Noble social amoeba maintain diversity

HOUSTON -- (September 30, 2009) -- In the seemingly simple world of the social amoeba, some cheat, giving advantage to their genetic kin. Why, then, do they not take over the population?

One possibility, said collaborating researchers from Baylor College of Medicine and Rice University, are mutations that confer the ability to resist cheaters. They describe this population in a report that goes online today in the journal Nature.

"These resistant strains are 'noble' in that they do not take advantage of the other strains," said Anupama Khare, a graduate student in the laboratory of Dr. Gad Shaulsky at BCM and the paper's first author. "In fact, they even reduce the cheater's ability to cheat such strains."

The paper is the latest chapter in the story of the Dictyostelium discoideum or social amoeba that is being unraveled in the laboratories of Shaulsky, professor of molecular and human genetics at BCM, Dr. Adam Kuspa, chair of biochemistry and molecular biology at BCM and Dr. Joan E. Strassmann and Dr. David C. Queller, professors in the department of ecology and evolutionary biology at Rice University.

Stalks and spores

Social amoebae live as single-cell organisms in normal times. When starved, they form multi-cellular organisms made up of stalks and spores. At one point in this process, some of the cells "sacrifice" themselves and become the dead stalks that support a body of spores, made up of living cells that keep the population alive. This population can be genetically diverse, and a roughly equal proportion of cells from the different gene pools sacrifice themselves to become stalks. That means that the different gene types survive in equal proportions.

Yet some amoeba "cheat" and as a result, more of that particular genotype survives. If this strategy always worked, these "cheaters" would take over the population.

Cheater resistant

To test their hypothesis that some amoebas are cheater resistant, the researchers mixed a population of mutated cells with a cheater strain and allowed them to develop into stalks and spores as chimeras. They thought that the cheater cells would exploit most of the cells in the mutant population, and that a high proportion of any cells left would be resistant to cheating. After several generations, their hypothesis proved true. Further studies showed that these "noble" social amoebae not only resisted the cheaters, they also did not themselves cheat on other amoebae.

"In this study Anu has demonstrated so clearly and cleanly such a response to cheaters at the molecular level. It is also very interesting that these resisters are noble, in the sense that they themselves do not exploit their ancestor," said Strassmann.

"The active cheating in social amobae is more similar to animal sociality, and is therefore a good model system for exploring the complex evolutionary dynamics of genes affecting cooperation, cheating and cheater-resistance," the authors wrote.

"The ones we identified don't abolish cheating completely," said Khare.

"Think of them as the firebreaks that prevent the flames from spreading," said Shaulsky. "Or they are like the people resistant or vaccinated against the flu that prevent it from spreading."

Lorenzo Santorelli of both BCM and Rice also took part in this work.

Funding for this research came from the National Science Foundation and the Cullen Foundation.