Evaluating Self-Incompatibility in Citrus by Histological and Molecular Assays

Self-incompatibility (SI) is a system used by flowering plants to prevent self-fertilization and promote outbreeding, ensuring genetic diversity. The system is regulated molecularly by the single polymorphic S-locus that expresses the male and female determinants that interact, inhibiting pollen germination or pollen tube growth when pollen from the same plant or genetically similar individuals reaches the stigma. Knowing the self-incompatible phenotype among cultivars is fundamental to ensuring fruit set or obtain seedless fruits in parthenocarpic species. Here we describe three techniques to measure self-incompatibility: (i) the self-incompatibility index (ISI); (ii) fluorescence microscopy to observe pollen germination and pollen tube growth after pollination; (iii) molecular analysis to assess diversity at the S-locus and expression of S-alleles.