Structural features of the s locus of apple
2012
Sassa, H. | Wang, S. | Minamikawa, M. | Kakui, H. | Kikuchi, S. | Koba, T.
Apple (Malus × domestica) exhibits gametophytic self-incompatibility (GSI) in which at least two specificity genes, pistil S and pollen S genes, located at the complex S locus region control self/non-self discrimination between pistil and pollen. The pistil S gene encodes a highly polymorphic extracellular ribonuclease, S-RNase, that is considered to function as a cytotoxin for the self pollen tube, resulting in arrested growth. Recent findings on Solanaceae, Antirrhinum and Prunus showed that pollen S genes of these plants encode F-box proteins (SLF/SFB), raising the possibility that SLF/SFB ubiquitinates non-self S-RNases for degradation to cope with their cytotoxic effect. We found that the apple S locus contains more than ten F-box genes that are related to each other and are specifically expressed in pollen, and we named these F-box genes S locus F-box brothers (SFBB). FISH (fluorescence in situ hybridization) analysis revealed that all the SFBB genes are physically close to the S-Rnase gene, and are located at the heterochromatic and subtelomeric region of the chromosome. The heterochromatic nature around the S locus would contribute to repress recombination at the region for maintenance of the GSI function. These findings suggest that many, if not all of the apple SFBB genes would function as component of pollen S to target non-self S-RNases, as has been demonstrated in the solanaceous plant petunia.
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