Goring, Daphne R. 2016. Nature plants (December 22).
A new player and mode of action has been discovered in the creation of a dominance hierarchy in the Brassicaceae self-incompatibility system.
When dominant and recessive versions of a particular trait are present, the variation is typically encoded by different forms or alleles of a single locus. An alternative scenario that has been long debated in the literature is the dominance modifier model1, where a secondary locus influences alleles of the first locus to modify dominant–recessive relationships. These modifiers were proposed to allow for adaptive change in the evolution of dominance1. The Brassicaceae self-incompatibility system represents a unique and real example of the dominance modifier model. The female S-LOCUS RECEPTOR KINASE (SRK) gene and the male S-LOCUS PROTEIN 11/S-LOCUS CYSTEINE-RICH (SP11/SCR) gene are the key regulators of this system2, and the linked SRK and SP11/SCR loci exist in a series of alleles termed S-haplotypes. Many of the S-haplotypes are co-dominant, and pollinations between flowers carrying the same S-haplotypes lead to pollen rejection by the self-incompatibility pathway3. A number of dominant–recessive relationships have also been documented4,5, and in these cases, for example, the male recessive S-haplotype is supressed, leading to pollen acceptance and fertilization in flowers sharing the same recessive S-haplotype. While this compromisd the outcrossing advantage of self-incompatibility, it can also increase the number of potential mates in a population. In this issue of Nature Plants, Yasuda et al.6 identify a new dominance modifier that targets the male SP11/SCR gene to regulate a dominance hierarchy in Brassica rapa class-II S-haplotypes.