Self-compatibility of 'Zaoguan' (Pyrus bretschneideri Rehd.) is associated with style-part mutations

The pear cultivar 'Zaoguan' (S(4)S(34)) is the a self-compatible descendant of 'Yali' (S(21)S(34)) × 'Qingyun'(S(4)S(9)). Two self-incompatible cultivars 'Xinya' and 'Yaqing', also S-genotyped as S(4)S(34) for the S-RNase gene, were used as controls. Field pollination data revealed that 'Zaoguan' displayed SC, whereas 'Xinya' and 'Yaqing' showed self-incompatibility (SI) upon self-pollination. Reciprocal pollinations between the varieties showed that most of the 'Zaoguan' flowers pollinated with 'Xinya' or 'Yaqing' pollen set fruits but that few of the 'Xinya' or 'Yaqing' flowers set fruit when pollinated with 'Zaoguan' pollen. The pollen performance was monitored with fluorescence microscopy, and we observed that 'Zaoguan' accepted self-pollen as well as 'Xinya' or 'Yaqing' pollen, whereas 'Xinya' or 'Yaqing' rejected self-pollen and 'Zaoguan' pollen. The S(34)-RNase but not the S(4)-RNase could be detected in all selfed progeny of 'Zaoguan'. Comparisons of the 2D-PAGE profiles of the stylar extracts from the three cultivars showed that the S(4)-RNase protein expressed normally, but the S(34)-RNase of 'Zaoguan' was not found. Thus, we concluded that the stylar S(34) products were defective in 'Zaoguan' and that the S (4)-allele functioned normally. The nucleotide sequences of the S(4)- and S(34)-RNase of 'Zaoguan' showed no differences from those of 'Xinya' or 'Yaqing', and they transcribed normally. These results indicate that SC in 'Zaoguan' was due to the loss of the S(34)-RNase caused by unknown post-transcriptional factors.