Spotlight on Dictyostelium Solitary and Sociable Life of a Cellular Slime Mold

Vegetative Cycle Dictyostelium discoideum, is a species of cellular slime mold included in the Amoebozoan group of protists. It spends most of its time in the vegetative state, as an individual haploid cell dividing by mitosis to produce new haploid cells. It feeds on bacteria found in the soil or on moist fallen leaves. When the bacterial supply becomes limited, the organism transitions either to a social cycle or a sexual cycle.

Social Cycle In the social cycle, individual cells aggregate together, signaling each other through the release of cyclic AMP (cAMP). The aggregate becomes a multicellular “slug” capable of moving toward light, heat or humidity. The “slug” forms a fruiting body that releases spores that form new amoeboid cells. Approximately 20% of the cells form the stalk of the fruiting body, lifting the remaining cells to a beneficial place for spore production and dispersal.

Social Cycle

Sexual Cycle For the sexual cycle, single cells aggregate in response to cAMP and sex pheromones. Two cells of opposite mating types fuse, and then begin consuming the other aggregated cells. Before they are consumed, some of the prey cells form a cellulose wall around the entire group. Elimination of the prey cells leads to a large diploid cell called a macrocyst. The macrocyst undergoes meiosis and releases haploid amoeboid cells.

Sexual Cycle

Intersecting Life Cycles of Dictyostelium discoideum The life cycles of Dictyostelium discoideum. Most of its life, this haploid social amoeba undergoes the vegetative cycle, preying upon bacteria in the soil, and periodically dividing mitotically. When food is scarce, either the sexual cycle or the social cycle begins. Under the social cycle, amoebae aggregate to cAMP by the thousands, and form a motile slug, which moves towards light. Ultimately the slug forms a fruiting body in which about 20% of the cells die to lift the remaining cells up to a better place for sporulation and dispersal. Under the sexual cycle, amoebae aggregate to cAMP and sex pheromones, and two cells of opposite mating types fuse, and then begin consuming the other attracted cells. Before they are consumed, some of the prey cells form a cellulose wall around the entire group. When cannibalism is complete, the giant diploid cell is a hardy macrocyst which eventually undergoes recombination and meiosis, and hatches hundreds of recombinants. Not drawn to scale. CC Creative Commons Attribution – Share Alike 3.0, David Brown & Joan E. Strassmann http://dictybase.org/

Questions to Answer What type of structure do Amoebozoans like “Dicty” use for movement? What is the usual food source for “Dicty” ? Under which environmental conditions does the social cycle occur? What chemical signal attracts individual cells to aggregate during the social cycle?

Questions to Answer What part of the life cycle shows altruism for some “Dicty” cells? Which part of the life cycle shows cannibalism for some “Dicty” cells? From the fruiting body, are spores produced by mitosis or meiosis? From the macrocyst, are spores produced by mitosis or meiosis?