Molecular Basis of Self-incompatibility in Fruit Tree Species in Prunus

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It has been known that the specificity of the self-incompatibility system in Prunus is determined by the pistil- and pollen-determinants, S-RNase (S-ribonuclease) and SFB (S haplotype-specific F-box protein), respectively; however, the molecular mechanism underlying the self-incompatibility reaction is still unclear. This study was conducted to elucidate the molecular basis of the gametophytic self-incompatibility system in Prunus (Rosaceae). To this end, we cloned and sequenced the Slocus region of several self-compatible mutant S haplotypes of cherries, almonds, Japanese apricots, and Japanese plums. It appeared that most of the self-compatible S haplotypes encode truncated dysfunctional SFB, indicating that mutation in SFB confers self-compatibility in Prunus. Molecular cloning of the self-compatible peach also indicated that SC in peach is conferred by mutation in its SFB. These observations are in contrast to the solanaceous self-incompatibility system, in which dysfunction in the pollen-determinant leads to self- and cross-incompatibilities. Furthermore, genetic and molecular analyses using the tetraploid sour cherry demonstrated that the presence of two different pollen S alleles in a pollen grain does not confer self-compatibility in Prunus. Namely, the well-known phenomenon of competitive interaction is ablsent in Prunus, as opposed to the solanaceous system. Based on these results, we have proposed the one-allele match model to explain the molecular basis of self-incompatibility system in Prunus.