Complex biological systems should often reconcile conflicting selection pressures. Specifically, in systems relying on molecular recognition, molecules should recognize particular partners, but avoid others. Here we study how such selection pressures shape the evolution of the self-incompatibility system in plants. This system inhibits self-fertilization using specific molecular recognition between proteins, expressed in the plant female and male reproductive organs.
We study the impact of these opposing selection pressures on the amino acid frequencies in these proteins’ recognition domain. We construct a theoretical framework enabling promiscuous recognition between proteins and multiple partners each, as found empirically, and employ stochastic simulations. We find asymmetric responses to selection affecting mostly the female, but not the male protein composition. Using large deviations theory, we well-approximate the simulated frequencies and find agreement with genomic data.
Our work offers a general theoretical framework to study the impact of multiple selection pressures, applicable to additional biological systems.