Characterization of a new apple S-RNase allele and its linkage with the Rvi5 gene for scab resistance

Apple (Malus 9 domestica Borkh.) is a species possessing S-RNase-based self-incompatibility. Knowledge of the S-genotype of cultivars is very important both for breeders and growers, in order to plan compatible crosses and cultivar combinations. Here we report a revision of the S-genotype of the cultivar Murray which is in disagreement with previous reports, and the characterization of the newly identified S-RNase allele. Moreover, Murray carries the Rvi5 gene for apple scab resistance, which was mapped at the bottom of linkage group 17, in a position close to that of the S-locus. To evaluate whether the segregation of S-alleles can influence that of Rvi5, we analyzed the linkage between the two loci in 215 progeny individuals from the cross Galaxy 9 Murray. Our results suggest that the S-genotype of cultivars used as parents can dramatically influence the frequency of seedlings inheriting the resistance gene, and this aspect can be exploited to increase the efficiency of breeding programs aimed at pyramiding scab resistance genes.