Mutualisms, which are interactions between members of different species that benefit both parties, are found everywhere – from exchanges between pollinators and the plants they pollinate, to symbiotic interactions between us and our beneficial microbes. .
Natural selection – the process by which organisms best adapted to their environment tend to survive and produce more offspring – predicts, however, that mutualism is disintegrating. Individuals who benefit from the cooperation of others but not reciprocally (so-called cheaters) should appear and destabilize mutualism. Yet, to date, there is surprisingly little evidence of such fraud or destabilization.
A team of biologists from the University of California, Riverside, has now found solid evidence of this cheating. By focusing on the interaction between nitrogen-fixing bacteria, or rhizobia, and their host legumes covering about 530 miles of Californian habitat, the researchers found that natural selection in their populations studied favored deceptive rhizobia.
The study in Letters of ecology, is the first to discover strains of cheaters in natural populations and to show how natural selection favors them.
The researchers used a previously published database to quantify the abundance of different rhizobial strains in the landscape. They focused on natural rhizobial populations of the genus Bradyrhizobium and native annuals, Acmispon strigosusthat these bacteria live. Within these datasets, they found that the less rhizobia bring benefits to their host plants, the more rhizobia are common.
"Our data show that natural selection promotes deceptive rhizobia and corroborates predictions that rhizobia can often upset plant defenses and evolve to harness hosts," said Joel Sachs, a professor of biology at the Department of Medicine. evolution, ecology and biology of organisms, which led the research team.
Sachs explained that beneficial bacteria are increasingly seen as essential for human health as well as for crop and livestock productivity. However, it is difficult to understand how these bacterial services vary in natural systems and the forces that modulate them.
"In crop plants, in particular, agronomists have tried – and failed – for decades to design organic fertilizers for crops based on beneficial bacteria," he said. "Similar challenges have been encountered when applying bacteria in other host systems: probiotics, for example, rarely affect host microbes." Our dataset suggests a potential flaw. in these approaches, bacteria, with their own evolving interests, can destabilize these interactions. "
In their article, researchers show how the benefits of bacterial symbionts vary over time and in space and how fast these systems can evolve.
"We often consider that bacteria services are fixed, but that's not at all true," Sachs said. "Just as every human being varies considerably in almost every trait we can measure, bacterial populations are even more variable, and understanding this variation and its drivers will be essential to effectively exploit these bacteria for our own purposes."
Already, his team is actively working to better understand how beneficial bacteria can be applied to improve plant growth. Preliminary data show that it is crucial to choose carefully among bacterial variants in order to avoid using harmful strains.
"Simply applying beneficial bacteria to a crop will often not be enough, as strains of exploitation are likely to hide within these populations," Sachs said.
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Letters of ecology (2019). onlinelibrary.wiley.com/doi/full/10.1111/ele.13247