Correlation of stylar ribonuclease zymograms with incompatibility alleles in sweet cherry

Protein stylar extracts of 16 cultivars of sweet cherry (Prunus avium), from the 10 different incompatibility groups to which incompatibility alleles have been assigned, were separated on acrylamide gels using isoelectric focusing (IEF) and were stained for ribonuclease activity. When two cultivars from the same incompatibility group were analyzed they gave identical zymograms and the cultivars of the 10 different incompatibility groups gave in all eight distinct zymograms. The ribonuclease polymorphism could be correlated with the reported S allele constitutions of the cultivars. Three ribonuclease bands were identified that each consistently corresponded to one of the six known incompatibility alleles (S-1, S-2 and S-6), a fourth band apparently corresponded to S-3 and to the combination of S-4 and S-5, and a fifth band to S-4 and S-5 in other combinations. Thus, it seems that S alleles of cherry have ribonuclease activity and that IEF is useful for distinguishing S allele constitutions. The ribonuclease pattern of 'Summit', a cultivar of unknown incompatibility group, indicated its incompatibility genotype to be S1S2, and this was confirmed by controlled pollination. The same band corresponded to S-4 and S-4', the mutant allele in self-compatible cultivars. IEF and ribonuclease staining promise to be useful tools for exploring the incompatibility relationships of cherry cultivars and perhaps of other self-incompatible Prunus crops.