Ameobae keep it in the family
In social amoebae such as Dictyostelium discoideum, cells aggregate to form a multicellular slug that migrates and then forms a fruiting body, which contains live spores (which go on to make new amoebae) and dead stalk cells. Unlike animals where all the cells descend from one fertilized egg, social amoeba fruiting bodies can contain cells with different genotypes. This potential for chimaerism creates a conceptual problem in that “cheater” cells could arise that preferentially become reproductive spores and force the victims to become stalk cells and die. One way that amoebae could avoid being cheated is if they recognize and preferentially aggregate with genetically similar cells while avoiding genetically distant cells – a process called kin discrimination.
New research shows that starving “social amoebas” seek the support of genetically similar “kin” when they form multi-cellular organisms to ensure survival. The amoeba 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. The new study 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. This indicates that Dictyostelium discoideum has social behavior. These single cells aggregate based on genetic similarity, not true kinship. This demonstrates a discrimination between “self” and “non-self” similar to that seen in the immune systems of higher organisms.
Kin discrimination increases with genetic distance in a social amoeba. 2008 PLoS Biol 6(11): e287
In the social amoeba Dictyostelium discoideum, thousands of cells aggregate upon starvation to form a multicellular fruiting body, and approximately 20% of them die to form a stalk that benefits the others. The aggregative nature of multicellular development makes the cells vulnerable to exploitation by cheaters, and the potential for cheating is indeed high. Cells might avoid being victimized if they can discriminate among individuals and avoid those that are genetically different. We tested how widely social amoebae cooperate by mixing isolates from different localities that cover most of their natural range. We show here that different isolates partially exclude one another during aggregation, and there is a positive relationship between the extent of this exclusion and the genetic distance between strains. Our findings demonstrate that D. discoideum cells co-aggregate more with genetically similar than dissimilar individuals, suggesting the existence of a mechanism that discerns the degree of genetic similarity between individuals in this social microorganism.
Tags: Biology, Microbiology, Science
