Self-incompatibility.

Self-incompatibility helps plants to avoid the combination of gametes that are too genetically similar, and it is controlled by interactions between proteins of pollen and the pistil. (a) In gametophytic self-incompatibility, compatibility between pollen and pistil is determined by the haploid genotype of the pollen. In this case, S protein is located in the pollen cytosol. (b) In sporophytic self-incompatibility, compatibility between pollen and pistil is determined by the sporophyte that produced the pollen and contributed S proteins to its coat.