108 



The Nature of Biological Diversity 



thoracic glands of insects — into the last instar larva of a chironomid. 

 The hormone elicited one new puff and induced the disappearance of 

 an old puff on one of the chromosomes. These changes normally oc- 

 cur when the insect metamorphoses. Thus the hormone ecdyson, 

 which stimulated metamorphosis, possihly achieves its effects, at 

 least in part, hy inducing chromosome differentiation which may 

 manifest changes in gene function. 



normal development 



transplanted 



prepupanum 



postpuparium 



preblastoderm 



blastoderm 



X 



FIG. 6. Comparison of regions of salivary-gland chromosomes of Drosophila 

 busckii at two stages in development and after incubation for 3 hours in cytoplasm 

 from preblastoderm and blastoderm embryos. Note the changes in puff develop- 

 ment in regions 3 and 6. (Figure from Kroeger. 1960.) 



We saw previously, during the discussion of tyrosinase synthesis, 

 that the response of a gene to activating stimuli is highly specific for 

 the state of cell differentiation in mice, and prohahly in all organisms. 

 Apparently suhstances from the cytoplasm enter the nucleus and 

 induce changes in the structure and function of receptive chromo- 

 somes, as shown hy the work on dipteran chromosomes. Except for 

 ecdyson, the nature of these substances is completely unknown, but 

 two leading candidates are proteins and ribonucleic acid or some com- 

 bination between them. 



