segmental duplication encompassing S-haplotype triggers pollen-part self-compatibility in Japanese pear (Pyrus pyrifolia)

Self-compatible mutants of self-incompatible crops have been extensively studied for research and agricultural purposes. Until now, the only known pollen-part self-compatible mutants in Rosaceae subtribe Pyrinae, which contains many important fruit trees, were polyploid. This study revealed that the pollen-part self-compatibility of breeding selection 415-1, a recently discovered mutant of Japanese pear (Pyrus pyrifolia) derived from γ-irradiated pollen, is caused by a duplication of an S-haplotype. In the progeny of 415-1, some plants had three S-haplotypes, two of which were from the pollen parent. Thus, 415-1 was able to produce pollen with two S-haplotypes, even though it was found to be diploid: the relative nuclear DNA content measured by flow cytometry showed no significant difference from that of a diploid cultivar. Inheritance patterns of simple sequence repeat (SSR) alleles in the same linkage group as the S-locus (LG 17) showed that some SSRs closely linked to S-haplotypes were duplicated in progeny containing the duplicated S-haplotype. These results indicate that the pollen-part self-compatibility of 415-1 is not caused by a mutation of pollen S factors in either one of the S-haplotypes, but by a segmental duplication encompassing the S-haplotype. Consequently, 415-1 can produce S-heteroallelic pollen grains that are capable of breaking down self-incompatibility (SI) by competitive interaction between the two different S factors in the pollen grain. 415-1 is the first diploid pollen-part self-compatible mutant with a duplicated S-haplotype to be discovered in the Pyrinae. The fact that 415-1 is not polyploid makes it particularly valuable for further studies of SI mechanisms.