Going against the family: Perturbation of a greenbeard pathway leads to falsebeard cheating

Greenbeards facilitate cooperation by encoding a perceptible signal, the ability to detect it, and a tendency to help others that display it. Falsebeards are hypothetical cheaters that display the signal without being altruistic. Despite many examples of greenbeards, evidence for falsebeards is scarce. The Dictyostelium discoideum tgrB1-tgrC1 allorecognition pathway encodes a greenbeard. It allows development, which yields fruiting bodies with altruistic stalks that increase spore dispersal. Here we show that cells lacking rapgapB, a tgrB1-tgrC1 signaling element, cheat by avoiding the stalk fate and generating more spores in chimeras than in pure populations. rapgapB ^- cells cheat only on partners with compatible tgrB1-tgrC1 allotypes, suggesting that beard display and recognition are intact but decoupled from altruism. The rapgapB ^- falsebeard provides a model to study greenbeard maintenance and subversion.