Heteromorphic incompatibility retained in self-compatible plants produced by a cross between common and wild buckwheat.

•   Common buckwheat (Fagopyrum esculentum) is a dimorphic self-incompatible plant with either a pin or a thrum flower. It is considered that self-incompatibility, flower morphology, and pollen size are governed by the S supergene. •   Here, we produced self-compatible buckwheat lines by an interspecific cross between F. esculentum and F. homotropicum with embryo rescue. •   The flower morphology of these lines was long homostyle and pollen size was similar to that of thrum. Pollen size of F1 plants produced by a cross between a pin plant and the self-compatible plant was similar to that of the self-compatible line and segregated together with flower morphology without exception. The pollen tubes of the self-compatible plants were compatible with styles of the pin plants but incompatible with the styles of thrum plants. However, the pollen tubes of pin flowers were incompatible with the styles of self-compatible plants, but the pollen tubes of thrum flowers were compatible with the styles of self-compatible plants. •   These results indicate that the self-compatibility allele, Sh , retains heteromorphic incompatibility and suggest that the Sh allele was derived from recombination in the S supergene.