Sequence divergence and loss‐of‐function phenotypes of S locus F‐box brothers genes are consistent with non‐self recognition by multiple pollen determinants in self‐incompatibility of Japanese pear ( Pyrus pyrifolia )

Summary The S‐RNase‐based gametophytic self‐incompatibility (SI) of Rosaceae, Solanaceae, and Plantaginaceae is controlled by at least two tightly linked genes located at the complex S locus; the highly polymorphic S‐RNase for pistil specificity and the F‐box gene ( SFB / SLF ) for pollen. Self‐incompatibility in Prunus (Rosaceae) is considered to represent a ‘self recognition by a single factor’ system, because loss‐of‐function of SFB is associated with self‐compatibility, and allelic divergence of SFB is high and comparable to that of S‐RNase . In contrast, Petunia (Solanaceae) exhibits ‘non‐self recognition by multiple factors’. However, the distribution of ‘self recognition’ and ‘non‐self recognition’ SI systems in different taxa is not clear. In addition, in ‘non‐self recognition’ systems, a loss‐of‐function phenotype of pollen S is unknown. Here we analyze the divergence of SFBB genes, the multiple pollen S candidates, of a rosaceous plant Japanese pear ( Pyrus pyrifolia ) and show that intrahaplotypic divergence is high and comparable to the allelic diversity of S‐RNase while interhaplotypic divergence is very low. Next, we analyzed loss‐of‐function of the SFBB1 type gene. Genetic analysis showed that pollen with the mutant haplotype S 4sm lacking SFBB1 ‐ S 4 is rejected by pistils with an otherwise compatible S 1 while it is accepted by other non‐self pistils. We found that the S 5 haplotype encodes a truncated SFBB1 protein, even though S 5 pollen is accepted normally by pistils with S 1 and other non‐self haplotypes. These findings suggest that Japanese pear has a ‘non‐self recognition by multiple factors’ SI system, although it is a species of Rosaceae to which Prunus also belongs.