SCHLICHTING, Carl D.: On the Origins of Differentiation: Development as a Reaction Norm

How cell types in multicellular organisms came to be differentiated is still an open issue. Here I offer a model that examines early events in the growth of a multicellular organism from a reaction norm perspective. I suggest that the origins of some cell differentiation patterns were originally passive outcomes of environmental effects. As interior cells’ contact with the external environment was diminished, their patterns of gene expression would be altered, due to changes in concentrations of externally supplied substances. Later, as multicellular growth continued, the relationships of cell layers to each other would also shift, producing concentration gradients of signalling molecules. These gradients would emanate both from the external cell layer towards the inside, and from internal cell layers to adjacent layers. Differentiation then arises initially as a byproduct of the changing patterns of gene expression, and the increasingly complex mixtures and changing concentrations of substances passing among layers. Subsequent evolutionary change will operate to stabilize the expression patterns in those cell layers whose phenotypes provide a fitness advantage to the organism.