Résumé
Berry size is an important trait both for yield (table grapes) and in relation to quality (wine grapes). The final size of an organ is dependent on cell number, cell size and/or number of carpel. Indeed it has been shown that cell number is a major determinant of fruit size variability in Solanaceae and Prunus, whereas a preliminary study on a small sample of grapevine genotypes suggested that the variability of berry size resulted from cell volume enlargement. At present only few genes involved in fruit size variation have been identified, mainly in Solanaceae and none in grapevine. Our aims are: (1) to check, in a large sample of grapevine genotypes, the relative importance of cell volume enlargement, number of cell and carpel in berry size variation, (2) to develop association genetics approaches with candidate genes for berry size and (3) to study the DNA polymorphism along the region carrying the fleshless berry locus (flb) in order to detect possible selection pressures in cultivated varieties. The flb locus was identified in a grapevine natural mutant which present berries with a size reduced by 20 times and circular seeds similar to wild grapevine berries. The ovaries and the berries of 3 subpopulations of cultivated Vitis vinifera genotypes (279), which represent the genetic and phenotypic variability of berry size, have been phenotyped. Preliminary results on a small sample of individual (27) suggested that the final berry size was determined at early stage of berry development. The ovarian size just before fertilization was variable and proportional to the final berry size. However the ovarian cell number or size seemed to explain the final berry size differently according to sub-populations. Several mechanisms and probably several genes would be involved in the variation of the berry size in these sub-populations. These data are currently under validation on the whole set of 279 individuals. In parallel, 44 candidate genes, orthologous to genes known to be involved in flower or fruit development and/or co-localizing with berry size QTLs (Quantitative Trait Locus) and/or expressed during grapevine berry development, have been partially or totally sequenced in 47 genotypes which reflect all the genetic diversity of grapevine. Several thousands of SNPs (Single Nucleotide Polymorphims) have been identified. A subset of these SNPs will be used for genotyping the 3 sub-populations using the SNPlex technology, in order to detect a possible correlation between genotypic and phenotypic variation. Finally, in order to test the hypothesis that flb is one of the genes selected during grapevine domestication, 69 gene fragments along flb locus (1Mbp) were sequenced in a sample of wild and cultivated Vitis vinifera. Our results revealed a small zone showing more polymorphism in wild genotypes than in cultivated genotypes and a large zone of 171Kb showing either more polymorphism in cultivated genotypes than in wild genotypes. These intervals will be studied more in details using two approaches: gene expression studies and an improvement of the genotype sampling for the DNA polymorphism studies.