Apple S locus region represents a large cluster of related, polymorphic and pollen-specific F-box genes

Gametophytic self-incompatibility (GSI) of Rosaceae, Solanaceae and Plantaginaceae is controlled by a complex S locus that encodes separate proteins for pistil and pollen specificities, extracellular ribonucleases (S-RNases) and F-box proteins SFB/SLF, respectively. SFB/SLFs of Prunus (subfamily Prunoideae of Rosaceae), Solanaceae and Plantaginaceae are single copy in each S haplotype, while recently identified pollen S candidates SFBBs of subfamily Maloideae of Rosaceae, apple and Japanese pear, are multiple; two and three related SFBBs were isolated from each S haplotype of apple and Japanese pear, respectively. Here, we show that apple (Malus × domestica) SFBBs constitute a gene family that is much larger than initially thought. Twenty additional SFBB-like genes/alleles were isolated by screening of a BAC library derived from S3S9 genotype, and tentatively named MdFBX1-20. All but one MdFBX showed S haplotype-specific polymorphisms. All the polymorphic MdFBXs were completely linked to S-RNase in 239 segregants. In addition, FISH revealed that the monomorphic gene MdFBX11 is also located near S-RNase, and the S locus is located in a subtelomeric region of a chromosome and is not close to the centromere. All MdFBXs were specifically expressed in pollen, except for a pseudogene MdFBX4 that showed no expression in any organs analyzed. Phylogenetic analysis revealed that the closest relatives of most MdFBXs were from a different S haplotype, suggesting that proliferation of MdSFBB/FBXs predates diversification of the S haplotypes.