Dominance Hierarchy Arising from the Evolution of a Complex Small RNA Regulatory Network. Brassicaceae

Sporophytic self-incompatibility (SI) is a genetic system that evolved in hermaphroditic plants to enforce outcrossing (by preventing self-fertilization) and involves a polymorphism in which multiple alleles are maintained and display dominance/recessivity relationships. In the genus Arabidopsis, SI is controlled by a single genomic region “the S-locus”, which contains two tightly linked genes that encode the pollen S-locus cysteine rich (SCR) and stigma S-locus receptor kinase (SRK) recognition proteins whose ligand-receptor molecular docking leads to self-pollen rejection. The prevention of fertilization through self-pollination in the Brassicaceae plant family is determined by the genotype of the plant at this self-incompatibility locus (S-locus). The many alleles at this locus exhibit a dominance hierarchy that determines which of the two allelic specificities of a heterozygous genotype is expressed at the phenotypic level.

In this work, we’ll report how we demonstrated that dominance/recessivity interactions between alleles of the gene controlling pollen self-incompatibility specificity in the plant Arabidopsis halleri is controlled by a complex combination of small RNAs produced by dominant alleles and their target sites in recessive alleles that collectively result in a simple linear dominance hierarchy. The control of such pairwise dominance/recessivity interactions in multi-allele self-incompatibility systems has been a classical puzzle since the pre-molecular days of genetics, and our results now provide a clear molecular basis in this example of balancing selection.