Doing their part by not doing their part
Freeloaders can be good for a community, yeast experiments suggest
By Tina Hesman Saey
Web edition : Tuesday, September 14th, 2010
CHEATERSA new study of cooperation in yeast finds that a mix of freeloaders and hard workers — who make important molecules — can outgrow colonies in which each yeast contributes equally. Eric L. Miller
Cheaters may not only prosper, they can make life better for everyone else too, a new study of cooperation in yeast reveals.
Common wisdom and scientific research on cooperation agree that the world is a much better place when everyone pulls their own weight, says Laurence Hurst, an evolutionary systems biologist at the University of Bath in England and a coauthor of the new study. That notion, known as the conspiracy of doves, has “largely gone untested because it was so obviously right,” he says.
But the long-held assumption that too many slackers spoil the broth for everyone turns out not to be true in all cases. In a new study published September 14 in PLoS Biology, Hurst and his colleagues show that mixed communities of hard-working yeast and loafers can outgrow cultures in which all yeast contribute equally.
In the new study, yeast cultures were grown in broth containing sucrose as a sugar source. To eat sucrose, yeast first have to slice the sucrose molecule into more easily digestible glucose and fructose molecules with an enzyme called invertase.
In wild populations, invertase-making yeast live side-by-side with yeast that don’t make the enzyme and so must rely on the invertase-makers to cut up the food. Most models of cooperation would predict that mixed cultures of invertase-making “cooperator” yeast and nonproducing “cheaters” would not fare as well as communities of enzyme-producing yeast alone. But when study coauthors R. Craig MacLean of the University of Oxford and Duncan Greig of University College London and the Max Planck Institute for Evolutionary Biology in Plön, Germany, grew cultures of the two types of yeast, flasks containing a mix of cooperator and cheater yeast grew faster and denser than ones with invertase-making yeast alone. The result had the researchers scratching their heads and asking, “How can this be?” says Ivana Gudelj, a mathematical systems biologist at Imperial College London.
Gudelj and postgraduate student Ayari Fuentes-Hernandez created a mathematical model that explains the puzzling result. The seemingly freeloading yeast in the mixed cultures actually help their enzyme-producing counterparts use sucrose more efficiently by indirectly slowing its conversion to glucose, the researchers found.
This is important because yeast use levels of glucose, not sucrose, as a way to control how much invertase is produced. So, on their own, enzyme-producing yeast make a lot of invertase and quickly burn through their food supply, limiting growth. By taking up any extra glucose, cheater yeast help the other yeast limit expensive invertase production, which forces more efficient use of sucrose and slower, but steadier, growth.
The model predicts that cheaters can be helpful when three conditions apply: resources are squandered when abundant, organisms can’t predict the amount of cooperation needed and cooperators get a greater share of the resources than cheaters do. Those conditions probably apply to a variety of real-world situations, the researchers say.
“I think this is a useful experiment,” says Laurent Keller, an evolutionary biologist at the University of Lausanne in Switzerland who was not involved with the research. Most studies of cooperation are conducted under more restricted laboratory conditions, he says, which may not bear much resemblance to ever-changing natural conditions.
The new study shows that changing environmental conditions can alter the way organisms interact with each other, perhaps even turning a cheater into a cooperator in certain circumstances, he says. “This interaction can be more complex than what people think.”
For example, the researchers showed that in unshaken flasks, glucose doesn’t diffuse far from enzyme-making yeast so cooperators get a bigger spoonful of sugar than nonproducers do. When flasks are shaken so that glucose is more evenly distributed, the benefit of having cheaters around is lost.
在一个人人合作无间的社区，如果混入几个游手好闲的人，也许对社区中的每一个人都有利。对酵母培养物的合作研究揭示了这一发现。 常识认为，如果社区中的每一个人都各司其职，没有人吃白饭，那么世界将会变得更美好。然而英格兰Bath大学的进化生物学家Laurence Hurst和同事进行的一项研究却得出了相反的结论，他们发现混合了努力工作和无所事事的酵母培养物比其它类型的培养物成长更快。数学生物学家Ivana Gudelj和研究生创造了一个数学模型去解开谜团。研究人员发现，看起来只知道占便宜的酵母能通过放慢葡萄糖转化的速度，帮助它们的同类更有效的使用蔗糖。这一改变至关重要，因为酵母是根据葡萄糖的浓度去控制产生多少蔗糖。如果蔗糖产生和消耗过快会限制它们的成长。"
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