Pleiotropic win-win mutations can rapidly evolve in a nascent cooperative community despite unfavorable conditions

Cooperation, paying a cost to benefit other individuals, is widespread. Cooperation can be promoted by pleiotropic “win-win” mutations which directly benefit self and partner. Previously, we showed that “partner-serving” should be defined as increased benefit supply rate per intake benefit (Hart & Pineda et al., 2019). Here, we report that “win-win” mutations can rapidly evolve even in nascent cooperation under conditions unfavorable for cooperation. Specifically, in a well-mixed environment we evolved engineered yeast cooperative communities where two strains exchanged costly metabolites lysine and hypoxanthine. Among cells that consumed lysine and released hypoxanthine, ecm21 mutations repeatedly arose. ecm21 is “self-serving”, improving self’s growth rate in limiting lysine. ecm21 is also “partner-serving”, increasing hypoxanthine release rate per lysine consumption and the steady state growth rate of partner. ecm21 also arose in monocultures evolving in lysine-limited chemostats. Thus, even without any pressure to maintain cooperation, pleiotropic win-win mutations may readily evolve.