Pseudomonas aeruginosa Raw sequence reads

Animals use signals to coordinate a wide range of behaviours, from feeding offspring to predator avoidance. This poses an evolutionary problem, because individuals could potentially signal dishonestly to coerce others into behaving in a way that benefits the signaller. Theory suggests that honest signalling is favoured when individuals share a common interest and signals carry reliable information. However, whilst many studies have manipulated signals, to examine how this influences behaviour, it has not been possible to directly test how the behaviour of signallers and receivers evolve in response to manipulation. Here, we exploit the opportunities offered by signalling between bacteria (quorum sensing). Using experimental evolution, phenotypic and genomic approaches, we show that: (1) a reduced relatedness, and therefore reduced common interest between interacting individuals, leads to the relative breakdown of signalling, via both reduced signalling and a reduced response to signal; (2) signal interference selects for lower levels of signalling. More generally, whilst our results provide clear support for signalling theory, we did not find evidence for the previously predicted coercion at intermediate relatedness, suggesting that mechanistic details can alter even the qualitative nature of specific predictions. Furthermore, populations evolved under low relatedness caused less mortality and damage to insect hosts, showing how signal evolution in bacterial pathogens can drive the evolution of virulence in the opposite direction to that usually predicted by theory.