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

Abstract

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 'Ceremeño', 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 S(21), 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 S(21), so it was named S(21)*. S-genotypes assigned to 'Abugo' and 'Ceremeño' were S(10)S(21)* and S(21)*S(25) respectively, of which S(25) is a new functional S-allele of European pear. Reciprocal crosses between cultivars bearing S(21) and S(21)* indicated that both alleles exhibit the same pollen function; however, cultivars bearing S(21)* had impaired pistil-S function as they failed to reject either S(21) or S (21)* pollen. RT-PCR analysis showed absence of S(21)* -RNase gene expression in styles of 'Abugo' and 'Ceremeño', suggesting a possible origin for S(21)* pistil dysfunction. Two polymorphisms found within the S-RNase genomic region (a retrotransposon insertion within the intron of S(21)* and indels at the 3'UTR) might explain the different pattern of expression between S(21) and S(21)*. Evaluation of cultivars with unknown S-genotype identified another cultivar 'Azucar Verde' bearing S(21)*, 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.