The quorum sensing diversity within and between ecotypes of Bacillus subtilis

Ecological sociobiology is an emerging field that aims to frame social evolution in terms of ecological adaptation. Here we explore the ecological context for evolution of quorum sensing diversity in bacteria, where social communication is limited to members of the same quorum sensing type (pherotype). We sampled isolates of Bacillus subtilis from soil on a microgeographical scale and identified three ecologically distinct phylogenetic groups (ecotypes) and three pherotypes. Each pherotype was strongly associated with a different ecotype, suggesting that it is usually not adaptive for one ecotype to ‘listen’ to the signalling of another. Each ecotype, however, contained one or more minority pherotypes shared with the other B. subtilis ecotypes and with more distantly related species taxa. The pherotype diversity within ecotypes is consistent with two models: first, a pherotype cycling model, whereby minority pherotypes enter a population through horizontal genetic transfer and increase in frequency through cheating the social interaction; and second, an occasional advantage model, such that when two ecotypes are each below their quorum densities, they may benefit from listening to one another. This is the first survey of pherotype diversity in relation to ecotypes and it will be interesting to further test the hypotheses raised and supported here, and to explore other bacterial systems for the role of ecological divergence in fostering pherotype diversity.