers along a 10.07 Mb region of chromosome 2, including the color locus. We demonstrated that at least six haplotypes could account for the loss of anthocyanin biosynthesis. Four of them resulted from the replacement of sections of the ‘colored’ haplotype, sized from 31 kb to 4.4 Mb, by the homologous sections of the ‘white’ haplotype mutated at the color locus. This transfer of information between the two homologous sequences resulted in the partial homozygosity of chromosome 2, associated in one case with a large deletion of 108 kb-long. Moreover, we showed that, in most cases, somatic mutations do not affect the whole plant; instead, they affect only one cell layer, leading to periclinal chimeras associating two genotypes. Analysis of bud sports of Pinot gris support the hypothesis that cell layer rearrangements in the chimera lead to the homogenization of the genotype in the whole plant. 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.
Pinot is one of the most ancient grapevine varieties made up of a large panel of clones, most of them used to produce very different wines with specific oenological characteristics in different vineyards around the world. This great diversity of clones, which is due to spontaneous somatic mutations that have occurred over time, makes Pinot a fascinating subject of study. It is the reason why we have undertaken a study focused on the color locus to identify the mutations responsible for color variation in a large panel of Pinot gris and Pinot blanc clones. The results we obtained shed light on large-scale molecular events that account for the loss of anthocyanin biosynthesis, such as chromosome replacement and deletion. These mutations first multiplied and, depending on the cell layer in which they occurred, lead to chimeras. Occasionally, cell layer rearrangements homogenize the whole plant. Clonal polymorphism of grapevine varieties results from a succession of such molecular and cellular mechanisms that are the driving forces behind the genetic drift of clones and the evolution of the grapevine genome.