Mutually beneficial interactions between species provide key services and resources for most ecological communities. Maintaining traits that benefit a separate, unrelated species requires a potentially delicate balance of costs and benefits, but most species that host mutualists are equipped to prevent them from taking advantage of the interaction. That doesn’t mean that mutualists never walk away from mutualism — as seen in a new study of the bacteria that fix nitrogen for one native California wildflower, mutualists may go their own way pretty frequently.
The study’s authors, collaborators at the University of California Riverside and Oregon State University led by Camille Wendlandt and Kelsey Gano-Cohen, cultured strains of Bradyrhizobium bacteria associated with Acmispon strigosus, a wildflower that grows across much of California. Bradyrhizobium fixes nitrogen in the root nodules of A. strigosus, providing fertilizer in poor soils, and the plant provides it with sugar as a payback — but populations of the bacteria vary in their productivity, and the host plant has a demonstrated capacity to throttle back rewards to Bradyrhizobium if it doesn’t deliver. Gano-Cohen, Wendlandt et al. cultured 85 Bradyrhizobium strains found inside A. strigosus root nodules, on the surface of A. strigosus roots, or in soil near A. strigosus, inoculated them onto replicates of a single inbred line of the host plant to measure their ability to initiate symbiosis and fix nitrogen, and collected DNA sequence data from multiple loci to reconstruct relationships among the strains.
Across 10 sites in widely varying habitats, only one of 62 strains isolated from within A. strigosus root nodules proved unable to initiate symbiosis under experimental conditions — the authors suggest this strain made its way into a nodule by piggybacking on another strain’s infection process. Another four that failed to form nodules with any inoculated plants were isolates from root surfaces. Six isolates formed nodules, but did not provide effective service to their host plant, either in terms of overall plant growth or incorporation of atmospheric nitrogen detected using stable isotope analysis.
In the phylogeny of the 85 strains estimated from the genetic data, non-nodulating and ineffective strains were interleaved with fully functioning mutualistic strains, indicating independently losses of nodulation and nitrogen fixation function. As Gano-Cohen, Wendlandt, et al. note, this is consistent with loss of mutualism being beneficial to Bradyrhizobium, over the short term, but less consistently so over the longer term: non-mutualistic strains pop up via mutation, flourish for a bit, and then disappear from the community associated with A. strigosus.
There’s a bias introduced by the way in which strains were collected and surveyed; if newly arising non-mutualistic strains find success in the broader soil community they wouldn’t necessarily be picked up near A. strigosus roots. How rhizobia fare outside of symbiosis, and whether giving up the capacity for nodulation helps in that context, remains a bit of an unknown in studies of this mutualism. The data presented in this paper are consistent, though, with pretty successful host control of the mutualism — whatever happens to strains that stop helping A. strigosus, a strong majority of Bradyrhizobium found in close association with the plant provide the services they’re "hired" to do.
Gano-Cohen KA, CE Wendlandt, K Al Moussawi, PJ Stokes, KW Quides, AJ Weisberg, JH Chang, et al. 2020. Recurrent mutualism breakdown events in a legume rhizobia metapopulation. Proc. Royal Soc. B 287:20192549. doi: 10.1098/rspb.2019.2549