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Figure 1.  Genetic control of gametophytic self-incompatibility (GSI) in Malus. The S-locus is composed of two tightly linked components, found in the pollen and pistil respectively. In GSI, the pollen self-incompatibility phenotype is controlled gametophytically, i.e., the genotype of the haploid pollen itself (gametophyte) determines its incompatibility type. For example, the pollen composition of a certain pollen donor plant is phenotypically half S₁ and half S₂. In the female parent, two alleles are co-dominant and both are expressed in the pistil. Pollen inhibition occurs when there is a match between the donor pollen S-haplotype and either of the two haplotypes present in the pistil, producing an incompatible reaction that inhibits the growth of the 'self' pollen tube growth. Three types of reactions can occur during a cross: (a) incompatible; neither of the two gametes will germinate, (b) semi-compatible; half the donor pollen will be inhibited and the other half will germinate and grow normally, (c) compatible; all pollen will germinate and grow normally.

Figure 2.  High resolution melting (HRM) curve profiles of seven S-allele-specific markers. Amplification curves of real-time PCR marker assays (left panels), HRM difference plots, where the derivative fluorescence signal (dF/dT) is plotted as a function of temperature (right panels). Each colour represents a specific S-genotype as shown by the legends. Light grey represents samples that were not amplified in the real-time PCR.

Figure 3.  Frequency of S-alleles and S-genotypes of the 183 apple advanced selections of the PFR’s breeding programme. Inner plot shows the percentage frequency distribution of S-alleles from the total 366 alleles observed among the 183 genotypes tested. Outer plot represents the absolute frequency of each S-genotype. All outer slices not showing a percentage value in the figure represent 0.3% respectively.