Re-examination of the self-incompatibility genotype of apple cultivars containing putative 'new' S-alleles

Recently, the self-incompatibility (S-) genotypes of 56 apple cultivars wer e examined by protein analysis, which led to the identification by Boskovic and Tobutt of 14 putative 'new' S-alleles, S12 to S25. This paper reports a re-examination of the S-genotypes of some of these cultivars through S-al lele 'specific' PCR and sequence analysis. The results obtained by this ana lysis indicated that the number of S-alleles that are present in apple is p robably smaller than the number proposed by Boskovic and Tobutt. The existe nce of three 'new' S-alleles (S20, S22 and S24) was confirmed. The existenc e of two other putative 'new' S-alleles (S23 and S25) was, however, contrad icted. The coding sequences of the S-alleles that correspond to the SIO and the S25 ribonuclease bands as well as those corresponding to the S22 and t he S23 ribonuclease bands were shown to be identical in sequence. Interesti ngly, the S-allele corresponding to the S22 and the S23 ribonuclease bands shared a high sequence identity (99% identity) with S27, which was previous ly cloned and sequenced from Baskatong, but which was not included in the a nalysis conducted by Boskovic and Tobutt. Both S-alleles only differ in poi nt mutations, which are not translated into differences in amino-acid seque nce. To our knowledge, this is the first report of two S-alleles that diffe r at the nucleotide level but still encode for identical S-RNases. The impl ications of these observations for determining the S-genotypes of plants by PCR analysis or protein analysis are discussed.