The evolution of self-incompatibility: a molecular voyeur's perspective

This review summarises current understanding of the evolution of self-incompatibility inferred from DNA sequence analysis. Self-incompatibility in many plant families is controlled by a single, highly polymorphicS-locus which, in the Solanaceae, encodes an allelic series of stylar ribonucleases known as the S-RNases. PCR approaches are a convenient way to examine the diversity of S-RNase sequences within and between wild populations of a self-incompatible species and provide a unique view into the species' current and historic population structure. Similar molecular appoaches have also been used to show that S-RNases are involved in self-incompatibility in families other than the Solanaceae. A model for the evolution of ribonuclease-based self-incompatibility systems is discussed.     

S-RNases in apple are expressed in the pistil along the pollen tube growth path

 The molecular bases of self-incompatibility have been intensively studied in a restricted number of model species, but for most families the expression and distribution of S-proteins is unknown. In this work, pistil cryosections from apple were used for in situ detection of S-proteins. Two specific antibodies, one against the S3-protein and another against all apple S-proteins were used. S-proteins were shown to be localised in the intercellular space of the transmitting tissue, both in the stigma and style, which agrees with the proposed mechanism of action for S-RNases in gametophytic self-incompatibility. Some intracellular labelling was also observed in all ovary sections, confined to one layer of the nucellus surrounding the embryo sac, but this labelling was found to be non-S-allele-specific. Nevertheless, the signal in the ovary was tissue-specific, which may indicate that some component not encoded by the S-locus but similar to S-proteins was detected. To the best of our knowledge this is the first report on the precise distribution of S-RNases in a rosaceous species. Received: 15 July 1998 / Revision accepted: 29 December 1998