A Comparative Analysis on Introduction to Multicellular Organisms With Active Differentiation and Spatial Pattern |

ABSTRACT:

The processes by which multicellular organisms first emergedfrom their unicellular ancestors are fundamental to the biology of complex,differentiated life forms. Previous work suggests that reproductive division oflabor between specialized germ and soma cells was central to this evolution insome cases. Here, we assess the potential of the digital life platform Avida toexamine the trade-off between survival and replication in multicellularorganisms. Avida uses a grid of selfreplicating computer programs capable ofmutation and evolution to address biological questions computationally. Wemodel our digital organisms after the Volvocales, a flagellated order ofphotosynthetic green algae that includes both unicellular and multicellularspecies. We show that, given selective pressures similar to those experiencedby the Volvocales in nature, digital organisms are capable of evolvingmulticellularity within the Avida platform. The strategies we observed thatbest handled the trade-off between survival and replication involved germ cellsproducing sterile, somatic offspring. These strategies are similar to those observed involvocine algae, which suggests that digital platforms, such as Avida, areappropriate to use in the study of reproductive altruism. The origin ofmulticellular organisms and the mechanism of development in cell societies arestudied by choosing a model with intracellular biochemical dynamics allowingfor oscillations, cell–cell interaction through diffusive chemicals on atwo-dimensional grid, and state-dependent cell adhesion. Cells differentiatedue to a dynamical instability, as described by our “isologous diversi.cation”theory. A .xed spatial pattern of differentiated cells emerges, where spatialinformation is sustained by cell–cell interactions. This pattern is robustagainst perturbations. With an adequate cell adhesion force, active cells arereleased that form the seed of a new generation of multicellular organisms,accompanied by death of the original multicellular unit as a halting state. Itis shown that the emergence of multicellular organisms with differentiation,regulation, and life cycle is not an accidental event, but a naturalconsequence in a system of replicating cells with growth. Development is the powerful process involving a genome inthe transformation from one egg cell to a multicellular organism with many celltypes. The dividing cells manage to organize and assign themselves special,differentiated roles in a reliable manner, creating a spatio-temporal patternand division of labor. This despite the fact that little positional informationmay be available to them initially to guide this patterning.