all strains were derived from haploid cells by4741 ( mating type a , euroscarf ) . cooperator strain has an intact suc2 gene , defective his3 gene ( his31 ) , and yfp expressed constitutively by the adh1 promoter ( inserted using plasmid prs401 containing met15 ) . the mutant cheater strain lacks the suc2 gene ( euroscarf y02321 , yil162w::kanmx4 ) , has an intact his3 gene , and has tdtomato expressed constitutively by the pgk1 promoter ( inserted using plasmid prs301 containing his3 ) . 2 and 5a were done using a strain containing yfp driven by the suc2 promoter ( inserted using plasmid prs306 containing ura3 ) . co - culture experiments were performed in 5ml batch culture at 30c using synthetic media ( minus histidine ) supplemented with 5% sucrose and variable concentrations of glucose and histidine . the 20% sucrose stock solution was filter - sterilized and stored with 1 mm tris ph 8.0 to prevent acid - catalyzed autohydrolysis . serial dilutions were performed daily ( 23 hours of growth ) such that the starting optical density was 0.0025 , corresponding to ~150,000 cells . equilibrium data in figures 1c and 3b , c were recorded after five days of competition between the two strains . all strains were derived from haploid cells by4741 ( mating type a , euroscarf ) . cooperator strain has an intact suc2 gene , defective his3 gene ( his31 ) , and yfp expressed constitutively by the adh1 promoter ( inserted using plasmid prs401 containing met15 ) . the mutant cheater strain lacks the suc2 gene ( euroscarf y02321 , yil162w::kanmx4 ) , has an intact his3 gene , and has tdtomato expressed constitutively by the pgk1 promoter ( inserted using plasmid prs301 containing his3 ) . 2 and 5a were done using a strain containing yfp driven by the suc2 promoter ( inserted using plasmid prs306 containing ura3 ) . co - culture experiments were performed in 5ml batch culture at 30c using synthetic media ( minus histidine ) supplemented with 5% sucrose and variable concentrations of glucose and histidine . the 20% sucrose stock solution was filter - sterilized and stored with 1 mm tris ph 8.0 to prevent acid - catalyzed autohydrolysis . serial dilutions were performed daily ( 23 hours of growth ) such that the starting optical density was 0.0025 , corresponding to ~150,000 cells . equilibrium data in figures 1c and 3b , c were recorded after five days of competition between the two strains .
the origin of cooperation is a central challenge to our understanding of evolution13 . 
 microbial interactions can be manipulated in ways that animal interactions can not , thus leading to growing interest in microbial models of cooperation410 and competition11,12 . in order for the budding yeast s. cerevisiae to grow on sucrose 
 , the disaccharide must first be hydrolyzed by the enzyme invertase13,14 . 
 this hydrolysis reaction is performed outside of the cytoplasm in the periplasmic space between the plasma membrane and the cell wall . here 
 we demonstrate that the vast majority ( ~99% ) of the monosaccharides created by sucrose hydrolysis diffuse away before they can be imported into the cell , thus making invertase production and secretion a cooperative behavior15,16 . a mutant cheater strain that does not produce invertase is able to take advantage of and invade a population of wildtype cooperator cells . however , 
 over a wide range of conditions , the wildtype cooperator can also invade a population of cheater cells . 
 therefore , we observe coexistence between the two strains in well - mixed culture at steady state resulting from the fact that rare strategies outperform common strategies  the defining features of what game theorists call the snowdrift game17 
 . a model of the cooperative interaction incorporating nonlinear benefits explains the origin of this coexistence . 
 we are able to alter the outcome of the competition by varying either the cost of cooperation or the glucose concentration in the media . 
 finally , we note that glucose repression of invertase expression in wildtype cells produces a strategy which is optimal for the snowdrift game  wildtype cells cooperate only when competing against cheater cells .