Primary structural features of rosaceous S-RNases associated with gametophytic self-incompatibility

We isolated cDNA clones encoding five S-RNases (S_1-,_S3- , S_5-, S_6-, S_7-RNases) from pistils of Pyrus pyrifolia (Japanese pear), a member of the Rosaceae. Their amino acid sequences were aligned with those of other rosaceous S-RNases sequenced so far. A total of 76 conserved amino acid residues were stretched throughout the sequence, but were absent from the 51–66 region which was designated the hypervariable (HV) region. The phylogenetic tree of rosaceous S-RNases showed that S-RNase polymorphism predated the divergence of Pyrus and Malus. Pairwise comparison of these S-RNases detected two highly homologous pairs, P. pyrifolia S_1- and S_4-RNases (90.0%) and P. pyrifolia S_3- and S_5-RNases (95.5%). The positions of amino acid substitutions between S_1- and S_4-RNases were spread over the entire region, but in the pair of S_3- and S_5-RNases, amino acid substitutions were found in the 21–90 region including the HV region. The substitutions in this restricted region appear to be sufficient to discriminate between S₃ and S₅ pollen and to trigger the self-incompatible reaction.