Recognition specificity of self-incompatibility in Pyrus and Malus

Pyrus displays gametophytic self-incompatibility controlled by a single highly polymorphic gene complex termed S locus, which comprises a stylar-expressed gene (S-RNase) tighlty linked with a pollen expressed gene, that determines the specificity of the self-incompatibility locus. Deduced amino acid sequence of 'Meigetsu' S₈-RNase in Pyrus pyrifolia and 'Kuerlexiangli' S₂₈-RNase in P. sinkiangensis showed 100% identity. S₃-RNase in Malusspectabilis was also found to be similar to S₈-RNase in P. pyrifolia with 96. 9% identity in the deduced amino acid sequence. The intron, which is generally highly polymorphic between alleles, was also remarkably well conserved within these allele pairs. The intron of PpS₈-RNase showed 95. 3 and 91. 9% identity with PsS₂₈-RNase and MsS₃-RNase, respectively. Pollen tube growth in styles, pollen tube length in artificial media containing different S-RNases and segregation of S haplotypes in F₁ plants revealed commonality of the recognition specificity between PpS₈-RNase and PsS₂₈-RNase and between PpS₈-RNase and MsS₃-RNase. Results suggested that PpS₈-RNase, PsS₂₈-RNase and MsS₃-RNase have maintained the same recognition specificity after the divergence of the two species and that amino acid substitutions found between PpS₈-RNase and MsS₃-RNase do not alter the recognition specificity.