106 The Nature of Biological Diversity 



Further studies show that the cytochemical properties of nucleo- 

 histone in proliferating tissues differ from those found in physiolog- 

 ically active cells. Both the stainahle phosphate groups of DNA and 

 the stainahle basic groups of histone decrease in nonreplicating cells, 

 suggesting that these groups are masked by residual protein, which is 

 known to increase in physiologically active cells (Alfert, 1958). In 

 addition to pointing out the changing chemical composition of 

 chromosomes during development, these and other studies also em- 

 phasize that chromosome replication is more than just DNA replica- 

 tion. The minimum replicating unit is nucleohistone (Bloch and 

 Godman, 1955 ) , but other constituents of chromosomes are also 

 duplicated during mitosis, though less completely and less consistently. 

 Presumably the variable constituents of chromosomes are the leading 

 candidates for the role of differential gene activators or inhibitors, 

 and thus we must look to protein or RNA. These constituents can be 

 shown to vary in different cells at various stages in differentiation, but 

 at this gross level of observation no correlation with the function of 

 specific genes is possible. Among animals only the chromosomes of 

 Drosophila are well enough known to offer hope of identifying chem- 

 ical changes at known genetic loci. In addition to the characteristic 

 banded pattern of the giant chromosomes of Diptera, it has been 

 known for many years that these chromosomes also exhibit localized 

 areas of enlargement, or puffs (Beermann, 1959) . These puffs (Fig. 5) 

 are distributed along the chromosomes in distinctive patterns that 

 are characteristic for the cell type and for the stage of its differentia- 

 tion (Becker, 1959). They are commonly thought to indicate regions 

 of heightened gene activity, but at any rate they are a clear indication 

 of chromosome differentiation and may be related to differential 

 gene function. Kroeger (1960) in a recent study sought to alter the 

 puffing pattern experimentally. He simply placed nuclei from Dro- 

 sophila salivary gland cells into cytoplasm taken from young embryos 

 of two different ages. After incubation in this younger, less differen- 

 tiated cytoplasm, the chromosomes of the nuclei were examined for 

 their puffing pattern. His results (Fig. 6) may be summed up suc- 

 cinctly by stating that the puffing patterns changed and were charac- 

 teristic for each cytoplasm. Some puffs regressed and new ones 

 appeared. Thus the chemical environment of the chromosome elicits 

 changes in chromosome morphology. The effective substances in 

 Drosophila embryo cytoplasm are still unknown, but in another or- 

 ganism — the dipteran, Chironomus tentans — a specific substance has 

 been shown to change chromosome puffing patterns. Clever and Karl- 

 son (1960) injected ecdyson— a hormone obtained from the pro- 



