Berry firmness is one of the most important quality traits in table grape production and, consequently, a key aspect for table grape breeding programs. To identify the genes determining the berry firmness in grapes, a progeny of Ruby Seedless × Sultanina (n = 137) was evaluated during three consecutive seasons. Results showed that even though the heritability was ~90%, season had an important effect on this trait. Quantitative trait loci (QTL) analysis and genetic mapping showed that the determinants for this trait are distributed in linkage groups 8 and 18. This is the first time that a stable QTL for berry firmness across seasons has been identified on linkage group 8. This QTL is mainly given by a male allelic and additive effect. Together, these two QTLs explained ~27.6% of the phenotypic variance, with confidence intervals of up to 10 cM. Among the tens of genes found in these two QTLs, we highlight a cation/calcium exchanger, a xylosyltransferase, a probable cellulose synthase, and a putative invertase. This study shows that berry firmness has a clear genetic basis. These results could also be used for the development of markers to assist table grape breeding.
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