Use of semi-compatible crosses in breeding for apple scab resistance

Gametophytic self-incompatibility is generally considered a hurdle for breeding programs, as it reduces the number of possible cross combinations. The wide variety of S-haplotypes in apple (Malus × domestica) reduces the probability of fully incompatible crosses, while the occurrence of semi-compatible combinations is more frequent. The yield in such combinations does not vary much with respect to full compatibility, but noteworthy a single S-haplotype from the male parent (pollen donor) will be transmitted to the progeny. This aspect should be carefully taken into account in breeding programs when one or more genes of interest are placed in proximity to the S-locus, as the segregation of the paternal alleles in the progeny will skew from the expected 1:1 ratio, reducing the frequency of the allele linked to the rejected S-haplotype. Among the genes linked to the S-locus in the rosaceous fruit trees, one of the most important is the apple scab resistance gene Rvi5. This gene, introduced in Malus × domestica from M. micromalus and × M. atrosanguinea 804, was fine-mapped in two large segregating populations and proved to be placed in close proximity to the S-locus, on the bottom of chromosome 17. Therefore, knowledge of the S-genotype of parental cultivars can be exploited to plan semi-compatible crosses that dramatically increase the frequency of the resistance gene in the progeny, and to avoid those combinations that conversely would reduce it.