Did rhizobia associated with invading legumes escape bacteriophage enemies?

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It is crucial to understand the factors that promote biological invasions because they threaten valued environmental and economic resources. Many plants in the legume family are invasive weeds, perhaps because they cooperate with bacteria called rhizobia to obtain a reliable supply of nitrogen. For example, the legume French broom and its rhizobia have become invasive in America after being introduced from Europe. Introduced species might become invasive by escaping enemies that plague them at home. Like other bacteria, rhizobia are often attacked by viruses. Did French broom become invasive in America because its partner rhizobia escape viruses that plagued them back home? To begin answering this question, rhizobia and associated viruses will be collected from European (native) and American (introduced) populations of French broom. If American populations of French broom rhizobia have escaped their enemies, they should be infected by fewer viruses. However, American rhizobium viruses might have evolved to infect the introduced rhizobia. To test this idea, rhizobia and associated viruses will be collected from a native legume and the viruses will be tested to discover if they can infect rhizobia associated with French broom. This work will help understand how viruses could be used to manipulate host populations. A postdoctoral scholar will be trained in research and communication skills as part of this project.For release from bacteriophage infection to explain invasiveness of rhizobia and their partner legumes, bacteriophages must control rhizobium populations in their home range but not in the invasive range. This pattern requires that bacteriophages be specialized and more easily infect sympatric than allopatric rhizobia. As a first step toward addressing this issue, the host ranges of rhizobial bacteriophages collected from three European and three American populations of Genista monspessulana will be assessed in vitro by inoculation onto single-isolate "lawns" of Bradyrhizobium genotypes isolated from these same populations. The number of bacteriophages isolated from native and non-native range populations will also be compared. To test whether indigenous American bacteriophages might control introduced Bradyrhizobium populations, the host ranges of bacteriophages isolated from bradyrhizobia associating with the native legume, Lupinus arboreus, will also be tested against Bradyrhizobium genotypes isolated in the survey described above. This work will test whether the Enemy Escape Hypothesis applies to an economically and ecologically important microbe associated with an invasive legume. It will further examine how bacteriophage host ranges evolve in response to shifts in host population changes. In so doing, it will enhance scientific understanding of how bacteriophages affect mutualistic interactions between hosts and beneficial bacterial symbionts.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.