Self-incompatibility of Prunus tenella and evidence that reproductively isolated species of Prunus have different SFB alleles coupled with an identical S-RNase allele

Many species of Prunus display an S-RNase-based gametophytic self-incompatibility (SI), controlled by a single highly polymorphic multigene complex termed the S-locus. This comprises tightly linked stylar- and pollen-expressed genes that determine the specificity of the SI response. We investigated SI of Prunus tenella, a wild species found in small, isolated populations on the Balkan peninsula, initially by pollination experiments and identifying stylar-expressed RNase alleles. Nine P. tenella S-RNase alleles (S-1-S-9) were cloned; their sequence analysis showed a very high ratio of non-synonymous to synonymous nucleotide substitutions (K-a/K-s) and revealed that S-RNase alleles of P. tenella, unlike those of Prunus dulcis, show positive selection in all regions except the conserved regions and that between C2 and RHV. Remarkably, S-8-RNase, was found to be identical to S-1-RNase from Prunus avium, a species that does not interbreed with P. tenella and, except for just one amino acid, to S-11 of P. dulcis. However, the corresponding introns and S-RNase-SFB intergenic regions showed considerable differences. Moreover, protein sequences of the pollen-expressed SFB alleles were not identical, harbouring 12 amino-acid replacements between those of P. tenella SFB8 and P. avium SFB1. Implications of this finding for hypotheses about the evolution of new S-specificities are discussed.