Social amoeba and the origin of multicellularity


Social amoebae, also called cellular slime molds or dictyostelids, are free living amoeboid cells in soil. What is special about these organisms is that they change their “shapes” in response to circumstantial conditions. When the food is exhausted, they become multicellular: a hundred thousand cells (in the case of Dictyostelium discoideum) gather and construct a motile slug, and finally culminate into a tiny fungus-like fruiting body composed of a spore ball atop and a supportive stalk. This process is an asexual development. When the environment is dark and submerged, the cells enter the sexual cycle; they maturate as gametes and fuse with appropriate mating partners to form dormant and highly resistant structures called macrocysts.

Although unique in life cycle as above, they share most of their developmental essence, such as morphogenetic movement, cell adhesion, and differentiation, with other complex organisms. The simplicity of dictyostelids is of great advantage for analytical studies, and the representative species, D. discoideum, has contributed greatly to reveal the molecular mechanisms underlying those common features of development. This is why D. discoideum is called a model organism for development (and for other basic biology as well). Even though molecular biological techniques have been so advanced that the difficulties in analyzing complicated higher organisms have been considerably reduced, the value of the model system should remain intact. In fact, the new techniques seem to have enhanced the D. discoideum studies much farther than those of other organisms. At the same time, however, it should be noted that their simplicity alone is important beyond convenience. Namely, they can be regarded as prototypes of multicellularity, so that we may ask why and how multicellular systems have been established during evolution.

In this issue, reviews on the recent achievements on the developmental mechanisms of the social amoeba are gathered, being arranged roughly in accordance with fundamental steps of development as cAMP signaling, cell-cycle checkpoint, quorum sensing, intracellular signaling for chemotaxis, cell–cell and cell–substrate adhesion, and control of cell differentiation. Readers will also find the reviews of studies from emerging concepts or new approaches on topics such as oscillation, stochasticity of cell-fate determination, transcriptional changes both at genome-wide and single-mRNA scales, dedifferentiation, and social behavior. Finally, recent studies in the sexual process are dealt with in two reviews.

I hope this collection of reviews on the development of the social amoeba will help us share the knowledge on the common mechanisms of development and consider the origin of multicellularity.