A General Mechanism of Differentiation Based on Morphogenetic Studies in Ciliates

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Abstract
Ciliate phenotype is shown to be under macronuclear control. Evidence is adduced indicating the presence of many subnuclei within the macronucleus, each subnucleus possessing a complete genome. In certain spp. different subnuclei undergo divergent fates: genic function is retained in some, but reduced or lost in others. This occurs in a predictable pattern, is probably due to differential effects of the cytosome. The role of the ectoplasmic kinetosomes in morphogenesis is reviewed. A variety of data show that the characteristic differentiation pattern of a ciliate is maintained by morphostatic barriers and metabolic limiting conditions which canalize different kinetosomes into an ordered hierarchy of functions. Morphogenesis is equivalent to reestablishment of this hierarchy following its partial destruction. A mechanism is outlined by which this may be achieved. Evidence is presented indicating that diversification of the subnuclei can be brought about by the regionally correlated diversification of differentiated kinetosomes. The implications of the data are discussed in the light of modern hypotheses bearing on problems of differentiation. The concept presented is shown to have tentative applicability both to protozoa and to multicellular organisms.