Disentangling strictly self-serving mutations from win-win mutations in a mutualistic microbial community

Mutualisms can be promoted by win-win mutations which directly benefit self (self-serving) and partner (partner-serving). Intuitively, partner-serving phenotype could be quantified as the benefit supply rate to partner by an individual. Here, we demonstrate the inadequacy of this thinking, and propose an alternative measure. Specifically, we evolved well-mixed mutualistic communities where two engineered yeast strains exchanged essential metabolites lysine and hypoxanthine. Among cells that consumed lysine and released hypoxanthine, a chromosome duplication mutation seemed win-win: it improved cell9s affinity for lysine, and increased hypoxanthine release rate per cell. However, increased release rate was due to increased cell size accompanied by increased lysine consumption per birth. Consequently this mutation is solely self-serving, since a fixed amount of intake lysine leads to an identical total hypoxanthine release rate - either by more numerous lower-releasing ancestors or fewer higher-releasing mutants. By extension, individuals with reduced benefit production rates may not be cheaters.