Genetic versatility of incompatibilty in plants

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Abstract
Flowering plants have at least 11 different self-incompatibility systems; most, if not all, are oppositional systems in which an incompatible reaction occurs between like alleles in pollen and style. Higher fungi have complementary systems in which compatibility is due to the presence of unlike alleles in the mating cells. This is shown to be a natural consequence of the contrast between haploid pollen entering a diploid style in a plant, and a dikaryotic cell presenting one nucleus to enter a haploid monokaryotic cell of a fungus. From the genetic dominance, co-dominance, and negative complementation (competition), the polymeric state of the S gene product is deduced as: a polymer in pollen and a monomer in the style of gametophytic systems, with the exception of a pollen co-polymer in grasses and a pollen monomer or co-polymer in Tradescantia paludosa; in the multiallelic sporophytic system a polymer is presumed in both pollen and style. Four-gene systems are discussed and evidence is given for a multigenic system in Raphanus sativus. A new model system based on three or more genes but with complementation between unlike alleles to give compatibility, as in the fungi, is applied to flowering plants, and a workable system is obtained which equilibrates at 0.235 self-compatible and 0.765 self-incompatible plants.