Rapid evolutionary changes may feedback to influence community ecology because genetic changes in one species might alter the strength of ecological interactions with other species, such as competitors, enemies, or mutualists. Although an increasing number of studies have documented rapid evolutionary changes, it is only recently that ecologists started to consider rapid evolution as a hypothesis to explain ecological patterns. Here we conducted a laboratory experiment to study how glyphosate, the most commonly used herbicide in US agriculture, impacts the soybean-rhizobium mutualism over both ecological and evolutionary time scales. We hypothesized that: 1) glyphosate reduces the abundance and population growth rates of rhizobia; and 2) glyphosate selects for glyphosate-tolerant rhizobia. We conducted an artificial selection experiment utilizing 20 field-collected rhizobia strains to determine whether rhizobia can evolve increased glyphosate tolerance. Replicates of each strain were streaked onto two types of agar media, containing either no glyphosate or 5 mM glyphosate. After re-culturing the rhizobia strains for multiple generations, we grew each strain in liquid media with or without glyphosate to estimate maximum growth rate using a spectrophotometer.
Our data suggest that rapid evolution of glyphosate tolerance is likely. Glyphosate reduced the maximum growth rate of rhizobia strains on average by 69 %, but some strains were less affected by glyphosate. Given this genetic variation, rhizobia in fields regularly exposed to glyphosate have the potential to evolve increased glyphosate tolerance. We expect that the effects of glyphosate on rhizobia might result in reduced soybean yields due to reductions in rhizobia abundance or because glyphosate tolerance may be costly in terms of reduced population growth rates and/or reduced nitrogen fixation. Understanding the potential negative effects of glyphosate on rhizobia and the resulting consequences for soybean yields is crucial for sustainable agriculture, especially because the use of herbicide and herbicide-tolerant soybean have increased dramatically in the past decade.