What the social lives of brewer’s yeast say about evolution

As any good beer brewer knows, the yeast used in fermentation stick together in large clumps consisting of thousands of cells that settle out where they are easily removed. Brewers had even traced this behavior to a gene that encodes a sticky protein that sits on the surface of yeast cells. But despite the fact that yeast are a major laboratory "workhorse," any further exploration of their social lives had remained almost entirely neglected. Indeed, the "domesticated" yeast commonly studied in genetics labs have had any social tendencies bred out of them. But a new report in the November 14th issue of the journal Cell, a Cell Press publication, reveals that wild yeast strains are actually an excellent model for understanding another realm of science entirely: how evolution driven by "survival of the fittest" can yield apparently "unselfish" cooperative behavior.

The researchers led by Kevin Verstrepen of Harvard University now find that the yeasts' clumping behavior serves to protect those fortunate enough to find themselves in the inner part of the aggregations from stressful conditions, including antimicrobial agents and alcohol. Those on the outer layer in essence wind up sacrificing themselves for the good of the group.

Evolutionarily speaking, Verstrepen said, such a system should favor "cheaters" who might stand to gain protection from their sticky peers without investing in any sticky stuff themselves. In fact, such cheaters arise quite often, but they don't fare so well when trouble comes.

The FLO1 gene that encodes the adhesive protein also offers built-in protection against those lacking the protein. FLO1 positive cells preferentially stick to each other and, in so doing, they actively exclude yeast cells without.

" One gene does it all," Verstrepen said. It singlehandedly promotes cooperation and protects against cheaters.

Cooperation has been a challenge for evolution by natural selection because individuals are predicted to act in a way that maximizes the number of offspring they themselves produce, the researchers explained. Costly behaviors that invest in a common good, therefore, should be disrupted by cheaters that save on the cost of cooperation but reap the rewards of others' investments. Since cheaters would be better off than those who cooperate, they should eventually take over and cooperation would be lost.

That generally doesn't happen because many instances of cooperation, such as alarm calling in birds or charitable acts by people, ultimately benefit both helper and helped. In other cases, animals come to the aid of their relatives--still a good strategy when it comes to passing their genes on to the next generation, which is what it's all about.

But a scientist by the name of William Hamilton who launched a field known as sociobiology in the 1960s realized there could be another way. He proposed that cooperation is also possible if a single gene that drives the tendency to cooperate also preferentially directs cooperation to other carriers of the gene. Such genes were later dubbed "green beard genes," the green beard being the recognizable "tag" that enables organisms to direct their interactions to other gene carriers.

The new work reveals FLO1 to be one of only a very few green beard genes known in nature, Verstrepen said. In fact, they showed, yeast of different species will even stick together so long as both carry FLO1.

" This system epitomizes the notion of the selfish gene that can, at least temporarily, act to increase its own frequency irrespective of evolutionary interests of other genes in the genome, an idea popularized in [Richard] Dawkin's [book] "The Selfish Gene," which expounded a gene-centered rather than individual-centered view of evolution, the researchers wrote. "The example of FLO1 is particularly telling because it counters the common misconception that selfish genes always result in selfish organisms: FLO1 is a 'selfish' green beard gene that drives an act of remarkable cooperation."

While the work is most important for the insights it offers evolutionary theory, he added, it might have some practical implications as well.

The researchers found that the cooperative yeast don't produce the sticky protein all the time. Rather, they wait until they sense a chemical that tells them plenty of other yeast are around. Brewers might take advantage of this as a way of better controlling the behavior for optimal beer brewing.

More importantly, Vertrepen said, disease-causing yeast also clump into biofilms that are resistant to drug treatment. The wild Brewer's yeast may serve as a model for learning more about how to combat these infectious microbes.

" Common yeast infections are essentially harmless," he said. But other pathogenic yeast can be deadly in those with compromised immune systems as a result of AIDS, organ transplant or cancer treatment. "Thousands of people die every year not from AIDS or cancer, but from yeast infections. Once in the bloodstream, it's a very deadly disease."

Source: Cell Press