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)

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.