Production of an S RNase with dual specificity suggests a novel hypothesis for the generation of new S alleles

Gametophytic self-incompatibility in plants involves rejection of pollen when pistil and pollen share the same allele at the S locus. This locus is highly multiallelic, but the mechanism by which new functional S alleles are generated in nature has not been determined and remains one of the most intriguing conceptual barriers to a full understanding of self-incompatibility. The S-11 and S-13 RNases of Solanum chacoense differ by only 10 amino acids, but they are phenotypically distinct (i.e., they reject either S-11 or S-13 pollen, respectively). These RNases are thus ideally suited for a dissection of the elements involved in recognition specificity. We have previously found that the modification of four amino acid residues in the S-11 RNase to match those in the S-13 RNase was sufficient to completely replace the S-11 phenotype with the S-13 phenotype, We now show that an S-11 RNase in which only three amino acid residues were modified to match those in the S-13 RNase displays the unprecedented property of dual specificity (i.e., the simultaneous rejection of both S-11 and S-13 pollen), Thus, S12S14 plants expressing this hybrid S RNase rejected S-11, S-12, S-13, and S-14 pollen yet allowed S-15 pollen to pass freely. Surprisingly, only a single base pair differs between the dual-specific S allele and a monospecific S-13 allele, Dual-specific S RNases represent a previously unsuspected category of S alleles. We propose that dual-specific alleles play a critical role in establishing novel S alleles, because the plants harboring them could maintain their old recognition phenotype while acquiring a new one.