One of the most ecologically significant mutualisms occurs between leguminous plants and nodule-forming bacteria, wherein the bacteria fix atmospheric nitrogen to a biologically available form in exchange for carbon from the plant. Intricate and highly specific crosstalk between host plants and rhizobia is required for recognition, nodule formation and macromolecule exchange. Despite the potential fitness gains partners can derive from cooperation, legumes and rhizobia can also benefit from antagonistic or cheating strategies. In fact, recent research demonstrates that the peptides and proteins that underlie legume-rhizobium communication can evolve to maximize one partner’s fitness at the expense of the other. Understanding the distribution of antagonistic traits across populations of mutualists is key to evaluating the importance of antagonistic coevolution in mutualisms. To test whether recently discovered molecular determinants of cooperation or cheating are segregating in a wild and invasive legume-rhizobium symbiosis, we examine natural frequencies the rhizobium peptidase, hrrP, which can confer a massive increase in fitness to a strain, and prevent the benefit of nitrogen fixation for the host.
Results/Conclusions
We evaluated 36 wild populations of Ensifer medicae rhizobia using a PCR-based screen for the hrrP locus. Out of 480 strains, 33 (6.8%) carried hrrP. Of these hrrP-positive strains, a disproportionate amount originated from the native range of the host legume Medicago polymorpha (8.2%) as compared to those from the invaded range of the host (2.9%). This suggests that rhizobia in the invaded range of this symbiosis are less likely to carry this antagonistic factor. The global distribution of this antagonistic peptidase molecule in wild populations suggests that ongoing antagonistic coevolutionary dynamics with co-occurring populations of the host legume plays a role in the evolutionary ecology of this mutualism. Further study will involve characterization of the effect of hrrP positive strains on fitness of the legume host in both native and invaded ranges.