Two Different Prunus SFB Alleles Have the Same Function in the Self-incompatibility Reaction

Many species in the families of Rosaceae, Solanaceae, and Scrophulariaceae exhibit gametophytic self-incompatibility, a phenomenon controlled by two polymorphic genes at the S-locus, style-S (S-RNase) and pollen-S (SFB). Sequences of both genes show high levels of diversity, characteristic of genes involved in recognition of self-incompatibility systems in plants. In this study, S (24) -RNase and SFB (24) alleles were cloned from Prunus armeniaca cv. Chuanzhihong (Chinese apricot). Sequence comparisons of deduced amino acid sequences revealed that the P. armeniaca S (24) -haplotype has different SFB alleles, but shares a single S-RNase allele with P. armeniaca S (4) -haplotype. Moreover, P. armeniaca S (24) -RNase haplotype has a single and three different alleles with S (1) -RNase of P. tenella (dwarf almond) and S (1) -RNase of P. mira (smooth pit peach), respectively. The functionalities of SFB (24) and SFB (4) have been evaluated by pollen tube growth and controlled field tests of P. tenella and P. mira. Genetic analysis of the two intercrosses showed that progenies segregated 1:1 into two S-genotype classes, which is consistent with the expected ratio for semi-compatibility. These findings imply that the allelic function of the S (24) -haplotype is identical to that of the S (4) -haplotype in a self-incompatibility reaction. Thus, these two Prunus S-haplotypes are in fact two neutral variants of the same S-haplotype. The evolution of the S-allele is also discussed in terms of both functions and differences between S (24) - and S (4) -haplotypes in Prunus.