The families Ranunculaceae and Chenopodiaceae, presumably belonging to a dicot evolutionary branch with ancestral basis shared with the monocots, both show a mechanism of self- and cross-incompatibility involving more than one incompatibility locus (S-locus) and requiring that all of the S-genes acting on the haploid pollen grain be matched in the pistil. This type of incompatibility system was previously considered to be unique for the grasses. However, whereas two S-loci with such complementary interaction have been established in the grasses, there are at least three S-loci in Ranunculus acris, and in Beta vulgaris there are at least four.
The observations strongly support the theory of the incompatibility genes being ancient constituents of the breeding systems of the angiosperms. Most probably a complex type of incompatibility control was already present at the presumed common ancestral basis of the monocots and the investigated dicot evolutionary branch. Self-incompatibility systems involving more than one S-locus may occur more frequently than hitherto expected. This has important implications for plant breeding which bear on the possibilities of reconstructing an incompatibility system from different forms within a normally self-fertilizing species and, thereby, to bring about obligatory cross-fertilization.