An Hypothesis of Gene Interaction at the S Locus in Self-Incompatibility Systems of Angiosperms

1. In Angiosperms, the common types of S-allele interaction that are found in self-incompatible systems controlled by multiple alleles at a single locus are independent action, partial or complete dominance, and mutual weakening. These are discussed in terms of a reaction system with the following properties: (a) each S allele governs the production of a specific incompatibility substance; (b) this substance must be present in a threshold quantity before it causes incompatibility; (c) heterozygotes produce two such substances, one for each S allele. 2. Independent action in a heterozygote is believed to result if the limits to production are high enough for each of the two allele-specific substances to be produced in super-threshold quantities. 3. Dominance is explained in terms of unequal competition between the reaction systems controlled by the two alleles for some limited factor. Nonlinear dominance relations can occur in populations because super-threshold quantities of incompatibility substances, although unequal, have equal potency in heterozygotes. 4. Mutual weakening is attributed to secondary limits that apply to the activities of only certain S alleles. 5. The hypothesis suggests moderate limits to S-allele action in the pollen of sporophytic species, but more restrictive limits in the pollen of artificial tetraploids with gametophytic systems. These correspond to the differences in the time of S-allele action in the pollen. 6. The postulated moderate limits in the stigmas of sporophytic species correspond to the narrow zone of pollen inhibition. In contrast, the absence of limits in the styles of gametophytic species agrees with the extended zone of pollen-tube inhibition.