Pinot blanc and Pinot gris arose as independent somatic mutations of Pinot noir

In grapevine (Vitis vinifera L.), accumulation and fixing of somatic mutations represent frequent events, allowing growers to select and propagate new cultivars. A great deal of somatic mutations does not affect the entire meristem but only a portion of it (chimeras). Resulting from the layered structure of the meristem, chimeras are composed by two genetically distinct tissue layers placed adjacent to one another. Among spontaneous somatic mutations occurred in grapevine, those affecting the berry colour locus are the most documented. Grape berry colour is due to the presence of a single pigment family, the anthocyanins, which largely vary in concentration and composition depending on the grape cultivar. In many plants anthocyanin biosynthesis is controlled by regulatory genes belonging to the Myb family of transcription factors. Two Myb-related transcription-factor genes, VvMybA1 and VvMybA2, regulate anthocyanin biosynthesis in V. vinifera grapes. Inactivation of these two functional genes, through the insertion of the Gret1 retrotransposon in VvMybA1 promoter and through a non-synonymous SNP present in the VvMybA2 coding region, gives rise to a white berry phenotype. Recently, several genetic and genomic studies revealed that the colour locus is a cluster of four Myb-like genes located on chromosome 2. As one of the founder varieties and cultivated worldwide, Pinot had several chances to undergo somatic mutations. Most of these affected the ancestral black berry colour, and gave rise to cultivars such as in Pinot blanc and Pinot gris. The most established evolutionary model is that Pinot blanc arose from Pinot gris which arose from Pinot noir, even if the relationship between Pinot blanc and Pinot gris has not yet been fully explored. Pinot gris is reported to be a periclinal chimera of Pinot noir, but also in this case the exact nature of the genetic modification remains to be determined. Our study has questioned this evolutionary model. Taking advantage of a layer-specific structural analysis of the berry colour locus in 4 Pinot noir, 10 Pinot blanc and 13 Pinot gris clones, along with its naturally derived chimeras or sports, we provide an evolutionary explanation of the somatic mutations that have affected the locus of berry colour. Through the study of the structural dynamics along the chromosome 2, a very large deletion (ca. 4,300 Kb) present in a single Pinot gris cell layer was identified and characterized, while a short deletion (ca. 150 Kb) present in both Pinot blanc cell layers was detected. Within the same cultivar, all clones shared the same deletion pattern. As recently observed in other Pinot noir clones, we can suppose that the structural changes that occurred in Pinot gris and in Pinot blanc were stress-mediated, resulting in the activation of a mobile genetic element. Theoretically, the origin of a colourless berry mutant can be ascribed to two distinct models: i) the sequential model, where the black-skinned berry ancestor gave rise to the grey-skinned which in turn gave rise to the white-skinned berry mutant, and ii) the parallel model, where the black-skinned berry ancestor gave rise to the grey-skinned and the white-skinned berry mutants separately. Here, we propose the parallel model as the evolutionary model for the formation of Pinot berry colour somatic variants. According to this novel model, the somatic mutants Pinot gris and Pinot blanc arose from the ancestral Pinot noir cultivar independently. We name this parallel model as the “Pinot-model”, distinctly from the previously reported sequential “CabSau-model”. Moreover, these results elucidated the relationship between Pinot blanc and Pinot gris. Finally, we suggest the name Pinot verdâtre for the unpigmented bud sport of Pinot gris, holding a peculiar genetic make-up and a green-like phenotype. Our findings represent a breakthrough towards the full understanding of the mechanisms behind the formation of white, grey, red, and pink grape cultivars, the overall phenotype of which determines a specific enological aptitude.