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

We isolated cDNA clones encoding five S-RNases (S-1-, S-3-, 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 S-3 and S-5 pollen and to trigger the self-incompatible reaction.