S-allele genotype of apple cultivars and establishment process for self-incompatibility

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( 1 ) The S-locus genes ( Sd-, Sh-, Se- and Si-RNase ) in the pistil were cloned and sequenced from the apple (Malus x domestica Borkh. ) cultivars. We developed an S-allele-specific polymerase chain reaction ( PCR )-restrictioon fragment length polymorphism ( RFLP ) analysis method for identification of the S-alleles. Using that method, we analyzed S-genotype of 136 apple cultivars and selections.( 2 ) The St- and Sg'-RNase CDNA were cloned from Malus trcmsitoria. S-allele genotypes of nine Malus species were identified by S-allele-specific PCR-RFLP analyzes.( 3 ) New S-RNases, Sg and Sh-RNase, and Sf-RNase show a strong similarity ( 93-94 % ) to each other at the deduced amino acid level. There are only two to six amino-acid differences among HV regions responsible for the S-allele specificity of Sf, Sg and Sh.( 4 ) Sg'-RNase differed by one amino acid from that of Sg-RNase. A difference of one amino acid within HV regions produced by natural mutation was not enough to generate a new S-allele. However, the model ofMaton et al. ( 1999 ) on establishing novel S-alleles was supported by one amino acid difference since S_g-allele could recognize both S_g- and S_g- alleles.( 5 ) Based on the phylogenetic analysis of S-RNases, it was speculated that the polymorphic S-RNases in apple were generated before divergence into the genera Malus and Pyrus.