The process by which new phenotypic traits can emerge in a population is a challenging question in the field of evolutionary biology, especially for phenotypes controlled by complex genetic networks requiring coordinated molecular changes between interacting partners. Addressing this issue requires an integrated understanding of both the genotype-to-phenotype map and the fitness landscape, which are hard to determine empirically for most phenotypes. To shed light on this biological process, we focused on the self-incompatibility system in outcrossing Arabidopsis species. Self-incompatibility is a reproductive system by which hermaphrodite flowering plants recognize and specifically reject self-pollen. In the Brassicaceae, it is based on a molecular lock-and-key mechanism involving two genes (the male SCR and the female SRK gene). Both display large allelic series and are tightly linked in a small non-recombining region which ensures strict haplotypic association between co-adapted alleles. The existence of this large diversity of S-haplotype has been recognized early on, and several theoretical models have been proposed for how it might arise. However, each of them has received some criticism so that, we currently still don’t have a good understanding of how new S-alleles come into existence.
We decided to decipher this mechanism by travelling in time. An ancestral resurrection approach is ongoing, whereby we are currently regenerating in planta the ancestral SRK alleles of two closely related but functionally distinct A.halleri haplotypes by genetic transformation into A.thaliana. Native SCR and SRK alleles from both haplotypes have already been transformed and successful restoration of the self-incompatible phenotype has been validated through cross-pollination experiments. We are now generating plants expressing the ancestral SCR and SRK alleles, which we will use to functionally challenge the ancestral components with their two extant descendant alleles. This will enable us to determine the mutational path followed to create a new functionally divergent self-incompatible allele.