Identification, Evolutionary Patterns and Intragenic Recombination of the Gametophytic Self Incompatibility Pollen Gene (SFB) in Tunisian Prunus Species (Rosaceae)

Plum (Prunus L.) is a species which exhibits a gametophytic self-incompatibility system GSI as the majority of species belonging to the Rosaceae family. The self-incompatibility system is genetically controlled by a single and multiallelic locus named the S locus which contains at least two linked genes, one is expressed in pistil named S-RNase gene, the second is expressed in pollen called SFB gene (S-haplotype-specific F-box). In this work, 16 Tunisian plums cultivars were considered. The PCR amplification of the SFB gene and the automatic sequencing of amplicons have identified the SFB alleles of each variety. The analysis of both nucleotide and peptide sequences shows the high degree of polymorphism in SFB alleles. Moreover, the mutational events show a high proportion of transversions and nonsynonymous mutations. Furthermore, sequence analysis has proved that the intragenic recombination can occur within the SFB gene. Regions characterizing the SFB gene: the F-Box conserved domain, and three variable and two hypervariable regions were detected. These (hyper) variable regions seem to be hydrophilic or at least not strongly hydrophobic. Dendrograms were basically constructed on nucleotide, and peptide sequences provide relationships and similarity of cultivars representing the same allele independently from geographic origin and provide a valuable scenario on the evolutionary history of the SFB gene. Moreover, the neutrality tests show that the SFB gene is the seat of positive selection that plays in favor of the rare alleles. Finally, the study of the relationship between the SFB gene and the S-RNase gene shows that both genes are contiguous and have an opposite transcription direction. To conclude, the study of self-incompatibility genes, as ideal markers of the self-incompatibility system, forms an essential node for understanding the origin and evolution of this system.