Reciprocal cross-feeding between bacteria can limit the emergence of metabolic dependencies

alongside corresponding monocultures for 650-800 generations. While coculture conditions required obligate exchange of nitrogen and carbon, additional cross-feeding of adenine and iron likely occurred. Contrary to expectations, dependencies for iron and unknown compounds emerged in monocultures, but expected iron and adenine dependencies were not observed in cocultures. Low expression of iron scavenging and adenine synthesis genes in cocultures suggested that cross-feeding repressed expression, thereby lowering gene cost. Thus, although cross-feeding can sometimes make costly genes dispensable, there are also cases where cross-feeding lowers gene cost, thereby promoting gene retention.