Signatures of antagonistic pleiotropy in a bacterial flagellin epitope

Immune systems respond to “non-self” molecules termed microbe-associated molecular patterns (MAMPs). Microbial genes encoding MAMPs have adaptive functions and are thus evolutionarily conserved. In the presence of a host, these genes are maladaptive and drive antagonistic pleiotropy (AP) because they promote microbe elimination by activating immune responses. The role AP plays in balancing the functionality of MAMP-coding genes against their immunogenicity is unknown. To address this, we focused on an epitope of flagellin that triggers antibacterial immunity in plants. Flagellin is conserved because it enables motility. Here, we decode the immunogenic and motility profiles of this flagellin epitope and determine the spectrum of amino acid mutations that drives AP. We discover two synthetic mutational tracks that undermine the detection activities of a plant flagellin receptor. These tracks generate epitopes with either antagonist or weaker agonist activities. Finally, we find signatures of these tracks layered atop each other in natural Pseudomonads.