The Evolution of Differentiation by William S. Bullough (Auth.)

By William S. Bullough (Auth.)

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In the closely related soil amoeba Naegleria the individuals remain solitary in the presence of adequate bacterial food, become flagellate when excess water is in the soil, and aggregate within a cyst in adverse conditions (Fig. 7 ; and see F I G . 7. The various forms of the soil amoeba Naegleria gruberi. ) Willmer, 1963). These various phases of activity, or differentiation, are evidently controlled by trigger mechanisms that are activated by environmental conditions, and it may be presumed that they are mediated through changed patterns of gene activity.

Certainly it is probable t h a t the message to commence replication must originate in the "mitosis operon" and pass to the centrioles, perhaps in the form of an effector molecule. Thus part of the process of differentiation for mitosis may involve the activation of certain plasmagenes. KINETOSOMES I n the protozoans there is clear evidence of a close relationship between centrioles and the kinetosomes (basal bodies or blepharoplasts) t h a t underlie flagella and cilia. Structurally these two organelles are closely similar (see Grimstone, 1961), and in the flagellates, but not the 48 THE EVOLUTION OF DIFFERENTIATION ciliates, they are closely connected.

1958) and of the ciliate Tetrahymena (Seaman, 1960,1962). However, doubts that the evidence for the existence of kinetogenes is indeed adequate have been expressed by Kimball (1964) and Pitelka and Child (1964). Certainly the evidence is inadequate to support any conclusion that the phases of differentiation shown by the protozoans may depend on the periodic activation of kinetogenes, although it can be suggested, for instance, that the growth of flagella in wet conditions by the soil amoeba Naegleria (Willmer, 1963) may involve activation of this type.

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