Figure 2.

Schematic model of the S locus. The S locus contains at least two genes, one encoding the male S determinant, which is expressed in anther tapetum surrounding the pollen grains during development, and the other encoding the female S determinant, which is expressed in the pistil (stigma). Both the genes encoding male and female S determinants are inherited as one segregating unit. The number of S alleles is thought to be over 50 to 100 in Brassica species.^167–169) When both male and female S determinants are derived from the same S alleles, the incompatible response occurs. In contrast, in the case of a combination of different S alleles from the male and female parents, the compatible response is observed.