How leaking and overproducing resources affect the evolutionary robustness of cooperative cross-feeding.

Cooperative cross-feeding, a resource-exchange mutualism between microbes, is ubiquitous; however, models suggest it should be susceptible to cheating. Recent work suggested two novel mechanisms that could allow cross-feeders to exclude cheaters, even in the absence of tight coupling between cooperative organisms. The first is pattern formation, where cross-feeders form regular patterns so that their resources are separated and cheaters cannot obtain both. The second mechanism is neighbor uncertainty, where demographic stochasticity separates resources so cheaters cannot obtain both. Here we use a stochastic spatial model to test whether those mechanisms are evolutionarily stable, or whether they will collapse under gradual evolution towards reduced resource production. The answer depends on whether a microbe can make the resource for itself without sharing it. If it cannot (i.e. if producing more of a resource means sharing more of a resource), then both mechanisms continue to function. In this case, resource production directly benefits the individual, and cooperation is a byproduct. If microbes can make the resource without sharing it (i.e. if production is an altruistic trait), then neighbor uncertainty completely fails, and pattern formation is weakened. In this case, the costly trait has no direct benefit to the individual, and can only persist if cooperative organisms become associated with their partner. Thus, the novel mechanisms, which operate without tight associations, falter. These results have implications for synthetic ecology, as they suggest that how cross-feeding is engineered will impact its evolutionary stability.