Production of an S RNase with Dual Specificity Suggests a Novel Hypothesis for the Generation of New S Alleles

Correspondence to: Daniel P. Matton, mattond{at}irbv.umontreal.ca (E-mail), 514-872-9406 (fax)

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 S₁₁ and S₁₃ RNases of Solanum chacoense differ by only 10 amino acids, but they are phenotypically distinct (i.e., they reject either S₁₁ or S₁₃ 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 S₁₁ RNase to match those in the S₁₃ RNase was sufficient to completely replace the S₁₁ phenotype with the S₁₃ phenotype. We now show that an S₁₁ RNase in which only three amino acid residues were modified to match those in the S₁₃ RNase displays the unprecedented property of dual specificity (i.e., the simultaneous rejection of both S₁₁ and S₁₃ pollen). Thus, S₁₂S₁₄ plants expressing this hybrid S RNase rejected S₁₁, S₁₂, S₁₃, and S₁₄ pollen yet allowed S₁₅ pollen to pass freely. Surprisingly, only a single base pair differs between the dual-specific S allele and a monospecific S₁₃ 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.

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