The Mutated S1-Haplotype in Sour Cherry Has an Altered S-Haplotype–Specific F-Box Protein Gene

Gametophytic self-incompatibility (GSI) is an outcrossing mechanism in flowering plants that is genetically controlled by 2 separate genes located at the highly polymorphic S-locus, termed S-haplotype. This study characterizes a pollen part mutant of the S(1)-haplotype present in sour cherry (Rosaceae, Prunus cerasus L.) that contributes to the loss of GSI. Inheritance of S-haplotypes from reciprocal interspecific crosses between the self-compatible sour cherry cultivar Ujfehértói Fürtös carrying the mutated S(1)-haplotype (S(1)'S(4)S(d)S(null)) and the self-incompatible sweet cherry (Prunus avium L.) cultivars carrying the wild-type S(1)-haplotype revealed that the mutated S(1)-haplotype confers unilateral incompatibility with a functional pistil component and a nonfunctional pollen component. The altered sour cherry S(1)-haplotype pollen part mutant, termed S(1)', contains a 615-bp Ds-like element within the S(1)-haplotype-specific F-box protein gene (SFB(1)'). This insertion generates a premature in-frame stop codon that would result in a putative truncated SFB(1) containing only 75 of the 375 amino acids present in the wild-type SFB(1). S(1)' along with 2 other previously characterized Prunus S-haplotype mutants, S(f) and S(6m), illustrate that mobile element insertion is an evolutionary force contributing to the breakdown of GSI.