F-Box Genes and the Evolution of the S-Locus in the Pyrinae

The Malus (apple) and Pyrus (pear) genera belong to the subtribe Pyrinae, formerly known as the subfamily Maloideae. Like many other species of the Rosaceae, they exhibit a gametophytic self-incompatibility (GSI) mechanism which is genetically determined by a single multigenic and highly polymorphic locus (S-locus). The S-locus controls pollen-pistil recognition through different genes; a stylar ribonuclease (S-RNase) is the female (pistil) determinant whereas its male (pollen) counterpart(s) might consist of one or more F-box proteins. The present study reports the characterization of S-locus F-box genes from a group of apple and pear (European, Japanese and Chinese pear) cultivars, selected on the basis of their S-allele composition. The S-locus structure and diversity in the Pyrinae is analyzed considering the number of F-box genes characterized for each S-haplotype, their genetic linkage to the S-RNase and their phylogenetic relationships within and among species. Our results provide new insight into the evolution of the S-locus within this taxonomic group, and the role that the different S-locus F-box genes might have in self-incompatibility