[Self-organization in the ontogeny of multicellular organisms: a computer simulation]

The progress in understanding the patterns of evolution of ontogeny is hindered by the fact that many features of ontogeny are counterintuitive (as well as the features of other processes related to self-organization, self-assembly, and spontaneous increase in complexity). The basic principle of ontogeny of multicellular organisms is that it is the process of self-assembly of ordered multicellular structures by means of coordinated behavior of many individual modules (cells), each of which follows the same set of"rules" encoded in the genome. These rules are based on the genetic regulatory networks. We hypothesize that many specific features of ontogeny that seem nontrivial or enigmatic are, in fact, the inevitable consequences of this basic principle. If so, they do not need special explanations. In order to verify this hypothesis, we developed the computer program "Evo-Devo" based on the above principle. The program is designed to model the self-assembly of ordered multicellular structures from an aggregation of dividing cells that originate from a single original cell (zygote). Each cell follows a set of rules of behavior ("genotype") that can be specified arbitrarily by the experimenter, and is the same for all cells in the embryo (each cell is programmed in exactly the same way as all other cells). It is not allowed to specify rules for groups of cells or for the whole embryo: only local rules that should be followed at the level of a single cell are possible. The analysis of phenotypic implementation of different genotypes revealed several features which are present in the ontogeny of real organisms and are regularly reproduced in the model. These include: inherent stochasticity; inescapable necessity of development of stabilizing adaptations based on negative feedback in order to decrease this stochasticity; equifinality (noise resistance) resulting from these adaptations; the ability of ontogeny to respond to major perturbations by generating new morphological structures that differ from the "normal" ones, but have similar level of complexity; the similarity of phenotypic manifestations of different mutations; channeling of possible evolutionary transformations of ontogeny; Waddington's creodes; high probability of destabilization of ontogeny (e.g., because of mutations); the possibility of a new morphological character to appear initially as a rare anomaly (low penetrance of many mutations); pleiotropy of mutations affecting ontogeny; spontaneous emergence of morphogenetic correlations; integrity of the developing organism. The fact that these features are regularly reproduced in the model implies that they are probably the inevitable consequences of the basic principle of ontogeny of multicellular organisms formulated above.