74 Discussion 



that those transmissible differences, that you find as distinguishing the 

 two clones that you finally isolate, occur at the time that you separate 

 these two cell? Could there not have been occasional large differences 

 in the norm of reaction arising by larger changes during the growth of 

 the large clones; which would mean that you might have physiological 

 individuality occurring in every cell division, and genetical changes 

 occurring perhaps less frequently but with high probability at some time 

 in the growth of the clone (although considering the very small magni- 

 tude of the physiological change which is involved here, I would be 

 willing to accept their occurrence during cell division)? 



Hughes: In certain stated cases with some particular cells, the daughter 

 cells differ from each other sufficiently for one to be a lethal and the other 

 to survive. That happens with sufficient frequency, and could be bred 

 for. My view is that if you can show that a single individual cell is 

 different from its twin sister, then mutation must be a very frequent 

 thing since it is picked up in my populations of from 100-300 cells. 



Lederberg: A difference between sister cells is not necessarily a trans- 

 missible difference. Although such transmissible differences may arise, 

 they may be superimposed upon the much more frequent physiological 

 fluctuation — one cell gets more cell wall than the other. We know that in 

 cell division the two cells are not necessarily exactly the same size. You 

 don't know that every step of selection involves a genotypic difference; 

 only an occasional one need have done so. 



Hughes : In the case of sensitivity to the environment whereby the cell 

 becomes long in form, there is only one division — that of the parent cell. 

 In that case you subculture the actual cell which varied, and not a clone. 



Demerec: I agree with Dr. Hughes that practically no two bacterial 

 cells are the same as far as their genetic constitution is concerned. In 

 our laboratory, in keeping stocks of different mutants we try to avoid 

 passing the stock through a single-colony transfer, because we are afraid 

 that by so doing we might change the other properties of the bacteria. 

 Each bacterial cell has a large number of genes, any one of which may be 

 able to mutate; some cells mutate with higher and some with lower 

 frequency. If we had a test sensitive enough to pick up all the gene 

 mutations that occur, we should get a mutation rate very much higher 

 than that obtained when we observe mutations of a single gene. 



Cavalli-Sforza: I am rather worried that Dr. Hughes found such a 

 great deal of variation in using the micromanipulator to select single 

 cells. When you use the much simpler method of isolation by plating 

 you don't find such a tremendous variation. I wonder. Dr. Hughes, if 

 you can give an explanation for that. Do you think that one would 

 perhaps get the same amount of variation from cell to cell if isolation 

 were made in the standard way ? Also, w hat is the accuracy of measure- 

 ment that can be obtained with your method ? You grow your colonies 

 directly in an oil-chamber, so there may be considerable variation in 

 local conditions that may affect the estimation of growth. Also, can you 

 say exactly how many cells die in your selection experiments with 

 penicillin? Do the majority die? 



Hughes: It depends on what level you choose; you can choose a 20 per 



