Computing an organism

The “social amoebae” of species Dictyostelium discoideum roam individually through the soil as long as their bacterial food is present. The social phase for these cellular slime molds begins when the food supply is exhausted. After some hours, the assemblage of amoebae aggregate into several large groups, each of which forms a worm-like slug that propels itself toward heat and light. This brings the slug to the surface of the soil, where it executes a sophisticated internal ballet that eventually results in a fruiting body, an elegant stalk, formed by dead cellulose-walled cells, atop of which perches a bag of spores. The whole structure is about a millimeter high. Now in this issue of PNAS Marée and Hogeweg (1) have provided a computer simulation of the frog-prince transformation of slug into fruiting body. Scientists have been intensively studying D. discoideum for decades, as a model system in developmental biology. (Dictyostelium turned up 47,500 entries in a Google search.) A tremendous boost to these studies occurred when it was discovered that the initial aggregation is induced by the pulsatile secretion of a chemoattractant that turned out to be none other than cAMP, one of a few major second messengers in mammalian physiology (2). Dictyostelium has become a “hydrogen atom” paradigm in development, for instead of hundreds of cell types as in humans, the slime mold has only two (principal) types, stalk and spore, in a ratio that is controlled over a wide range of sizes. What causes aggregation and then slug formation and motion? How are the proportions of differentiated cells controlled? And how is the morphogenetic movement organized so that it provides the appropriate geometric structure of spore-on-stalk? Marée and Hogeweg have provided a computer simulation of the frog-prince transformation of slug into fruiting …