Identification of S-genotypes in 18 pear accessions and exploration of the breakdown of self-incompatibility in the pear cultivar Xinxue

Self-incompatibility is a genetic mechanism in flowering plants that promotes outcrossing and prevents inbreeding. In this study, the reported 15 S-RNase alleles were isolated from 18 pear accessions that contained 14 different S-genotypes. The S-RNase alleles were only expressed in the style, not in the root, stem, leaf, fruit, sepal, and pollen. Pollen tube grew into self-styles and self-pollinated fruit set involved the breakdown of self-incompatibility in cv. Xinxue. However, the two S-RNase alleles in cv. Xinxue had identical amino acid sequences to those self-incompatible cultivars, and S5- and S6-RNase were normally expressed in style, indicating that the loss of self-incompatibility in cv. Xinxue likely resulted from pollen-part mutation. Further, S5S5 and S6S6 genotyped individuals were identified in self-pollinated progeny, indicating that S5 and S6 genotyped pollen were compatible with the self-styles of cv. Xinxue. A genetic analysis showed that the segregation ratio of S5S5, S5S6, and S6S6 was approximately 0:2:1 (χ2 = 3.505 < χ20.05, 2 = 5.99), which did not fit the theoretical ratio of 1:2:1 (χ2 = 13.340 > χ20.05, 2 = 5.99), suggesting that the breakdown of self-incompatibility could be caused by modified factor(s) located outside of the S-locus. These results are useful for parental assignment and understanding self-incompatibility reaction.