Beer Yeast Yield Discovery

A team of Harvard scientists made a significant breakthrough in the study of evolutionary biology, pinpointing the origin of cooperative behavior in yeast. Their work, published in Cell magazine two weeks ago, provides important information on the proliferation of genes that allow organisms to cooperate only with each other.

The group was led by biology professor Kevin J. Verstrepen, who studied at the University of Belgium’s Center of Malting and Brewing. “One of the things I learned there,” said Verstrepen, “is that yeast cells have a tendency to stick together at the end of fermentation.”

This phenomenon, called “flocculation,” fascinated Verstrepen. As he pursued his Ph.D. at MIT, he became interested in looking at flocculation. “From the yeast’s perspective—why would they want to stick together in the first place?”

In the lab, Verstrepen and his Harvard team isolated the gene that was responsible for yeast clumping and named it “FLO1.” They also discovered the evolutionary purpose of this gene: to provide “a very simple defense mechanism,” according to Verstrepen.

“These yeast form a big clump of cells,” he explained, “and the outer cells protect the inner cells from a toxic agent in the environment.” Verstrepen also found that the toxin ethanol, produced by yeast as a byproduct of cell metabolism, has harmful effects that can be avoided by flocculating. This helps explain how fermentation works.

But another important application of his research initially escaped Verstrepen. “I was presenting this work here at Harvard and in the audience was an evolutionary biologist by the name of Kevin Foster,” Verstrepen said. “He started jumping around and crying out that this is a ‘green beard gene.’”

The “green beard gene” was initially proposed by Oxford evolutionary biologist William D. Hamilton in 1964. The idea helps explain how cooperative behavior can evolve among organisms. Biologists have often asked how cooperation could succeed when “cheaters”—organisms that did not express the cooperative gene—could benefit from other organisms’ cooperation.

A green beard gene is a gene that allows organisms to cooperate only with other organisms that also have the gene. These organisms can identify each other as easily as if each possessed a green beard.

While a handful of such genes were previously identified in ants and slime molds, the FLO1 yeast research now represents “the most detailed genetic analysis of green beard genes to date,” Verstrepen said.

David C. Queller, a professor in the Department of Ecology and Evolutionary Biology at Rice University, said of the research, “I find it interesting that an organism as well-studied as yeast has these green-beard genes. Perhaps once we look close enough, we might find them to be more common, at least in microbial systems. That’s not something that we expected.”

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