Table 1.
Step-Wise Process leading to New SI Specificitya
| Step No. | Mutation | Haplotype | Phenotype |
|---|---|---|---|
| SxF−SxM | Self-incompatible (specificity x), cross-compatible with all non-x alleles | ||
| 1 | SxF⇒SxyF | ⇓ | |
| SxyF−SxM | Self-incompatible (specificity x), cross-compatible with all non-x alleles | ||
| 2 | SaM⇒SyM | ⇓ | |
| SaF−SyM | Self-compatible, pistil cross-compatible with all non-a alleles, but pollen incompatible with SxyF | ||
| 3 | SaF⇒SyF | ⇓ | |
| SyF−SyM | Self-incompatible (specificity y), cross-compatible with all non-y alleles, complete new specificity |
a
An evolutionary model for SI is depicted. The model assumes separate but tightly linked pollen and pistil genes; note that the second mutation does not occur in the same haplotype as the first (but in a haplotype with pistil allele SaF).