Self-compatibility in loquat (Eriobotrya japonica Lindl.) is possibly due to S6-RNase mutation

Loquat (Eriobotrya japonica Lindl.), a member of the Rosaceae, carries the RNase-dependent gametophytic self-incompatibility (GSI) fertilization system. However, cultivars that carry the S6 allele are self-compatible (SC). In the present study, the S6ins-RNase and four S6-SFB genes were analyzed. Near the 3' end of the S6ins-RNase open reading frame, a 24-nucleotide in-frame insertion was identified. Transcription of S6ins-RNase mRNA in SC cultivars Avri, Yehuda and Akko 1 was lower than that of the coupled S-RNase (S2, S3 and S4, respectively). Computer modeling of the putative S6ins-RNase protein positioned the 8-amino acid insertion as an external ring on the protein's surface, facing the catalytic site, and possibly preventing RNA degradation. Accordingly, RNase activity gel analysis showed no activity for the S6ins-RNase protein. In the four cloned S6. SFBs (EjSFBB1a-S6, EjSFBB1b-S6, EjSFBB4-S6 and EjSFBB8-S6), which correspond to three SFB brother subtypes, no mutation was observed. Thus, the SC of the loquat S6 haplotype is due to inactivity of the S6ins-RNase.