Yeast cells

Colonies of yeast cells offer insights into the evolution of cooperative species. (W. Blake Gray )

Ever feel like you were the one doing all the work while everyone else just sat back and enjoyed the fruits of your hard labor?

Well then you might appreciate the plight of those lowly yeast cells that work overtime breaking down table sugar into glucose and fructose while other free-loading fungi soak up the nutrients and proliferate wildly.

In a paper published Tuesday in the journal PLOS Biology, researchers at the Massachusetts Institute of Technology used yeast to investigate the consequences of widespread "cheating" among microbial societies.

Authors wondered: If slacker yeast cells grew bigger and faster than "cooperative" yeast cells, and this trend continued over generations, would their colony eventually collapse?

The answer was surprising and provided valuable insight into the evolution of cooperative species, as well as the risks of extinction.

In order to determine the fate of free-loading yeast, assistant professor of physics Jeff Gore and biophysics postdoctoral fellow Alvaro Sanchez created a mathematical model, then tested the models' conclusions in a series of experiments performed on real yeast communities.

In one experiment, six yeast colonies were made up entirely of so-called cooperator yeast cells -- cells that all had the abilility to break down table sugar into usable nutrients for themselves and other cells. This was the experiment's control group.

Six other yeast colonies, however, were a mix of cooperator yeast cells and cheater yeast cells -- cells that lacked the ability to produce enzymes that break down table sugar. These colonies were said to be "invaded" by cheater cells, who simply consumed food and reproduced.

The cheater cells grew bigger and faster than the cooperator cells, as the cooperator cells expended valuable energy breaking down sugar, or doing work.  

Despite this imbalance, the colonies did not collapse. Instead, the two groups reached a point of sustained equilibrium.

In fact, the invaded colonies were nearly the same size as the colonies that consisted only of hard workers, the authors wrote.

Or at least that was the case when the mixed microbial societies existed in benign environments, where food was plentiful. Once the mixed groups were exposed to stress, things got dicey.

When researchers diluted the colonies' sugar supply, all six  cooperator-only colonies survived. However, all six mixed colonies went extinct.

"The presence of cheaters in the population therefore reduces the resilience of the population, even if the productivity of the population is unchanged," the authors concluded.

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