Heteromorphic incompatibilityThis is caused by differences in the lengths of stamens and style (called heterostyly) (Figure 5.5). In one flower type called the pin, the styles are long while the anthers are short. In the other flower type, thrum, the reverse is true (e.g. in Primula). The pin trait is conditioned by the genotype ss while thrum is conditioned by the genotype Ss. A cross of pin (ss) × pin (ss) as well as thrum (Ss) × thrum (Ss) are incompatible. However, pin (ss) × thrum (Ss), or vice versa, is compatible. The condition described is distyly because of the two different types of style lengths of the flowers. In Lythrum three different relative positions occur (called tristyly).

       Homomorphic incompatibilityThere are two kinds of homomorphic incompatibility: gametophytic and sporophytic (Figure 5.6).Gametophytic incompatibilityIn gametophytic incompatibility (originally called the oppositional factor system), the ability of the pollen to function is determined by its own genotype and not the plant that produces it. Gametophytic incompatibility is more widespread than sporophytic incompatibility. Gametophytic incompatibility occurs in species such as red clover, white clover, and yellow sweet clover. Homomorphic incompatibility is controlled by a series of alleles at a single locus (S1, S2, …, Sn) or alleles at two loci in some species. The system is called homomorphic because the flowering structures in both the seed‐bearing (female) and pollen‐bearing (male) plants are similar. The alleles of the incompatibility gene(s) act individually in the style. They exhibit no dominance. The incompatible pollen is inhibited in the style. The pistil is diploid and hence contains two incompatibility alleles (e.g. S1S3, S3, S4). Reactions occur if identical alleles in both pollen and style are encountered. Only heterozygotes for S alleles are produced in this system.Sporophytic incompatibilityIn sporophytic incompatibility, the incompatibility characteristics of the pollen are determined by the plant (sporophyte) that produces it. It occurs in species such as broccoli, radish, and kale. The sporophytic system differs from the gametophytic system in that the S allele exhibits dominance. Also, it may have individual action in both pollen and style, making this incompatibility system complex. The dominance is determined by the pollen parent. Incompatible pollen may be inhibited on the stigma surface. For example, a plant with genotype S1S2 where S1 is dominant to S2, will produce pollen that will function like S1. Furthermore, S1 pollen will be rejected by an S1 style but received by an S2 style. Hence, homozygotes of S alleles are possible.Incompatibility is expressed in one of three general ways, depending on the species. The germination of the pollen may be decreased (e.g. in broccoli). Sometimes, removing the stigma allows normal pollen germination. In the second way, pollen germination is normal, but pollen tube growth is inhibited in the style (e.g. tobacco). In the third scenario, the incompatibility reaction occurs after fertilization (e.g. in Gesteria). This third mechanism is rare.