Gene Activity Patterns and Cellular Differentiation

Puffing in giant chromosomes ot Diptera is considered to reflectthe pattern of active gene loci in these chromosomes. In anyone tissue only a relatively small portion of the total bands(about 10 to 20%) have been observed to form a puff at sometime or another in larval development. These patterns of "potentiallyactive" loci are tissue specific, though greatly overlapping.The actual rate of activity at these loci is controlled independentlyof each other and independently in each tissue by factors ofthe extranuclear metabolism. Puffing at some loci seems to berelated to specific cellular functions, such as secretion ofthe salivary glands. The activity of others may be related tomore basic metabolic processes. In relation to larval development,puffing patterns may change with changing cell functions orwith developmental processes in the cells themselves. In salivaryglands ofChironomus activity of DNAase and of acid phosphataseseems to change in relation to cell breakdown at the end ofthe pupal molt. Changes of acid phosphatase activity begin earlyin the last larval ins tar, but the enzyme is bound to lysosomesuntil metamorphosis. This suggests that the genes specificallyactive during metamorphosis have to interact with a longtermcontrol-system of development. The induction of metamorphosisis a sequential process, gene activations being among the firststeps in this sequence. The activation of these genes by ecdysoneis independent of protein synthesis. It is only the reactionof these genes that leads to the subsequent events in the cell,including the subsequent puff activations. This is shown bythe fact that they depend on early RNA synthesis as well ason protein synthesis. These results on puffing are discussedwith regard to the general problem of the relationships betweenpatterns of gene activity and differentiation.