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.