Indexed on: 26 Dec '15Published on: 26 Dec '15Published in: Quantitative Biology - Cell Behavior
The origin of multicellularity is a fundamental open question in biology. For multicellular organisms to evolve from an aggregate of unicellular organisms, cells with an identical genotype must first differentiate into several types. Second, this aggregate of distinct cell types should show better growth than that of an isolated cell in the environment. Third, this cell aggregate should show robustness in the number distribution of differentiated cell types. To reveal how an ensemble of primitive cells achieves these conditions, we developed a dynamical-systems model of cells consisting of chemical components with intracellular catalytic reaction dynamics. The reactions convert external nutrients to internal components for cellular growth, and the divided cells interact through chemical diffusion. We found that cells sharing an identical catalytic network spontaneously differentiate induced by cell-cell interactions, and then achieve cooperative division of labor, the mutual use of products among differentiated cell types, enabling a higher growth rate than that in the unicellular case. This symbiotic differentiation emerged for a class of reaction networks under the condition of nutrient limitation and strong cell-cell interactions. Then, robustness in the cell type distribution was achieved, while instability of collective growth sometimes emerged even among the cooperative cells when the internal reserves of chemical products is dominant. The simplicity and generality of the present mechanism suggests that evolution to multicellularity is a natural consequence of interacting cells with limited resources, being consistent with the behaviors and forms of several extant primitive forms of multicellularity, such as certain bacteria.