Eco-evolutionary feedback in cooperatively growing yeast populations

Background/Question/Methods

Eco-evolutionary feedback emerges when organisms are not only able to modify their environment, but also evolve in response to that environmental change and both processes occur at similar time-scales. Environmental modification is often a cooperative effort, requiring the concerted action of many individuals in the population. In this case, non-cooperative “cheater” individuals that benefit from the modified environment but do not contribute to its building, may proliferate, potentially causing population collapse. Here, we have examined experimentally whether a cooperatively growing microbial species (S. cerevisiae) shows an eco-evolutionary feedback involving competition between a cooperator, niche-building strain, and a “cheater” strain. Then, we have investigated whether this eco-evolutionary feedback contributes to the maintenance of the cooperative phenotype in the population, as well as the demographic stability of the population as a whole. In order to address this question, we have used laboratory cultures of yeast growing cooperatively in sucrose media, which is cooperatively transformed into glucose by the cells in the population. We experimentally altered initial population size and cooperator fraction, and performed daily dilutions to track simultaneous changes in population size and composition as a function of time.

Results/Conclusions

We have experimentally demonstrated that evolutionary competition between a niche-building cooperator yeast strain, and a “cheater” strain that does not contribute to environmental modification but reaps its benefits, causes a concomitant change in population size and environmental composition that in turn feeds-back into the outcome of the evolutionary competition. This eco-evolutionary feedback rescues the population from collapse, and leads to a rich dynamic behavior that we have observed directly in the form of spiraling towards a co-existence fixed point. Our results also indicate that mixed cooperator-cheater cultures are less resilient to fast external environmental changes than pure cooperator cultures are. Finally, we show that cooperation may be favored by external environmental fluctuations.