The Expression of Self-compatibility in Almond May Not Only Be Due to the Presence of the S-f Allele

The pistil (S-RNase) and the pollen [S-haplotype-specific F-box protein (SFB)] components of the S-f allele, presumably conferring self-compatibility in almond {Prunus amygdalus Batsch [syn. P. dulcis (Mill.) D.A. Webb]}, were identified and sequenced in 'Ponc', a local Spanish almond cultivar, confirming their identity with the published sequences of these components. Despite the presence of the S-f allele, the 'Ponc' phenotype was self-incompatible as confirmed by different pollination tests, including self pollen tube growth, fruit set after self-pollination, and fruit set in bagged branches. However, the pistil and the pollen of 'Ponc' were fully viable when pollinated by a cross-compatible pollen or used on a cross-compatible pistil. The fact that 'Ponc' presents two different S-proteins with RNase activity may indicate an active function of its S-f-RNase, whereas in the self-compatible almond cultivars thus far studied, the S-f-RNase has been inactive. This activation indicates that the presence of the S-f allele may not be the exclusive source of self-compatibility in almond, and other factors may also be involved in the expression of almond self-compatibility.