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D. purpureum
Photo: Chandra Jack

Although the category “protist” or “protozoan” is used to refer to a number of single celled eukaryotes lacking a cell wall, the terms do not denote evolutionary relationship. Modern molecular phylogenetic evidence points to a wide diversity of simple eukaryotes, diverging at various points along the tree of life. The amoebozoa form part of a supergroup of eukaryotes that form a sister clade to the fungi and animals, having diverged more recently than the plants. Among the amoebozoa are the social amoebas or the dictyostelids, which exhibit the characteristic behavior of forming multicellular organisms under certain growth conditions. The dictyostelids actually comprise four major phylogenetic groups, representing evolutionary forms as diverse as the animal kingdom.

Most of what we know of the dictyostelids come from the study of one species: Dictyostelium discoideum, an important model organism for the study of chemotaxis, differentiation, cell-cell communication, and most recently as a host organism for pathogenic bacteria. D. discoideum is thus far the only social amoeba with a fully sequenced, mapped and assembled genome sequence. Dictyostelium purpureum is a distinct species from D. discoideum, although it belongs in the same phenotypic grouping. Like D. discoideum, it exhibits a robust multicellular life cycle, and shares some of the early signaling molecules with D. discoideum. Both species spend vegetative growth preying on bacteria in the soil, and when starved, aggregate to enter multicellular development that culminates into a fruiting body with resistant viable spores (D. purpureum is so named for its strikingly purple spores), and supporting stalk structures. Cells that form the stalk sacrifice themselves for the survival of the spores, and is one of the few known cases of evolved altruism. However, when mixed, the two species are able to separate out, and form distinct structures, indicating that even in these simple microbes, the ability to discriminate between friend and competitor is highly evolved. D. purpureum takes the distinction another step further by recognizing kin within the same species - clones will segregate out from one another in mixed systems, indicating a sophisticated recognition mechanism.

Additional dictyostelid genome sequences will drastically improve our ability to understand dictyostelid metabolism. Social amoebae are found in soil throughout the world, and no doubt form an important part of the different soil microbiomes. Amoebas are likely to be natural repositories of pathogenic bacteria such as Legionella or Mycobacterium. Possibly due to the aggressive selection pressures of different bacterial defenses, the social amoeba genomes display a large repertoire of polyketide synthase genes, modular genes best known for synthesis of antibiotic compounds, although in this case may be just a hint of a very varied secondary metabolite arsenal. The dictyostelids are the only known eukaryotes that use halogens as part of their natural metabolism, synthesizing and degrading chlorinated molecules as part of their signaling systems. Together with a robust resistance against ionizing radiation, these organisms form important candidates for bioremediation of chemically and radioactively contaminated environments.


Publication: TComparative genomics of the social amoebae Dictyostelium discoideum and Dictyostelium purpureum.Genome biology. 2011 Feb 28;12(2):R20. [Epub ahead of print]