UC San Diego

One important problem in ecology and evolution is how species interactions affect the evolution of community members, and vice versa, how evolution affects species interactions. To address this problem, researchers observe species interactions and their evolution in simplified model communities in the lab. A previous study carried out in a model two-species microbial mutualism between yeast Saccharomyces cerevisiae and alga Chlamydomonas reinhardtii found that a single adaptive mutation in yeast can affect yields of both species. My thesis builds on this result in two directions. First, to better understand these effects, I developed an experimental approach for estimating birth and death rates of both species in monoculture. Once extended to co-cultures, this approach will allow us to study how individual adaptive mutations affect basic life-history traits. Second, I helped carry out a long-term evolution experiment in the yeast-alga community in two conditions, one where yeast is obligately dependent on the alga and another where the dependence is facultative. I found that during the course of evolution, yields of both species repeatably increased in the facultative condition but the changes were more variable in the obligate condition. This work, together with the ongoing sequencing efforts, will help us understand the conditions in which co-evolution is more or less repeatable at different levels of biological organization.