Evolution

Cooperative Yeast Break Free

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Science  07 Jun 2013:
Vol. 340, Issue 6137, pp. 1143
DOI: 10.1126/science.340.6137.1143-b

Cooperation is pervasive in nature, but how it remains a successful evolutionary strategy in the face of defectors and free riders is not yet fully understood. This is especially true in models of the classical Prisoner's Dilemma game, in which, theoretically, cooperators should always lose to defectors.

Van Dyken et al. engineer baker's yeast, Saccharomyces cerevisiae, into cooperators and defectors and pit them against each other in a Prisoner's Dilemma game. They are mutated so they cannot take up sucrose from the medium. Cooperators secrete invertase, which breaks sucrose down into monosaccharides. Defectors are mutant for invertase. Neither cooperators nor defectors can take up sucrose, but both can take up monosaccharides. When a growth cost is imposed for cooperation, and defectors are able to achieve limited monosaccharide-independent growth, defectors initially outcompete cooperators. But as the colonies grow, the cooperator populations expand at the expense of the defectors. The cooperators form genetically demixed sectors, analogous to "genetic surfing" seen in frontier populations. Simulations support the idea that an expanding colony frontier favors (cooperative) genotypes that maximize group productivity and that this could apply to range expansions seen in many species, including humans.

Curr. Biol. 23, 919 (2013).

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