New results show that yeast populations grow better when a few individuals cheat the system
Yeast colonies with mooches, thieves and cheats actually grow faster and larger than colonies without these freeloading individuals, according to a study published 15th September in PLoS Biology, challenging the widely held belief that cheaters bring only bad news to cooperating populations.
Researchers found that when some yeast cheat their neighbors out of glucose, the entire population grows faster. Image: Eric Miller, Max Planck Institute of Evolutionary Biology "This is a most surprising result," said Laurence Hurst of the University of Bath in the UK, who coauthored the study. "The theory of cooperation was one of the best worked theories in all of evolution. Everyone assumed that it had to be the case that the world is better off when everyone cooperates."
The results may explain why yeast populations tolerate the presence of cheaters, added Michael Travisano, a biologist at the University of Minnesota, who was not involved in the research -- "because a mixed strategy is to everyone's benefit."
Most yeast secrete invertase, which hydrolyzes sucrose into fructose and glucose, their preferred food. However, some yeast are known to cheat the system. Cheater yeast don't secrete invertase and therefore don't contribute to the glucose production, yet they still eat the glucose that is generated by the rest of the population.
According to the theory of cooperation, which states that organisms are better off when everyone cooperates, yeast populations should be best off when all the yeast produce invertase. This would maximize the availability of glucose, which should enable more yeast growth. But when Hurst and his colleagues grew yeast populations with both producers and non-producers of invertase, this is not what they saw. Instead, the yeast grew the fastest and saw the highest population numbers when a proportion of the population was cheating.
One reason populations with cheaters grew better has to do with the yeast's inability to efficiently use abundant resources, Hurst said. "If you can hop down to your local McDonald's for a Big Mac, and it's very easy and very cheap, then you don't mind if you eat half of it and throw the rest away," Hurst said. "If you were starving in Africa, you wouldn't even imagine doing that." With the cheater yeast using up some of the available glucose, the cooperators are able to use the remaining resources much more efficiently, he said, allowing the population to grow larger and more quickly.
The team also modeled the experimental results in an effort to see whether their findings were specific to yeast growing on a petri dish, or whether they might apply to other organisms as well. The results showed that their experimental outcomes could be generalized -- cheats would benefit a population whenever certain criteria were met. These results imply that cooperation isn't always the most beneficial path for a population, Hurst said. Instead, the benefits of cooperation depend on the characteristics of the population itself. Under certain conditions, some amount of cheating is likely beneficial.
But the story is not a simple one, said Jeff Gore, a biophysicist at the Massachusetts Institute of Technology, who did not participate in the research. For example, "if the cheaters and cooperators are growing at different rates, the ratio of cooperators to cheaters won't be stable," Gore said. Thus, the population may be changing, and "you still have to ask what [it] is going to evolve to," and not just look at where it is now.
R.C. MacLean, et al., "A mixture of "cheats" and "cooperators" can enable maximal group benefit," PLoS Biol, 8(9): e1000486, 2010.