Rebecca Batstone

Department of Ecology and Evolutionary Biology, University of Toronto

Experimentally evolving partners: evidence for the rapid evolution of nodulation ability in the legume-rhizobium mutualism

Smithsonian Image

Traits that allow a focal mutualist to preferentially associate with or allocate rewards to higher quality partners (i.e., preference traits) are thought to maintain mutualism stability over evolutionary time as lower quality, potentially cheating, partners receive less benefits. Despite abundant evidence that preference traits are widespread, variation in partner quality persists in nature. In order to investigate the factors that drive variation in partner quality, I experimentally evolved two strains of the rhizobium Ensifer meliloti that differed in their ability to fix nitrogen, one being effective and the other ineffective, with five Medicago truncatula lines that previously showed marked variation in preference for the effective strain. I quantified the proportion of each strain associating with each plant line across five growing seasons (lasting c. two months each), and found that the proportion of nodules occupied by the ineffective strain was greater than predicted, even for plant lines thought to prefer the effective strain. As a follow-up, I inoculated the same plant lines with rhizobia isolated from the end of the evolution experiment (i.e., derived isolates) and those from the beginning of the experiment (i.e., ancestral isolates), and found that within only one year of rhizobia “evolution”, the ineffective strain evolved to be much better at nodulating all plant lines, and in some instances, increased in quality. The observed degree of rapid evolution for improved nodulation and partner quality explains why the ineffective strain persisted over time, and has important implications for legumes colonizing new habitats where mismatched partners may be present (e.g., during range expansions, biological invasions, and in agro-ecosystems).

comments powered by Disqus