Allopatric divergence of cooperators confers cheating resistance and limits the effects of a defector mutation

Studies of microbial social defectors that 'cheat' on cooperative genotypes generally focus on interactions with their cooperative parents, yet in nature defectors may meet diverse cooperators. Genotype-by-genotype interactions may constrain the ranges of cooperators upon which particular defectors can cheat, limiting the cheaters' spread and potentially the overall equilibrium frequency of cheaters. The bacterium Myxococcus xanthus undergoes cooperative multicellular development upon starvation, but some developmental defectors can cheat on cooperators, outcompeting them within mixed groups. We show that a defector disrupted at the signaling gene csgA has a narrow cheating range among diverse natural cooperators owing to antagonisms not specifically targeted at defectors. More strikingly, lab-evolved cooperators only slightly differentiated from the defector have allopatrically evolved beyond its cheating range by accumulating fewer than 20 mutations when development was not directly under selection. Cooperators might diversify not only with respect to which defectors cheat on them, but also in the potential for a particular mutation to reduce expression of cooperative trait or generate a cheating phenotype. We tested this by constructing a new csgA mutation in several highly diverged cooperators. The mutation generated very different sporulation phenotypes – from a complete defect to no defect – indicating that genetic background effects can limit the set of genomes for which a given mutation creates a defector and potentiates cheating. Our results suggest that natural populations feature geographic mosaics of cooperators diversified in susceptibility to cheating by any given defector and in the social phenotypes generated by any given mutation in a cooperation gene.