Abstract
In their natural habitat, bacteria are consumed by bacterivorous nematodes; however, they are not simply passive preys. Here we report a defensive mechanism used by certain bacteria to mobilize nematode-trapping fungi to kill nematodes. These bacteria release urea, which triggers a lifestyle switch in the fungus Arthrobotrys oligospora from saprophytic to nematode–predatory form; this predacious form is characterized by formation of specialized cellular structures or ‘traps’. The bacteria significantly promote the elimination of nematodes by A. oligospora. Disruption of genes involved in urea transport and metabolism in A. oligospora abolishes the urea-induced trap formation. Furthermore, the urea metabolite ammonia functions as a signal molecule in the fungus to initiate the lifestyle switch to form trap structures. Our findings highlight the importance of multiple predator–prey interactions in prey defense mechanisms.
Highlights
In their natural habitat, bacteria are consumed by bacterivorous nematodes; they are not passive preys
Consistent with a previous observation, we found that fresh cow dung induced trap formation in A. oligospora on water agar plates (Fig. 1a)
As bacteria constitute a significant portion of the cow dung biota, these results suggest that active metabolites from bacteria in cow dung may serve as inducers of trap formation in A. oligospora
Summary
Bacteria are consumed by bacterivorous nematodes; they are not passive preys. We report a defensive mechanism used by certain bacteria to mobilize nematode-trapping fungi to kill nematodes. These bacteria release urea, which triggers a lifestyle switch in the fungus Arthrobotrys oligospora from saprophytic to nematode– predatory form; this predacious form is characterized by formation of specialized cellular structures or ‘traps’. These specific strategies provide certain bacteria an advantage against their nematode predators These bacterial defense mechanisms in turn create selective pressure on nematodes, resulting in predator–prey arms race. A product of urea degradation, in turn serves as a signal molecule to initiate this lifestyle switch These findings provide a striking example of multiple predator–prey interactions, which play an important role for modulating the composition and population dynamics in nature
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