Molecular identification of two new self-incompatible alleles (S-alleles) in Chinese pear (Pyrus bretschneideri).

Self-incompatibility (SI) is an important intraspecific reproductive barrier in flowering plants. To identify the S-alleles of Chinese pear species (Pyrus pyrifolia, P. bretschneideri, P. ussuriensis and P. sinkiangenis etc.), S-RNase-specific PCR amplification, sequence analyses and field pollination tests were performed using two cultivars 'Jingxiang' and 'Esu' of P. bretschneideri as materials. Two new S-RNase genes were identified from the two cultivars. They were 1,122 bp and 1,058 bp in length, and designated as S37-RNase (GenBank accession no. DQ839238) and S38-RNase (GenBank accession no. DQ839239). By comparison of their deduced amino acid sequences with those of S1-to S36-alleles of Oriental pear, it can be seen that both the two new S-alleles had their conserved regions C1 and C2, but their hypervariable regions (HV) were quite different from those of the others. S37 showed a higher similarity (96%) to S38 in the amino acid sequences deduced from them, whereas both of them displayed the highest similarity (98%) to S15 and the lowest (63%) to S32. The two S-alleles had introns of 786 bp and 723 bp, respectively, similar in size to that of S15 (777 bp). Finally, the S-genotypes of 'Jinxiang' and 'Esu' were unambiguously determined as S34S37 and S15S38, respectively.