How can a blob of cytoplasm cheat? In more ways than you'd think.
"They might," says Queller, "have a mutation that makes an adhesion molecule less sticky, for example, so that they slide to the back of the slug, the part that forms spores."
"But there are tradeoffs," says Strassmann, "because if you're too slippery, you'll fall off the slug and lose all the advantages of being part of group."
Strassmann and Queller wondered if it would be possible to break the social contract among the amoebae by setting up conditions where relatedness was low, and each clonal lineage encountered mostly strangers and rarely relatives.
Together with then graduate student Jennie Kuzdzal-Fick, they set up an experiment to learn what happened to cheating as heterogeneous (low relatedness) populations of amoebae evolved.
"At the end of the experiment we assessed the cheating ability of the descendants by mixing equal numbers of descendants and ancestors, and checking to see whether the descendants ended up in the stalks or the spores of the fruiting bodies," says Strassmann.
They found that in nearly all cases the descendants cheated their ancestors. What's more, when descendant amoebae were grown as individual clones, about a third were unable to form fruiting bodies.
Many of the mutants, in other words, were "obligate" cheaters. Having lost the ability to form their own fruiting bodies, they were able to survive only by free-loading, or taking advantage of the amoebae that had retained the ability to cooperate.
This result, Queller and Strassmann say, shows that cheater mutations that threaten multicellularity occur naturally and are even favored--as long as the population of amoebae remains genetically diverse.
But the scientists were aware that obligate cheaters are either very rare or altogether missing among wild social amoe
|Contact: Cheryl Dybas|
National Science Foundation