V NUCLEIC ACIDS EMBRYONIC DEVELOPMENT 293 



The problem of DNA synthesis in lethal hybrids is obviously of even greater 

 interest; but it is a difficult one, because of the already mentioned difficulties in 

 estimating accurately the DNA content during early development. 



DNA synthesis in Ranafiisca has been studied in eggs fertilized with sperm pre- 

 viously treated with nitrogen mustard; they display exactly the same type of 

 lethality as the hybrids (Brachet, ig54b). The arrested blastulae have a 34% 

 lower DNA content than the control; afterwards, although development is entirely 

 blocked, a measurable DNA synthesis (24% increase) occurs during 2 days. In the 

 normal controls, which reach the early neurula stage, DNA synthesis is more 

 pronounced (33%). Similar results have been obtained by Shen (1954) on his 

 Triton X Salamandra hybrids: DNA synthesis in the lethal hybrid is definitely re- 

 duced, but not entirely stopped. 



All these data come from experiments where DNA was estimated with colori- 

 metric methods, which are unspecific and not very sensitive; but it is interesting 

 to note that the same conclusions have recently been reached by Gregg and Lov- 

 trup (1955), who used the microbiological method which enabled Hoff-Jorgensen 

 (1954) to study DNA synthesis during normal development of frog eggs. Their 

 experiments show that the DNA content of normal Rana pipiens embryos remains 

 apparently constant until the end of segmentation, at which point synthesis occurs 

 at a constant rate. In the lethal Rana pipiens <^ X Rana sylvatica 3 hybrids, synthesis 

 of DNA is practically normal, even though development has stopped in the gas- 

 trula stage; the DNA content of the hybrids ceases to increase further when it 

 has reached the value characteristic for controls at the neural fold stage. 



It is clear, from the experimental data, that one simple hypothesis of the mech- 

 anism of lethality has been disproved: if the eggs contain, as Hoff-Jorgensen 

 (1954) believes, a store of DNA or DNA precursors, one might have expected the 

 lethal embryos to stop development when their DNA reserves are exhausted. This 

 is obviously not the case, since DNA synthesis continues in the blocked embryos 

 of all the lethal combinations so far studied. 



These experiments further show that, while RNA synthesis and morphogenesis are 

 closely linked together, there is a dissociation between DNA synthesis and development : DNA 

 synthesis still proceeds when development has stopped. Other instances of dissocia- 

 tion between DNA synthesis and growth, obtained on very different material, will 

 now be examined. 



VI. FINAL REMARKS 



[d] Independence of DNA synthesis and protein synthesis 



We already know that RNA is directly involved in the formation of proteins 

 and that, in microorganisms at least, synthesis of new RNA molecules is a compul- 

 sory concomitant of induced enzyme synthesis (Pardee, 1954, 1955; Spiegelman 

 et al., 1955; Greaser, 1955a, 1955b; Ghantrenne, 1956). It is clear that the same 

 questions may be asked for DNA as for RNA: is synthesis of DNA necessary for 

 protein synthesis? Does DNA play a direct role in protein synthesis? 



The answer to these questions comes entirely from biochemical studies on 

 microorganisms, except for the already mentioned fact that, in blocked lethal 



Lileralure p. 2gg 



