Update on and review of incompatibility (S-) genotypes of apple cultivars

Apple cultivars display a self-incompatibility system that restricts self-fertilization and fertilization between cultivars bearing identical S-alleles. There has been considerable progress in identification of S-alleles in apple in recent years and methods are now available for the accurate S-genotyping of cultivars. Following a recently revised numerical identification system for apple S-alleles, we present the first extensive compilation of apple cultivars with their S-genotypes. This list contains data from our own investigations using S-allele-specific PCR methodology, including a number of new data, as well as published data from various other sources. Eighteen different S-alleles are discriminated, which allowed the determination of the S-genotypes for 150 diploid or triploid European, American, and Japanese cultivars. Many of these cultivars are cultivated worldwide for their fruit. Also included are a number of old, obsolete cultivars and a few nondomestic genotypes. We observed a wide variation in the frequency of S-alleles in the apple germplasm. Three S-alleles (S-2, S-3, and S-9) are very common in the cultivars evaluated, presumably as a result of the widespread use of the same breeding parents, and seven alleles are very rare (S-4, S-6,S-8,S-16,S-22,S-23,S-26).