What does it mean for an organism the width of a human hair to be domesticated? To find out, Kevin Verstrepen, a biologist from Harvard University, went straight to many peoples’ dream destination: the brewery. Here, Saccharomyces cerevisiae — the tame, simple, go-to laboratory model for nearly all plants and animals — lives a double life as the best-fermenting yeast in the world.
After talking with brewers, Verstrepen found that wild yeast behavior differed significantly from the yeast used by experimental biologists: “What I learned is that brewer’s yeast stick together at the end of fermentation,” Verstrepen says. “They start adhering to each other, clumping together, forming what we call flocs of cells. Lab yeast cells don’t do that.”
Brewer’s yeast. Credit: Caylan Larson
To find out why, Verstrepen and his team took the wild yeast back to the lab. There they discovered that the yeast were clumping together to protect themselves from stress. Toward the end of fermentation, the yeast produce enough alcohol to harm themselves. By clumping together, using a special adhesion protein, they create a nearly impenetrable barrier for alcohol and other toxins, with the yeast on the outside altruistically taking the brunt of the blow. “Outer cells protect the inner cells. Basically the chemicals cannot penetrate these flocs,” says Verstrepen.
This kind of collaboration is a deceptively sticky problem in evolutionary biology. Darwin struggled with it — and never resolved it. In any collaboration, every participant makes an investment and reaps the benefits. But how is this system protected from cheaters who bilk the rewards without contributing themselves?
Enter the so-called green-beard solution, first hypothesized by W.D. Hamilton. The theory goes that in order to join a mutually beneficial collaboration, one would need a signifier called a “green-beard gene.” In yeast this appears to be FLO1. Yeast that do not produce this adhesion protein cannot enter the floc.
Green-beard genes were thought to be nonexistent or extremely rare in nature, with only a few fuzzy examples thus far. The existence of a green-beard gene in yeast makes this model organism far more social — and complicated — than ever imagined. To date it provides the cleanest example of a green-beard gene in nature.
Do laboratory yeast ever encounter stress? “They don’t encounter as much stress as these feral strains, because actually, we treat our laboratory yeast very well,” says Verstrepen. Essentially, laboratories provide a toxin-free, risk-free environment, and the result has been domestication. “S. cerevisiae don’t behave like they would in the wild anymore. They behave like we would want them to behave in a test tube. We basically stopped evolution.”
FLO1 Is a Variable Green Beard Gene that Drives Biofilm-like Cooperation in Budding Yeast
Cell November 14, 2008
