In cruciferous plants, SI is determined by the S locus, which harbors genes encoding the stigma papilla cell‐specific S‐receptor kinase (SRK) and the pollen surface‐localized ligand of SRK, S‐locus cysteine‐rich protein/S‐locus protein 11 (SCR/SP11) (Schopfer et al., 1999; Suzuki et al., 1999). In B. rapa, the linear dominance hierarchy of class II S haplotype SCR alleles is controlled by SCR‐methylation‐inducing region 2 (Smi2) in the S locus; the Smi2 of the dominant S locus generates small interfering RNAs (siRNAs), which suppresses the expression of the recessive S‐locus SCR by siRNA‐mediated DNA methylation (e.g., BrS44 > BrS60 > BrS40 > BrS29, Yasuda et al., 2016). BoS15 and BrS60 are interspecific pairs between B. oleracea and B. rapa based on their high sequence similarity and incompatibility of B. oleracea BoS15 pollen with BrS60/BoS18 interspecific hybrid stigmas (Sato et al., 2006). ...BrS29 could be deduced to be recessive to BoS15.