Methylation of the S f locus in almond is associated with S-RNase loss of function

Self-compatibility in almond (Prunus dulcis) is attributed to the presence of the Sf haplotype, allelic to and dominant over the series of S-alleles controlling self- incompatibility. Some forms of the Sf haplotype, however, are phenotypically self-incompatible even though their nucleotide sequences are identical. DNA from leaves and styles from genetically diverse almond samples was cloned and sequenced and then analyzed for changes affecting S f -RNase variants. Epigenetic changes in several cytosine residues were detected in a fragment of 4,700 bp of the 5' upstream region of all self-compatible samples of the S f - RNases, differentiating them from all self-incompatible samples of S f -RNases analyzed. This is the first report of DNA methylation in a Rosaceae species and appears to be strongly associated with inactivation of the S f allele. Results facilitate an understanding of the evolution of self- compatibility/self-incompatibility in almond and other Pru- nus species, and suggest novel approaches for future crop improvement.