Social amoebae: Model system for social evolution and multicellular development
Dictyostelids ordinarily live as single-celled amoebae in the forest soil, where they hunt, engulf, and kill bacterial prey by phagocytosis. Upon starvation, the cells aggregate together and become a multicellular organism.
Hundreds of thousands of amoebae communicate using extracellular cyclic AMP, and then aggregate together to form a mound and eventually a migrating, multicellular slug. The slug can migrate for some time, moving towards light and heat, but eventually the cells of the slug transform into a structure called a fruiting body.
Stalk Cells Exhibit Altruism. Approximately 20% of the cells in the slug form a rigid stalk and die, while the remaining 80% rise to the top and form spores. Stalk cells are altruistic, in that they give up their lives to life the other cells up and out of the soil, helping them to disperse to a better location. Because multiple different strains can contribute to a single fruiting body, there can be conflicts over which cells will die and form the stalk, and which will live on as spores. Some strains cheat by contributing less to the stalk than other strains.
Cooperation & Conflict in Multicellular Organisms
Arms Race Dynamics in Natural Populations. We are currently analyzing the genome sequences of 20 natural isolates of D. discoideum. Our goal is to understand the patterns of sequence evolution at roughly a hundred candidate loci involved in social cheating and to test the hypothesis that conflict over spore-stalk allocation has led to co-evolutionary arms races and rapid evolution at these loci. We have found that genes involved in social cheating behaviors are different from other genes in the genome -- they show greater polymorphism, lower levels of population divergence, and greater haplotype structure, suggestive of balancing selection acting to maintain multiple alleles in a population.
In ongoing work, we are examining the patterns of cheating and cooperation in natural populations to determine the nature of coevolution between cheaters and cooperators. We carry out our studies of natural populations at our field site at the Mountain Lake Biological Station.
Kin discrimination in the social amoebae. Theory predicts that altruistic acts can be selected if they tend to benefit relatives - for this reason, a natural explanation for how Dictyostelium might prevent cheating is if individuals could recognize and preferentially cooperate with relatives. We have shown that D. disocideum is capable of self-nonself recognition, in that mixtures of genetically different strains tend to separate and form separate fruting bodies during development. Two highly polymorphic cell adhesion genes, tgrB1 and tgrC1, are important in mediating self-nonself recognition. We are currently examining the molecular evolution of the other genes belonging to this large gene family.