Biology subjects

Caroline B. Turner

Publications and source records attributed to Caroline B. Turner.

3 recordsLinked to original sources

bioRxiv2015

Replaying Evolution to Test the Cause of Extinction of One Ecotype in an Experimentally Evolved Population

In a long-term evolution experiment with Escherichia coli, bacteria in one of twelve populations evolved the ability to consume citrate, a previously unexploited resource in a glucose-limited medium. This innovation led to the frequency-dependent coexistence of citrate-consuming (Cit+) and non-consuming (Cit-) ecotypes, with Cit- bacteria persisting on the exogenously supplied glucose as well as other carbon molecules released by the Cit+ bacteria. After more than 10,000 generations of coexistence, however, the Cit- lineage went extinct; cells with the Cit- phenotype dropped to levels below detection, and the Cit- clade could not be detected by molecular assays based on its unique genotype. We hypothesized that this extinction event was a deterministic outcome of evolutionary change within the population, specifically the appearance of a more-fit Cit+ ecotype that competitively excluded the Cit- ecotype. We tested this hypothesis by re-evolving the population from one frozen sample taken just prior to the extinction and from another sample taken several thousand generations earlier, in each case for 500 generations and with 20-fold replication. To our surprise, the Cit- type did not go extinct in any of these replays, and Cit- cells also persisted in a single replicate that was propagated for 3,000 generations. Even more unexpectedly, we showed that the Cit- ecotype could reinvade the Cit+ population after its extinction. Taken together, these results indicate that the extinction of the Cit- ecotype was not a deterministic outcome driven by competitive exclusion by the Cit+ ecotype. The extinction also cannot be explained by demographic stochasticity, as the population size of the Cit- ecotype should have been many thousands of cells even during the daily transfer events. Instead, we infer that the extinction must have been caused by a rare chance event in which some aspect of the experimental conditions was inadvertently perturbed.

Evolutionary Biology↗

bioRxiv2015

Evolution of organismal stoichiometry in a 50,000-generation experiment with Escherichia coli

Organismal stoichiometry refers to the relative proportion of chemical elements in the biomass of organisms, and it can have important effects on ecological interactions from population to ecosystem scales. Although stoichiometry has been studied extensively from an ecological perspective, little is known about rates and directions of evolutionary changes in elemental composition in response to nutrient limitation. We measured carbon, nitrogen, and phosphorus content of Escherichia coli evolved under controlled carbon-limited conditions for 50,000 generations. The bacteria evolved higher relative nitrogen and phosphorus content, consistent with selection for increased use of the more abundant elements. Total carbon assimilated also increased, indicating more efficient use of the limiting element. Altogether, our study shows that stoichiometry evolved over a relatively short time-period, and that it did so in a predictable direction given the carbon-limiting environment.

Evolutionary Biology↗

bioRxiv2015

Evolution and coexistence in response to a key innovation in a long-term evolution experiment with Escherichia coli

Evolution of a novel function can greatly alter the effects of an organism on its environment. These environmental changes can, in turn, affect the further evolution of that organism and any coexisting organisms. We examine these effects and feedbacks following evolution of a novel function in the long-term evolution experiment (LTEE) with Escherichia coli. A characteristic feature of E. coli is its inability to consume citrate aerobically. However, that ability evolved in one of the LTEE populations. In this population, citrate-utilizing bacteria (Cit+) coexisted stably with another clade of bacteria that lacked the capacity to utilize citrate (Cit-). This coexistence was shaped by the evolution of a cross-feeding relationship in which Cit+ cells released the dicarboxylic acids succinate, fumarate, and malate into the medium, and Cit- cells evolved improved growth on these carbon sources, as did the Cit+ cells. Thus, the evolution of citrate consumption led to a flask-based ecosystem that went from a single limiting resource, glucose, to one with five resources either shared or partitioned between two coexisting clades. Our findings show how evolutionary novelties can change environmental conditions, thereby facilitating diversity and altering both the structure of an ecosystem and the evolutionary trajectories of coexisting organisms.\n\nEvolution does not produce novelties from scratch. It works on what already exists, either transforming a system to give it new functions or combining several systems to produce a more elaborate one.\n\n-Francois Jacob

Evolutionary Biology↗