Variable patterns of density-dependent survival in social bacteria

In numerous species of social animals and social microorganisms, fitness is positively dependent on population density, at least in some environments and over some density ranges. This Allee effect is observed in the cooperative bacterium Myxococcus xanthus during multicellular fruiting body development, during which the standard laboratory genotype sporulates less efficiently at lower population densities and produces no spores below a minimum threshold density. Here we demonstrate significant quantitative variation in Allee patterns among distinct natural isolates of M. xanthus. Isolates with similar developmental performance at intermediate population densities exhibit stark variation in performance at both very low and very high densities. Such variation has implications for evolutionary performance under fluctuating natural environments. It also suggests that distinct intraspecific populations of social animals and other social microbes with different selective histories may vary in the effects of density on social fitness.