Self-Incompatibility in Olive: a New Hypothesis on the S-Locus Genes Controlling Pollen-Pistil Interaction

Self-incompatibility is one of the most important barriers adopted by many flowering plants to prevent inbreeding, thus maintaining diversity within species. The elucidation of this reproductive constraint is crucial in olive (Olea europaea L.) because it may help to improve breeding strategies and orchard management. The available literature reports that a gametophytic self-incompatibility (GSI) system might exist in olive, even though neither cytological nor molecular data were provided. A molecular approach failed to find S-RNase (S-locus ribonuclease) and SLF (S-locus F-box containing protein), the genes responsible for GSI. In this paper, cyto-histological observations and bio-molecular evidence is presented, supporting the presence in olive of a sporophytic self-incompatibility (SSI) system. The main genes known to play a crucial role in SSI are SRK (S-locus receptor kinase) and SLG (S-locus glycoprotein), the female determinants, and SCR (S-locus cysteine rich protein), the male determinant. OeSLG and OeSRK genes cloned by PCR with degenerated primers were shown to be differentially expressed in flower organs of self-compatible (cv Frantoio) and self-incompatible (cv Leccino) genotypes by means of Real-Time PCR analysis. Since the same approach failed to isolate SCR, a 454 pyrosequencing library derived from flowers at different developmental stages was screened. One of the sequences displaying a conserved amino acid structure in terms of cysteine residuals, named OeSCR-like, proved to be specifically expressed in anthers at blooming stages. The overall data so far collected support the existence of an SSI system in olive, at least in the tested genotypes.