Molecular and Evolutionary Characterization of Pollen S Determinant (SFB Alleles) in Four Diploid and Hexaploid Plum Species (Prunus spp.).

In more than 60 families of angiosperms, the self- and cross-fertilization is avoided through a complex widespread genetic system called self-incompatibility (SI). One of the major puzzling issues concerning the SI is the evolution of this system in species with complex polyploid genomes. Among plums, one of the first fruits species to attract human interest, polyploid species represent enormous genetic potential, which can be exploited in breeding programs. However, molecular studies in these species are very scarce due to the complexity of their genome. In order to study the SFB gene [the male component of gametophytic self-incompatibility system (GSI)] in plum species, 36 plum accessions belonging to diploid and hexaploid species were used. A total of 19 different alleles were identified; 1 of them was revealed after analyzing sequences. Peptide sequence analysis allowed identifying the five domains features of the SFB gene. Polymorphism analysis showed a subtle difference between domesticated and open pollinated Tunisian accessions and suggested a probable influence of the ploidy level. Divergence analysis between studied sequences showed that a new specificity may appear after 5.3% of divergence at synonymous sites between pairs of sequences in Prunus insititia, 6% in Prunus cerasifera, 8% and 9% in Prunus domestica and Prunus salicina respectively. Furthermore, sites under positive selection, the ones more likely to be responsible for specificity determination, were identified. A positive and significant Pearson correlation was found between the divergence between sequences, divergence time, fixed substitutions (MK test), and PSS number. These results supported the model assuming that functionally distinct proteins have arisen not as a result of chance fixation of neutral variants, but rather as a result of positive Darwinian selection. Further, the role that plays recombination can not be ruled out, since a rate of 0.08 recombination event per polymorphic sites was identified.