Identification of seven S-alleles from the European pear (Pyrus communis) and the determination of compatibility among cultivars

The European pear (Pyrus communis) belongs to the Rosaceae, which carries the gametophytic self-incompatibility fertilization system (GSI). GSI is haplotype specific and controlled by the S-locus. The S-locus carries a multi-allelic gene that encodes for RNase (S-RNase), which is involved in executing pollen rejection in a specific manner. The different S-alleles also serve for molecular identification of the S-haplotypes. Orchards of trees carrying GSI contain at least two cultivars, which are fertilization-compatible with each other. However, compatibility may be full when the cultivars differ in both their S-loci or partial when they share one of the two S-loci. Semi-compatibility between cultivars can lead to yield reduction, since half of the pollinator's pollen is rejected. Identification of semi-compatibility according to yield levels is not always trivial, since many factors are involved in yield production. On the contrary, the application of molecular analysis, for the identification of S-RNase alleles can clearly determine the genetic compatibility between cultivars. Many S-alleles of Rosaceae fruit trees have been identified, but only a few of them are from European pear cultivars. In Israel the yield of the European pear is relatively low. To elucidate if semi-S-allele compatibility could provide the explanation, the S-allele content of the four cultivars grown in the Israel was determined. It was found that 'Spadona', the main cultivar carries the alleles Sj and Sk, and its pollinators, 'Coscia', 'Gentile' and 'Spadochina' carry Sb-Sk, Si-Sj and Sk-Sl, respectively. Thus, each of the pollinators shares one of the 'Spadona' S-alleles and therefore is semi-compatible with it. All of the alleles are new, apart from Sb, which has been identified previously by Zuccherelli et al. (2002). To identify potential fully compatible pollinators for 'Spadona', five additional cultivars that are being considered for introduction into the Israel pear orchard were analysed. It was discovered that 'Bon Rouge', 'Doctor Jill Guyot', 'Forelle' and 'Red Clapp' are semi-compatible and that 'Lawson' is fully compatible with 'Spadona'. In this latter analysis, three additional European pear S-alleles, Sm, Sn and So were identified.