In the new study, the researchers wanted to bring their experimental system closer to the complexity of natural environments. "Our approach is to start with the most simple system you can," Gore says. "Then we'd like to try to understand these more complicated interactions between different species, so as a baby step in that direction, we added a bacterial competitor."
This bacterial competitor consumes much of the sugar produced by the cooperative yeast, and also competes for other resources, such as nutrients. After these bacteria were added, the percentage of cooperators in the yeast population increased to about 45 percent.
This isn't because the yeast "decide" to become more cooperative, as humans might when faced with an external threat, Gore says. It's determined purely by genetics. "That's the point of doing experiments with microbial populations: There should be a mechanism that we can understand," he says.
In this case, the researchers found two mechanisms at work. First, in the face of extra competition for sugars, the cooperators have a slight advantage over cheaters because they have slightly easier access to the sugar they produce themselves.
A related mechanism involves population density. When bacteria are present, the overall yeast population stays smaller and is forced to spread out more, making it harder for the freeloaders to find food.
"The cheater cells can really spread when there are a lot of other yeast, because there's a lot of sugar out there for them. If there aren't very many yeast, then it's challenging for cheaters to spread," Gore says.
Though the experiment does not precisely replicate natural conditions, it does show that it is important to try to get as close
|Contact: Sarah McDonnell |
Massachusetts Institute of Technology