Pistil-function breakdown in a new S-allele of European pear, S21*, confers self-compatibility

European pear exhibits RNase-based gametophytic self-incompatibility controlled by the polymorphic S-locus. S-allele diversity of cultivars has been extensively investigated; however, no mutant alleles conferring self-compatibility have been reported. In this study, two European pear cultivars, 'Abugo' and 'CeremeA +/- o', were classified as self-compatible after fruit/seed setting and pollen tube growth examination. S-genotyping through S-PCR and sequencing identified a new S-RNase allele in the two cultivars, with identical deduced amino acid sequence as S21, but differing at the nucleotide level. Test-pollinations and analysis of descendants suggested that the new allele is a self-compatible pistil-mutated variant of S21, so it was named S21*. S-genotypes assigned to 'Abugo' and 'CeremeA +/- o' were S10S21* and S21*S25 respectively, of which S21 is a new functional S-allele of European pear. Reciprocal crosses between cultivars bearing S21 and S21* indicated that both alleles exhibit the same pollen function; however, cultivars bearing S21* had impaired pistil-S function as they failed to reject either S21 or S21* pollen. RT-PCR analysis showed absence of S21*-RNase gene expression in styles of 'Abugo' and 'CeremeA +/- o', suggesting a possible origin for S21* pistil dysfunction. Two polymorphisms found within the S-RNase genomic region (a retrotransposon insertion within the intron of S21* and indels at the 3'UTR) might explain the different pattern of expression between S21 and S21*. Evaluation of cultivars with unknown S-genotype identified another cultivar 'Azucar Verde' bearing S21*, and pollen tube growth examination confirmed self-compatibility for this cultivar as well. This is the first report of a mutated S-allele conferring self-compatibility in European pear.