The Prunus self-incompatibility locus (S locus) is seldom rearranged

Self-incompatibility enables flowering plants to discriminate between self- and non-selfpollen. In Prunus, the 2 genes determining specificity are the S-RNase (the female determinant that is a glycoprotein with ribonuclease activity) and the SFB (the male determinant, a protein with an F-box motif). In all Prunus S haplotypes characterized so far, with the exception of Prunus armeniaca S(2) haplotype, the 2 genes have opposite transcription orientations. Nevertheless, the relative transcription orientation observed in P. armeniaca S(2) haplotype has been postulated to be the one present in all S haplotypes from this species. We show that this is not the case by demonstrating that that the relative transcription orientation of the pollen and pistil genes of the P. armeniaca S(17) haplotype is that which is commonly found in Prunus. Using a phylogenetic approach, we show that the relative transcription orientation of the S-RNase and SFB genes is seldom changed (less than once every 380 million years). This contrasts with the Brassica sporophytic S locus where chromosomal rearrangements are often observed in the region between the pollen and pistil genes.