This week’s BEACON Researchers at Work blog post is by MSU faculty member Jen Lau.
1988 was a good year. MSU won the Rose Bowl; Miami Vice, 21 Jump Street, and Family Ties were still on prime-time, and I was a gangly, angsty adolescent entering middle school with a hideous perm and fluorescent clothes. Who could imagine 1988 being any more exciting? But it was–1988 also saw the beginnings of Lenski’s long term E. coli evolution experiment, one of the most exciting experiments in evolutionary biology. In the same year, sixty-five miles down the road, Kellogg Biological Station (KBS) became one of the first Long-Term Ecological Research (LTER) sites. And that’s how all the magic of 1988 collides. Unbeknownst to me back in 1988 with my permed hair and adolescent angst, that combination of scientific starts would motivate much of my research a quarter century later.
Just as Lenski’s long-term evolution experiment has taught evolutionary biologists much about the limitless potential for evolution, long-term ecological research sites have contributed an amazing amount to our understanding of community and ecosystem ecology. Less well-appreciated is the potential contribution of LTER experiments to the study of evolution. Since Lande & Arnold’s seminal 1983 paper, thousands of studies have measured natural selection on wild populations; yet, we still have a limited understanding of the selective agents causing that evolution or the evolutionary and genetic mechanisms driving evolution in local populations. That’s where LTER experiments come in–just like Lenski’s flasks, LTER sites contain replicated populations evolving for many generations, often under multiple experimentally manipulated ecological conditions. Although in most cases we do not have genotypes of the wild populations inhabiting these LTER sites back in 1988 tucked away in -80 degree freezers, we can still compare populations evolving in experimentally-controlled treatments to test for evolutionary changes in response to a known experimental factor.
My good buddy and collaborator Katy Heath (Univ. Illinois) and I are doing just that. We are using a long-term nitrogen (N) addition experiment set up by Kay Gross at the KBS LTER back in 1988 to test basic predictions about the stability of mutualisms (positive interactions between two organisms). In resource mutualisms, two organisms trade resources in a manner that benefits both partners. For example, in the legume-rhizobium resource mutualism, plants in the legume family trade carbon fixed through photosynthesis for N fixed by their rhizobium symbionts. Theory predicts that changes in the availability of traded resources can destabilize such resource mutualisms. For the legume-rhizobium mutualism, in particular, increased soil N is predicted to cause evolutionary changes that will destabilize this mutualism. First, high soil N availability is predicted to cause legumes to abandon the mutualism. Second, most theories also predict that high soil N will cause the evolution of less cooperative rhizobium mutualists that provide fewer growth benefits to their plant hosts.
By studying rhizobium populations that have been evolving since 1988 in N-addition vs. control plots at the KBS LTER, we find that N-addition has indeed caused the evolution of less cooperative rhizobia. When plants are inoculated with rhizobium strains isolated from N-addition plots they produce about 30% less aboveground biomass than plants inoculated with rhizobium strains isolated from lower N control plots (check out our recent paper). By sequencing our strains, we can test additional hypotheses about the genetic and evolutionary mechanisms driving these evolutionary shifts. We find that the evolution of reduced cooperation results from genetic changes on the pSym plasmid, an area of the bacterial genome containing numerous genes related to N-fixation and rhizobium-legume signaling. A variety of evolutionary mechanisms could cause such evolutionary changes: 1) High nitrogen may simply relax many of the selective pressures that exert purifying selection for high quality rhizobium strains in lower nitrogen environments (e.g., sanctions or partner choice), 2) Low quality rhizobia could actually be selected for in high N environments if there are trade-offs between rhizobium quality and survival in the soil environment. Patterns of nucleotide diversity at known symbiosis genes support the latter hypothesis; high nitrogen appears to actively select for low quality mutualists.
In short, by using an LTER experiment as an extra-large test tube for studying bacterial evolution, we can quantify evolutionary responses to known, experimentally manipulated environmental changes. What we find with the legume-rhizobium mutualism does not bode well for natural N-fixation. The legume-rhizobium mutualism was once responsible for nearly 98% of N inputs to terrestrial systems; our results suggest that less cooperative mutualists that are probably fixing much less N are actively favored by natural selection as synthetic nitrogen availability increases. What we don’t know are the limits to this evolution and just how widespread these phenomena are. Will rhizobium quality continue to decline just as Lenski’s strains continue to evolve, shifting this once important mutualism to parasitism? And is this phenomenon unique to KBS, or are other legume-rhizobium mutualisms facing similar evolutionary fates? Luckily for some gangly tween currently in middle school, hopefully these LTER studies will continue for another few decades, allowing for tests of evolutionary responses over even longer time scales. Luckily for Katy and me, there are numerous other LTER N-addition experiments all over the country for us to study next to determine the generality of our results.
And this is why next time you see me wandering around campus, I’ll be blaring some REM on my Sony Walkman, pegging my pants, and wearing fluorescent pink leg warmers and a matching scrunchie. Not only am I a typical academic fashion failure who hasn’t bought new clothes in decades, I’m also paying homage to the 80s because it was one of the happiest decades in recent American history and also because it was a windfall (accidentally or intentionally) for experimental evolution.
For more information about Jen’s work, you can contact her at jenlau at msu dot edu.