Breakdown of self-incompatibility of apple (Malus × domestica Borkh.) induced by gamma-ray mutagenesis

Self-compatibility (SC) in a naturally self-incompatible fruit tree species is an incredibly interesting trait for breeding objectives and a valuable tool to investigate the mechanism of the gametophytic self-incompatibility system. We focused on an apple (Malus × domestica Borkh.), in which studies on the SC remain largely unexplored because of a lack of spontaneous and/or induced self-compatible mutants, and performed gamma-ray mutagenesis and succeeded in producing several pollen-part self-compatible mutants. We revealed that SC in the mutants was not caused by a dysfunctional pollen S factor in either of the S-haplotypes but presumably by a translocation of an S-haplotype to a non-homologous chromosome, which allowed the mutants to produce S-heteroallelic pollen grains that overcome self-incompatibility through the competitive interaction between the two different S-factors in the pollen grain. The results from our study demonstrate that duplication of the pollen-S in a pollen grain causes the breakdown of the pollen function in Malus, like Pyrus of Rosaceae and Solanaceae, but not in the mutation of the pollen-S itself, like Prunus. The mutants and our finding will be useful for apple breeding programs.