Cheating is surprisingly easy
D. discoideum spend most of their lives as predatory single cells hunting bacteria through the leaf litter and upper soil layers of forests in eastern North America. But when they can't find bacteria and begin to starve, they gather to form fruiting bodies, a thin stalk of cells with a ball of spores at the top, like a miniature Space Needle. The amoebae that end up in the stalk die, giving up their lives to benefit the amoebae that become spores.
Importantly the cells that stream together to form the fruiting body can be clonal (genetically identical) or have two (or more) genetic makeups. If each clone in a two-clone fruiting body contributes half the cells to the spore body, both clones gain from cooperating because each must sacrifice fewer cells to the stalk.
But game theory suggests the clones should sometimes evolve strategies that allow them to gain the benefits of cooperation without paying the costs.
In 2008 Queller and Strassmann published a genome-wide screen of D. discoideum that found roughly 180 cooperation genes, genes that might produce cheaters if they mutated. The number of genes, and the number of different biological pathways they affected, suggested it might be easy to evolve cheating and difficult to control it fully.
At the time cheaters were believed to be held in check by mechanisms that made non-cooperation costly. The first D. discoideum cheater to be scrutinized, CheaterA, described in 2000, is not able to form fruiting bodies on its own. This is a crippling disability that would prevent it from surviving in the wild.
But the screen from 2008 selected only clones able to produce clonal fruiting bodies, thus passing a basic test of evolutionary fitness. These clones were what is called
|Contact: Diana Lutz|
Washington University in St. Louis