Sulfate-reducing bacteria are widespread and live in many environments. Some sulfate reducers are capable of growing in a mutualistic relationship with methanogens. In nature, diverse sulfate-reducing bacteria have repeatedly evolved into obligate mutualists. We want to study how this evolution occurred by using a model system for evolution in the lab between a sulfate reducer, Desulfovibrio vulgaris, and a methanogen, Methanococcus maripaludis. After allowing these two organisms to evolve for 1000 generations in an environment that forces their cooperation, 12 out of 22 D. vulgaris populations lost the ability to reduce sulfate (sulfate-minus genotypes). We want to test whether the ratio of sulfate-minus relative to sulfate-plus (a genotype still able to grow on sulfate) genotypes in coculture affects the fitness benefit of the sulfate-minus genotypes. To do this, a sulfate-minus and plus D. vulgaris were placed together and their relative abundance in coculture was tracked over time. The experiment will tell us if some genotypes have the ability to take over when they are rare. To track these changes, we are using a method called FREQ-Seq. FREQ-Seq is an inexpensive way of analyzing the frequency of certain genes or other DNA sequences in a mixed population over time. We can use this method to understand how these populations of D. vulgaris evolved. I originally predicted there would be a higher sulfate-minus to plus ratio at the end of the competition compared to the initial ratio. D. vulgaris is an interesting model organism for studying the evolution of obligate sulfate-reducer/methanogen mutualisms because it can live either alone as a sulfate-reducer or together with other organisms in a syntrophic relationship. The data we collected can help us better understand the mechanisms of natural selection that cause sulfate reducing populations to become obligate mutualists.

Mutualistic Relationships are critical for the stability of ecosystems but their mechanisms are poorly understood. Local adaptation is an indication of how specialized the organisms are to their mutualistic partner. Most experiments that study locally adapted species focus on host-parasite and trophic interactions, however, we focus on two species, Desulfovibrio vulgaris and Methanococcus maripaludis, existing in a mutualistic relationship. We used 6 independently evolved populations. The individual species in the co-evolved populations were separated and paired with a partner from a different evolved population to see if they grew faster with the partner they evolved with. We tested for local adaptation by frequently measuring the optical density of the cocultures. The OD measurements were used to calculate growth curves. The growth curves were compared to determine if the co-cultures were locally adapted. Our results indicated that neither species was locally adapted.