A new self-compatibility haplotype in the sweet cherry 'Kronio', S5' attributable to a pollen-part mutation in the SFB gene

''Kronio'' is a Sicilian cultivar of sweet cherry (Prunus avium), nominally with the incompatibility genotype S5S6, that is reported to be naturally self-compatible. In this work the cause of its self-compatibility was investigated.. Test selfing confirmed self-compatibility and provided embryos for analysis; PCR with consensus primers designed to amplify S-RNase and SFB alleles showed that the embryos were of two types, S5S5 and S5S6, indicating that S-6 pollen failed, but S-5 succeeded, perhaps because of a mutation in the pollen or stylar component. Stylar RNase analysis indicated active S-RNases for both S-5 and S-6. The S-RNase alleles were cloned and sequenced; and sequences; encode functional proteins. Cloning and sequencing of SFB alleles showed that S-6 was normal but S-5 had a premature stop codon upstream of the variable region HVa resulting in a truncated protein. Therefore, the self-compatibility can be attributed to a pollen-part mutation of S-5, designated S-5'', the first reported case of breakdown of self-incompatibility in diploid sweet cherry caused by a natural mutation at the S-locus. The second intron of the S-RNase associated with S-5'' contained a microsatellite smaller than that associated with S-5; primers designed to amplify across this microsatellite effectively distinguished S-5 from S-5''. Analysis of some other Sicilian cherries with these primers indicated that S-5'' is also present in the Sicilian cultivar ''Maiolina a Rappu'', and this proved to be self-compatible.