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Self-incompatibility is a common genetic system preventing selfing and enforcing outcrossing in hermaphroditic flowering plants. In Brassicaceae, self-incompatibility involves the segregation of a very large allelic series forming a simple linear, transitive dominance hierarchy. In this seminar, I will detail a set of experiments and theoretical models aiming at unraveling the evolutionary processes and molecular mechanisms by which dominance and recessivity is achieved between these numerous S-alleles. I will show that they involve the action of so-called "dominance modifiers" whose existence had been conjectured almost a century ago, but had remained elusive so far, and take the form of trans-acting small non-coding RNAs. I will describe our efforts to understand the regulatory pathway(s) by which these genetic elements cause
transcriptional silencing of the recessive S-alleles in heterozygote combinations.
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