Genetic Features of the Spontaneous Self-Compatible Mutant, ‘Jin Zhui’ (Pyrus bretschneideri Rehd.)

‘Jin Zhui’ is a spontaneous self-compatible mutant of ‘Ya Li’ (Pyrus bretschneideri Rehd. S(21)S(34)), the latter displaying a typical S-RNase-based gametophytic self-incompatibility (GSI). The pollen-part mutation (PPM) of ‘Jin Zhui’ might be due to a natural mutation in the pollen-S gene (S(34) haplotype). However, the molecular mechanisms behind these phenotypic changes are still unclear. In this study, we identified five SLF (S-Locus F-box) genes in ‘Ya Li’, while no nucleotide differences were found in the SLF genes of ‘Jin Zhui’. Further genetic analysis by S-RNase PCR-typing of selfed progeny of ‘Jin Zhui’ and ‘Ya Li’ × ‘Jin Zhui’ progeny showed three progeny classes (S(21)S(21), S(21)S(34) and S(34)S(34)) as opposed to the two classes reported previously (S(21)S(34) and S(34)S(34)), indicating that the pollen gametes of ‘Jin Zhui’, bearing either the S(21)- or S(34)-haplotype, were able to overcome self-incompatibility (SI) barriers. Moreover, no evidence of pollen-S duplication was found. These findings support the hypothesis that loss of function of S-locus unlinked PPM expressed in pollen leads to SI breakdown in ‘Jin Zhui’, rather than natural mutation in the pollen-S gene (S(34) haplotype). Furthermore, abnormal meiosis was observed in a number of pollen mother cells (PMCs) in ‘Jin Zhui’, but not in ‘Ya Li’. These and other interesting findings are discussed.