Experimental Evolution of a Synthetic Yeast Cooperative System

Cooperative interactions are observed at all levels of biological organization. Cooperation facilitates major transitions in evolution such as from unicellularity to multicellularity. However, the mechanisms behind the evolution of newly-formed cooperative systems are unknown.  I have used a synthetic cooperative system termed CoSMO (Cooperation that is Synthetic and Mutually Obligatory) as a model for incipient cooperation. CoSMO is an engineered yeast system consisting of two non-mating strains, differentially labeled with fluorescence, which rely on direct exchange of distinct metabolites. Evolved CoSMO cocultures exhibit an improvement in viability, defined as an increased ability to survive reductions in population density. I have determined that this viability improvement is heritable, by measuring the viability of reconstituted evolved CoSMO pregrown in rich media which allow cells to grow in the absence of cooperation. Furthermore, by mixing an evolved strain with its ancestral partner, I determined that a single evolved cooperator was responsible for community-level viability improvement. I am characterizing phenotypic changes in evolved cooperators that contribute to increased viability. The phenotypic changes can potentially be categorized as either “self-helping”, such as through increased starvation tolerance and higher affinity for metabolites, or “partner-helping”, such as through increased metabolite release. In all cases, I will identify which strategies are used by the cooperators in order to stabilize cooperation in CoSMO. Similar analyses on replicate populations will be performed to quantify the diversity of mechanisms that contribute to improved fitness.  Through CoSMO, we will gain insights into how cooperative systems evolve from their early stages.