Evolutionary ecology of predation by the soil bacterium, Myxococcus xanthus

The ability of a predator to kill prey is partially determined by features of the predatory environment. This relationship may be modified by the evolution of traits involved in searching for prey or handling (capturing, killing, and consuming) prey once they have been found. The course of such predatory evolution may depend on the same ecological variables that affect prey-killing ability. I have sought to better understand the relationships between ecological variables, predatory performance, and evolution in the soil bacterium Myxococcus xanthus by designing predation arenas that consisted of square petri dishes filled with buffered agar that had patches of prey bacteria (Escherichia coli or Micrococcus luteus) distributed in a grid on top of the agar. I used these predation arenas to test the effects of several ecological variables on predatory performance and on the evolution of predatory traits.; Assays with these arenas showed that predatory performance of M. xanthus is influenced by the prey species that is available, surface hardness, and food availability. In general, M. xanthus swarms expanded over a greater area such that they could attack more prey when resources were common compared to when they were scarce, regardless of whether the resources were prey patches or homogeneous distributions of synthetic nutrients. Resource level also modified the response of M. xanthus swarms to surface hardness. On low-nutrient surfaces M. xanthus swarmed faster on hard compared to soft agar. This ranking was reversed if nutrients were distributed at high concentrations. Examination of the swarming rate of motility mutants across a range of casitone and agar concentrations indicated that this result was caused by elevated swarming by the social gliding motility system at high nutrient concentrations and was facilitated by extracellular structures called fibrils.; I also used the predation arenas to test whether there was a trade-off between adapting to a prey-free environment and being a good predator. Eight populations that evolved in a liquid, prey-free environment for 1000 generations were all worse than the ancestor at encountering prey patches and killing prey in shaking liquid, indicating that adaptation to this environment generally involved loss of predatory ability.; Finally, predation arenas were used to test whether prey density affects the evolution of searching and handling of prey, and if the effects depend on the relative impact of these traits on the rate of prey consumption. As predicted, evolution of eight populations in a low patch-density environment for ∼100 generations consistently led to an increase in the rate at which patches were encountered by the swarm and a 7-fold overall increase in the rate of swarming across the surface between patches (searching). The degree of searching improvement of eight populations that evolved in a high patch-density environment was less pronounced (∼2-fold). Handling of prey patches improved slightly overall, but the extent of improvement was not affected by patch density, as had been predicted. These results show that searching improvements have a greater effect on fitness in the low-density environment where more searching is required for consumption of each patch.