Making the ultimate family sacrifice

There is no greater sacrifice than giving one's life for others, and a new study by Rice University biologists and Baylor College of Medicine (BCM) geneticists is helping narrow the search for genes that drive single-celled amoebae to stick close to their kin before altruistically giving their all. The study, which appears this week in PLoS Biology, focuses on the soil-dwelling amoeba Dictyostelium discoideum. These single-celled creatures eat bacteria, and as long as food is plentiful, they're content to live alone. But when food is scarce, D. discoideum band together by the thousands. About 20 percent of colony members sacrifice themselves to form a stiff stalk that the rest can climb to migrate and form a new colony elsewhere.

This altruistic behavior is a challenge for evolutionary biologists. Why, for example, haven't cheaters -- amoebae who avoid the stalk and focus all their energies on getting into the new colony -- squeezed out their altruistic brethren? What are the genetic checks and balances that guard against cheating?

"They seem to care how genetically similar their partners are," study co-author David Queller said of the new findings. "That's something that you see in other social organisms, and you'd expect to see it from theory, but it's still kind of surprising to see that behavior in an amoeba."

Queller, Rice's Harry C. and Olga K. Wiess Professor of Ecology and Evolutionary Biology, and colleagues examined how much mixing occurred between 16 strains of D. discoideum. The genetic similarities between strains varied, and the tests revealed that the more genetically akin two strains were, the more they mixed and worked together during colony formation.

Rice postdoctoral researcher Elizabeth Ostrowski, the study's lead author, said a number of studies have previously shown that genetically dissimilar strains of D. discoideum sometimes live side-by-side in the wild. Earlier studies also showed that unrelated strains could cooperate within the same colony. Ostrowski said scientists took this to mean that unrelated or distantly related strains of D. discoideum often joined forces in the wild.

Using modern genetic technology, Ostrowski was able to put that assumption to a more rigorous test than ever before. She said BCM co-author Mariko Katoh played a key role by designing experiments and developing the method of tagging wild strains of D. discoideum with different fluorescent markers to see how the strains cooperate spatially and temporally during colony formation. Using these markers and Ostrowski's precise genetic profiles, the research team was able to determine exactly how much mixing occurred between dissimilar strains.

Queller, Rice evolutionary biologist Joan Strassmann and Gad Shaulsky, professor of molecular and human genetics at BCM, previously collaborated on a genome-wide search for mutations that produce D. discoideum cheaters. The search identified more than 100 mutations that would allow D. discoideum to cheat by avoiding service in the stalk.

The new findings suggest that specific genes also help closely related individuals cooperate in ways that keep these cheaters in check.

"We're showing that strains may not be mixing as much as scientists previously thought," Ostrowski said.

Shaulsky said, "By segregating, they minimize the risk that cells of their genetic similarity will die."

Ostrowski said the results suggest that "cooperative" genes are likely those that produce the sticky adhesives that the amoebae secrete to grab onto one another during colony formation. She said follow-up research is already under way to find precisely which genes are involved.

Strassmann, Rice's Harry C. and Olga K. Wiess Professor of Ecology and Evolutionary Biology and chair of the Department of Ecology and Evolutionary Biology, said, "Cooperation is one of the success stories of the evolution of life, and we are just beginning to unlock the potential of Dictyostelium as a model system for studying both cooperation and conflict in social evolution."

Source: Rice University