Cooperative Sucrose Metabolism In Yeast Is A Snowdrift Game

Abstract

Understanding the conditions required for the initiation and maintenance of cooperation is a classic problem in evolutionary biology. In order for the budding yeast S. cerevisiae to grow on sucrose the disaccharide must first be hydrolyzed by the enzyme invertase. This hydrolysis reaction is performed outside of the cell, in the periplasmic space between the plasma membrane and the cell wall, suggesting that invertase production may represent a cooperative behavior. Here we demonstrate that the vast majority (∼99%) of the monosaccharides created by sucrose hydrolysis diffuse away before they can be imported, thus making invertase production and secretion a cooperative behavior. In competition experiments we find coexistence between the wildtype cooperator strain and a mutant cheater strain that does not produce invertase, implying that the interaction is governed by the snowdrift game in which the optimal strategy is the opposite of one's opponents. A simple model of the cooperative interaction incorporating nonlinear benefits is able to explain this coexistence and also produces a phase diagram predicting that the outcome of the competition can be altered by varying either the cost of cooperation or the glucose concentration in the media. We are able to confirm the predictions of this phase diagram and also find that increasing the availability of glucose can have the surprising effect of decreasing the growth rate of the culture. Finally, we have characterized the wildtype invertase production strategy and find that the response is appropriate for the snowdrift game--wildtype cells cooperate when competing against cheater cells but cheat when competing against cells that always cooperate.