Correlation of stylar ribonuclease isoenzymes with incompatibility alleles in apple

Fifty-six cultivars of apple were analysed for stylar ribonucleases; proteins were extracted from styles, separated by non-equilibrium pH gel electrofocusing and stained for activity. Excellent correlation was found between the ribonuclease bands revealed and the 11 known incompatibility, S, alleles, in 14 diploid cultivars genotyped in the classic work of Kobel by monitoring pollen tube growth after test crossing, and in 20 cultivars genotyped, at least partially, by more recent DNA methods. For 12 triploid cultivars studied by Kobel, the correlation was good but not perfect. Two apparent minor electrophoretic variants for S-10 were noted and, to distinguish them from each other and also from the electrophoretically similar S-3, isoelectric focusing was used. Ten cultivars were genotyped for the first time. In all, 14 ribonuclease bands that may correspond to the 'new' S alleles, S-12 to S-25, Were detected but these alleles should be regarded as provisional until confirmed by pollination tests, especially when the electrophoretic differences were only slight. Analysis of stylar ribonucleases is a convenient method of predicting S alleles in flowering material and thereby investigating incompatibility relationships. The polymorphism of the S locus makes it useful for checking the identity and parentage of cultivars.