Starving "social amoebas" seek the support of genetically similar "kin" when they form multi-cellular organisms to ensure survival, researchers from Baylor College of Medicine and Rice University in Houston report in a paper published today in the open-access journal PLoS Biology.
The amoeba, called Dictyostelium discoideum, usually lives as a single celled organism, and as long as it has sufficient food and a pleasant environment, it is happy to remain that way. However, when food supplies run low, the single celled, free living forms move toward one another to form an aggregate. This aggregated community of individuals then forms a multi-cellular organism, with the single cells adopting one of two roles; either spores, which can survive and reproduce; or dead cells, which form a stalk that lifts the spores above the ground, increasing the chances that the spores will disperse effectively to more favorable environments. Only cells that act as spores can pass on their genetic information to future single celled amoeba, and therefore evolutionary theory predicts that individual cells should prefer being spores to stalks – unless acting as a stalk cell can increase the success of genetically similar kin. New work, from the laboratories of Profs. Joan Strassmann and David Queller of Rice University and Gad Shaulsky of Baylor College of Medicine, investigated whether the cells are able to modify their behavior according to the presence of genetically similar allies in an aggregate.
It has already been shown in previous work that Dictyostelium cells sometimes "cheat" by avoiding becoming part of the dead stalk, thereby increasing the chances that their genes will be reproduced in future generations. In the laboratory, the scientists mixed cells from genetically distinct strains of the amoebas and found that they segregate into clusters of genetically similar "kin" after they have congregated into the multi-cellular formation. In this way, the researchers determined that Dictyostelium reduces the likelihood that it will become a stalk cell that will "die" to assist in the survival of a genetically distant individual.
"The big thing we found is that Dictyostelium discoideum have social behavior," said Dr. Mariko Katoh, an author on the report. "We didn't really know if they could discriminate when the genetic differences were small. That was the surprising part."
"These single cells aggregate based on genetic similarity, not true kinship," said Dr. Gad Shaulsky, professor of molecular and human genetics at BCM. This demonstrates a discrimination between "self" and "non-self" that is similar to that seen in the immune systems of higher organisms.
Citation: Ostrowski EA, Katoh M, Shaulsky G, Queller DC, Strassmann JE (2008) Kin discrimination increases with genetic distance in a social amoeba. PLoS Biol 6(11): e287. doi:10.1371/journal.pbio.0060287
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CONTACT:
Elizabeth Ostrowski
Rice University
Department of Ecology and Evolutionary Biology
MS-170
6100 Main Street, MS 170
Houston, TX 77005-1892
ostrowski@rice.edu
Journal
PLoS Biology