A modifier locus affecting the expression of the S-RNase gene could be the cause of breakdown of self-incompatibility in almond.

Self-compatibility has become the primary objective of most almond (Prunus amygdalus Batsch) breeding programmes in order to avoid the problems related to the gametophytic self-incompatibility system present in almond. The progeny of the cross 'Vivot' (S(23)S(fa)) x 'Blanquerna' (S(8)S(fi)) was studied because both cultivars share the same S(f) allele but have a different phenotypic expression: active (S(fa)) in 'Vivot' and inactive (S(fi)) in 'Blanquerna'. In addition, the microscopic observation of pollen tube growth after self-pollination over several years showed an unexpected self-incompatible behaviour in most seedlings of this cross. The genotypes of this progeny showed that the S(fi) pollen from 'Blanquerna' was not able to grow down the pistils of 'Vivot' harbouring the S(fa) allele, confirming the active function of this allele against the inactive form of the same allele, S(fi). As self-compatibility was observed in some S(8)S(23) and S(8)S(fa) individuals of this progeny, the S(f) haplotype may not always be linked to the expression and transmission of self-compatibility in almond, suggesting that a modifier locus may be involved in the mechanism of self-incompatibility in plants.