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 <i>Dictyostelium discoideum tgrB1-tgrC1</i> 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 <i>rapgapB</i>, a <i>tgrB1-tgrC1</i> signaling element, cheat by avoiding the stalk fate and generating more spores in chimeras than in pure populations. <i>rapgapB</i> ^<i>-</i> cells cheat only on partners with compatible <i>tgrB1-tgrC1</i> allotypes, suggesting that beard display and recognition are intact but decoupled from altruism. The <i>rapgapB</i> ^<i>-</i> falsebeard provides a model to study greenbeard maintenance and subversion.