Genetic features of a pollen-part mutation suggest an inhibitory role for the Antirrhinum pollen self-incompatibility determinant

Self-incompatibility (SI), an important barrier to inbreeding in flowering plants, is controlled in many species by a single polymorphic S-locus. In the Solanaceae, two tightly linked S-locus genes, S-RNase and SLF (S-locus F-box)/SFB (S-haplotype-specific F-box), control SI expression in pistil and pollen, respectively. The pollen S-determinant appears to function to inhibit all but self S-RNase in the Solanaceae, but its genetic function in the closely-related Plantaginaceae remains equivocal. We have employed transposon mutagenesis in a member of the Plantaginaceae (Antirrhinum) to generate a pollen-part SI-breakdown mutant Pma1 (Pollen-part mutation in Antirrhinum1). Molecular genetic analyses showed that an extra telocentric chromosome containing AhSLF-S (1) is present in its self-compatible but not in its SI progeny. Furthermore, analysis of the effects of selection revealed positive selection acting on both SLFs and SFBs, but with a stronger purifying selection on SLFs. Taken together, our results suggest an inhibitor role of the pollen S in the Plantaginaceae (as represented by Antirrhinum), similar to that found in the Solanaceae. The implication of these findings is discussed in the context of S-locus evolution in flowering plants.