Inheritance of Sf-RNase in Japanese apricot (Prunus mume) and its relation to self-compatibility

Self-compatible cultivars of Japanese apricot (Prunus mume Shieb. et Zucc.), a tree species that normally shows S-RNase-based self-incompatiblity, have a horticultural advantage over self-incompatible cultivars. Inheritance of self-compatibility and a common Sf-RNase allele that is observed in self-compatible cultivars was investigated using progenies from controlled crosses. Total DNAs were isolated from the parents and progenies of seven crosses that included at least one self-compatible cultivar as a parent. These DNAs were PCR-amplified with the Pru-C2 and PCE-R primer pair to determine S-haplotypes of the parents and progenies. A novel S-haplotype, S8, was found. In all crosses examined, the Sf-RNase gene was inherited from either the seed or pollen parent as a pistil S-allele in a non-functional S-haplotype. Self-compatibility of about 20 trees each from reciprocal crosses of 'Benisashi (S7Sf)' and 'Shinpeidayu (S3Sf)', and 26 selections from 16 different crosses was tested by pollination and pollen-tube growth studies. Cosegregation of the Sf-RNase allele and self-compatibility was confirmed with all but selection 1K0-26 (S3S7). Selection 1K0-26 (S3S7) that originated from 'Benisashi (S7Sf)' × 'Koshinoume (S3Sf)' appeared to be self-compatible even without the Sf-RNase allele. The possible role of pollen-S, a presumably existing pollen component of gametophytic self-incompatibility, is discussed.