Gene duplication and genetic exchange drive the evolution of S-RNase-based self-incompatibility in Petunia

Self-incompatibility (SI) systems in flowering plants distinguish self- and non-self pollen to prevent inbreeding. While other SI systems rely on the self-recognition between specific male- and female-determinants, the Solanaceae family has a non-self recognition system resulting in the detoxification of female-determinants of S-ribonucleases (S-RNases), expressed in pistils, by multiple male-determinants of S-locus F-box proteins (SLFs), expressed in pollen. It is not known how many SLF components of this non-self recognition system there are in Solanaceae species, or how they evolved. We identified 16-20 SLFs in each S-haplotype in SI Petunia, from a total of 168 SLF sequences using large-scale next-generation sequencing and genomic polymerase chain reaction (PCR) techniques. We predicted the target S-RNases of SLFs by assuming that a particular S-allele must not have a conserved SLF that recognizes its own S-RNase, and validated these predictions by transformation experiments. A simple mathematical model confirmed that 16-20 SLF sequences would be adequate to recognize the vast majority of target S-RNases. We found evidence of gene conversion events, which we suggest are essential to the constitution of a non-self recognition system and also contribute to self-compatible mutations.