ABSTRACT
Somatic variation contributes to phenotypic variation in multi-cellular organisms. In plants, which lack a germ line, variation affecting somatic cells can be transmitted to the next sexual generation (Walbot and Evans 2003). The shoot apical meristem (SAM), which directs the growth of the plant shoot, produces vegetative structures, i.e., leaves and lateral shoots, until environmental and developmental signals trigger the switch to reproduction (Bäurle and Dean 2006) and initiates the development of reproductive structures. This lack of germ line can generate potential problems and benefi ts. On one hand, gametes could carry mutations accumulated during somatic growth. A problem that is partially overcome by the selection against cell lineages carrying deleterious mutations, which are impaired in growth and development and so are less likely to contribute to gamete formation, and by the selection during the haploid gametophytic phase which is characteristic of plants (Pineda-Krch and Fagerstrom 1999; Walbot and Evans 2003). On the other hand, somatic variation contributes to the generation of genetic variation and this contribution can be very relevant in perennial species (Antolin and Strobeck 1985). In species amenable to vegetative reproduction like grapevine, somatic variation can eventually give rise to independent plant variants without going through the gametophytic phase. Somatic variation has a strong relevance in fruit trees where vegetative reproduction is commonly used to propagate those relevant phenotypes appearing as spontaneous sports. In grapevine, somatic variation has been the only source of genetic variation used by breeders to improve the phenotypic features of classical cultivars (This et al. 2006). This is why, somatic variation deserves special consideration in a book of grapevine genomics as a source of phenotypic variation of relevance in the genetic improvement of the species as well as in functional analysis.
