Non-self Recognition by Multiple F-box Genes in Self-incompatibility of Japanese Pear (Pyrus pyrifolia)

Many species of Rosaceae exhibit gametophytic self-incompatibility (GSI) in which pistil-part specificity is controlled by polymorphic ribonuclease called S-RNase.  S-RNase is considered to be uptaken both by self and non-self pollen tubes, and acts as cytotoxin to self pollen tubes only.  Recent studies identified F-box proteins as the pollen-part determinants.  In Prunus, a polymorphic F-box gene was identified by sequence analysis of the S locus region and named as SFB (S haplotype-specific F-box protein).  Mutations or deletion of SFB were shown to be associated with pollen-part loss of GSI.  SFB is predicted to recognize self S-RNase, and allow it to act as the cytotoxin for self pollen tube.  In Japanese pear, however, sequence analysis of the S locus region identified multiple F-box genes as the candidates for pollen determinants, and named SFBB (S locus F-box brothers).  Interhaplotypic divergence of SFBB was very low, while intrahaplotypic divergence was comparable to allelic divergence of S-RNase.  In addition, loss-of-function of SFBB was associated with no or small change on pollen-part specificity.  Pollen with S^4sm that was derived from S⁴ haplotype lacks SFBB1-S⁴, and was rejected by otherwise compatible S¹ pistil, while accepted by other non-self pistils.  Although S⁵ haplotype encodes truncated SFBB1 protein, S⁵ pollen was normally accepted by S¹ and other non-self pistils.  These findings suggest that GSI of Japanese pear is ‘non-self recognition by multiple factors’ type in which each SFBB targets subfraction of non-self S-RNases, and thus is similar to that of Petunia (Solanaceae) rather than to ‘self recognition by single factor’ system of Prunus.