A life of solitude and society
In the damp leaf litter and soil of Massachusetts forests lives a remarkable organism, the social amoeba Dictyostelium discoideum. For most of its life, it exists as a solitary, single-celled amoeba, moving through the soil and consuming bacteria. These individual cells divide and multiply as long as their bacterial food source is plentiful. When the food runs out, however, a dramatic transformation begins. Starvation triggers a chemical distress signal, and the amoebas start to secrete pulses of cyclic AMP (cAMP). This signal draws the scattered cells together, and they stream toward a central point to form a multicellular aggregate.
This new collective, known as a pseudoplasmodium or "slug," can be composed of up to 100,000 individual cells. The resulting slug is a motile organism, typically 2-4 millimeters long, that crawls as a single unit toward light and heat, cues that guide it to the soil surface for optimal spore dispersal. The slug moves by producing a cellulose sheath, essentially laying down its own track through which the collective of cells glides. During this mobile phase, which can last for several days, the cells within the slug begin to differentiate. Roughly the front third of the slug is composed of prestalk cells, while the rear two-thirds are prespore cells.
An altruistic sacrifice
The slug's migration ends in a final, cooperative act of self-sacrifice. When it reaches a suitable location, the slug stops migrating and begins a process called culmination, which takes about eight to ten hours to complete. It reorients itself vertically, and the prestalk cells at the front begin to form a rigid stalk made of cellulose. These cells, making up about 20% of the total population, die in the process, forming a sturdy, dead structure that lifts the remaining cells high into the air.
The elevated cells are the prespore cells, which now mature into hardened, resilient spores. They form a ball, called a sorus, at the very top of the stalk, which can be a few millimeters tall. This elevation is critical for dispersal, allowing wind or passing creatures to carry the spores to new, potentially food-rich environments. When conditions are favorable, each spore can hatch into a new, solitary amoeba, starting the life cycle all over again. This division of labor, where a fifth of the population perishes to ensure the survival and dispersal of the other 80%, is an example of altruism at the cellular level.