Asymmetrical diversification of the receptor-ligand interaction controlling self-incompatibility in Arabidopsis

How two-components genetic systems accumulate evolutionary novelty and become diversified in the course of evolution is a fundamental problem in evolutionary systems biology. In the Brassicaceae, self-incompatibility (SI) is a spectacular example of a diversified allelic series in which numerous highly diverged receptor-ligand combinations are segregating in natural populations. However, the evolutionary mechanisms by which new SI specificities arise in the first place have remained elusive. Using in planta ancestral protein resurrection, we demonstrate that two allelic variants currently segregating as distinct receptor-ligand combinations diverged through an asymmetrical process whereby one variant has retained the same recognition specificity as the (now extinct) ancestor, while the other has functionally diverged and now represents a novel specificity no longer recognized by the ancestor. Examination of the structural determinants of the shift in binding specificity suggests that allosteric changes may be an important source of evolutionary novelty in this highly diversified receptor-ligand system.