Cooperation can be found virtually everywhere: between different cell types in our body, different individuals in an ant colony, and different species in a mutualistic interaction. The extant cooperative systems have evolved for millions of years, and it is nearly impossible to retrace their evolutionary trajectories. We have created a novel, genetically tractable cooperative system that can be observed as it evolves, step-by-step, from its inception. The engineered system consists of two complementary types of yeast cells: the red-fluorescent cells require adenine to grow and release lysine and the yellow-fluorescent cells require lysine to grow and release adenine. Together, the two cell types form a cooperative system termed CoSMO (Cooperation that is Synthetic and Mutually Obligatory). A minimum cell density, which can be predicted mathematically, is required for CoSMO to be "viable," i.e. to grow from low to high density in the absence of adenine and lysine supplements. Upon evolution, this density requirement is relaxed by orders of magnitude. How has this incipient cooperative system evolved improved viability? Are the changes self-serving and/or partner-serving? Do partners coevolve?
The pervasiveness of cooperation is paradoxical because cooperative systems are threatened by "cheaters" that consume benefits without paying a fair cost. Transposons are cheaters of genomes; tumors are cheaters of multicellular organisms; even viruses can cheat each other when infecting a host. How can cooperative systems survive cheaters and evolve to their often sophisticated current forms? We have created a cheater-of-CoSMO - a cyan-fluorescent strain that requires lysine to grow but does not release adenine. CoSMO and the cheater-of-CoSMO will allow us to examine whether cheater-recognition mechanisms can evolve in a primitive microbial cooperation-cheating system.
Microbial communities are abundant in nature. They can digest industrial and municipal wastes, mediate biogeochemical cycling of elements such as carbon and nitrogen, or directly impact human health. A community can be defined by its "structure" and "function." "Structure" includes the relative abundance (composition) and the spatial distribution (pattern) of constituent populations. "Function" refers to community bioactivity (e.g. metabolism of environmental compounds, colonization of a host, or production of biomass) and community resilience against perturbations. The structure and function of a community are tightly coupled, both influenced by interactions among cells through their shared environment. We are examining community structure and function in engineered and simplified natural communities.