June 4, 2015
Is there an evolutionary advantage to social cheating?
Social cheating, a term used by behavioral ecologist to describe how one organism receives a benefit at the cost of one or more other organisms, is a phenomenon found in several different types of complex organisms. But does it also exist in simpler creatures?The answer is yes, and in new research published in Thursday’s edition of the journal Current Biology, researchers from Washington University in St. Louis, the University of Houston, and the Baylor College of Medicine have attempted to learn more about social cheating by observing the behavior in the social amoeba Dictyostelium discoideum (Dicty, for short).
“One of the major advantages of the social amoeba is that it is relatively easy to identify which genes influence which behaviors – something that is difficult to do in many other organisms,” first author Dr. Elizabeth Ostrowski, an assistant professor of biology and biochemistry at the University of Houston, told redOrbit via email.
“This organism is a unique relative to most other organisms in that cells aggregate to become multicellular,” she added, “and this means multiple genetic lineages can co-occur inside any given multicellular organism. This genetic diversity creates an environment where cheating can potentially flourish.”
Co-senior author Dr. Joan Strassmann, the Charles Rebstock Professor of Biology in Arts and Sciences at Washington University, added that Dicty “has a very interesting life cycle. As a solitary amoeba, it eats bacteria then divides by binary fission to make two and so on. But when it starves... the amoebae aggregate by the tens of thousands and form a multicellular body like a worm that crawls towards heat and light. Ultimately this slug forms a fruiting body.”
Determining if cheating is an effective strategy
One-fifth of the creature’s cells die to form a hardy stalk that the others swarm up, forming spores at the top where they can be easily dispersed by invertebrates, Dr. Strassman said, adding that the resulting social system is comparable to that of a wasp or bee colony, in which some members of the group die for the benefit of others.
Of particular interest, she told redOrbit, is “discovering the kinds of attributes that let some clones become spore and others become stalk, though most clones contribute some cells to both types of tissue. Another really important thing about this system is that, because the social stage comes from aggregation rather than through a single cell bottleneck like in human development, we expect there to be competition.”
When unrelated amoebae gather to form a fruiting body, some strains may overcontribute to the spores and undercontribute to the stalk, the researchers explained. These strains are the cheaters, and while the researchers knew that they existed in wild populations of these amoebas, they were not certain if this strategy could be successful when it came to natural selection.
As part of their research, the authors sequenced 20 Dicty strains that had been isolated from the soil in the eastern US. They went looking for variation in 140 genes that had previously been implicated in social behavior and compared them to the rest of the genome to see if the social genes were evolving differently. The 140 genes in question had previously been discovered to change a cooperating amoeba into a cheater once they are disabled.
“This large number of candidate genes was essential to the success of the project,” Dr. Ostrowski told redOrbit. “Also, by sequencing the entire genome of a large number of isolates, we could compare the sequence signatures of two classes of genes – those with roles in cheating behaviors and those without. This proved to be a really powerful way of detecting how genes with roles in cheating behaviors differ from other genes in the genome.”
Cheaters, cooperative strains locked in a virtual stalemate
Thanks to those sequencing efforts, the researchers found their answer. They were able to determine that neither the cheating variants of the social genes nor the more cooperative ones were able to gain an advantage over the other, said Dr. David Queller, the Spencer T. Olin Professor of Biology in Arts & Sciences at Washington University and co-senior author.
“What we found was a kind of stalemate, like trench warfare where the genes in question neither sweep positively through the population or get eliminated. This can tell us generally about how natural selection operates on social traits,” Dr. Strassman told redOrbit. She added that the study also showed that “social competition is a key feature of the species. If it were not, we would not find selection to operate differently on this set of genes than on general genes.”
“These results strongly suggest that social interactions among clones are important in nature,” added Dr. Queller. “We have done lots of experiments that show this is a great study system of how cooperation and cheating work in the lab. But we weren’t sure that these cheating-type interactions were important in nature... Now, we have some good evidence that these interactions between clones are important in nature.”
“Cheating is readily observed across diverse systems in nature, but there has been a lot of debate about whether cheating pays off in the long-term – in this system or in any other system,” Dr. Ostrowski concluded. “Molecular evolution approaches like the ones we used here are a powerful tool for addressing the long-term success of different variants across evolutionary timescales, timescales much larger than what we can observe ourselves.”