Analysis of the S-locus structure in Prunus armeniaca L.: Identification of S-haplotype specific S-RNase and F-box genes

The gametophytic self-incompatibility (GSI) system in Rosaceae has been proposed to be controlled by two genes located in the S-locus: an S-RNase and a recently described pollen expressed S-haplotype specific F-box gene (SFB). However, in apricot ( Prunus armeniaca L.) these genes had not been identified yet. We have sequenced 21 kb in total of the S-locus region in 3 different apricot S-haplotypes. These fragments contain genes homologous to the S-RNase and F-box genes found in other Prunus species, preserving their basic gene structure features and defined amino acid domains. The physical distance between the F-box and the S-RNase genes was determined exactly in the S2-haplotype (2.9 kb) and inferred approximately in the S-1-haplotype (<49 kb) confirming that these genes are linked. Sequence analysis of the 5' flanking regions indicates the presence of a conserved region upstream of the putative TATA box in the S-RNase gene. The three identified S-RNase alleles (S-1, S-2 and S-4) had a high allelic sequence diversity (75.3% amino acid identity), and the apricot F-box allelic variants (SFB1, SFB2 and SFB4) were also highly haplotype-specific (79.4% amino acid identity). Organ specific-expression was also studied, revealing that S-1- and S-2-RNases are expressed in style tissues, but not in pollen or leaves. In contrast, SFB1 and SFB2 are only expressed in pollen, but not in styles or leaves. Taken together, these results support these genes as candidates for the pistil and pollen S-determinants of GSI in apricot.