Abstracts (first author)
The paradigma of irreversibility in gametophytic self-incompatibility: the Malus and Prunus systems
In the present hypothesis, S-RNase-based gametophytic self-incompatibility (GSI) evolved once before the split of the Asteridae and Rosidae, but the Rosaceae Malus and Prunus GSI systems are different: in Prunus the S-RNase gene presents two introns and the S-pollen is a single F-box gene (called SFB). The two genes show evidence for a partially coevolved history, both presenting high levels of synonymous and non-synonymous divergence, as well as positively selected amino acid sites that account for the specificities present in natural populations. In Malus, the S-RNase gene has one intron, and multiple S-pollen F-box genes (called SFBBs) have been described. Levels of diversity at the S-RNase gene are 10 times higher than at the SFBB genes. Nevertheless, intra-haplotypic diversity of SFBB is similar to the S-RNase gene. Moreover, there is evidence for amino acids under positive selection only when intra-haplotype SFBB genes are analyzed. Thus, it is not surprising that different self-recognition mechanisms have been proposed: in Prunus self S-RNases are protected from degradation by the self-SFB protein, while in Malus each SFBB protein is predicted to interact with a subset of non-self S-RNases that mediates their degradation. Here, we perform a detailed characterization of S-RNase and F-box like genes present in the P. persica and M. domestica genomes. Phylogenetic analyses revealed three duplications that predate Malus and Pyrus speciation. The gene lineage determining GSI function are different in Malus and Prunus. Expression analyses in 12 Malus tissues revealed that two of the S-RNase lineage genes are expressed mostly in pistils, but one has acquired seed expression. In Malus the closest SFB-like gene shows the highest expression in stamen but is also expressed in seeds. An hypothesis for the evolution of Malus and Prunus GSI is presented. Implications of the polyphyletic S-RNase GSI evolution on uncharacterized GSI systems are discussed.