Discussion 271 



the ploidy. This has been found in all types of cells which have been 

 investigated so far. However, in spite of the fact that several full 

 papers have been written by others on this subject, a major difficulty 

 remains which Dr. B. Ephrussi and I encountered in our original work. 

 When ploidy increases, not only does the number of hits increase 

 accordingly, but also the radioresistance of the individual unit which 

 undergoes the hit. The slope of the straight part of the survival curve 

 decreases. 



Let us consider the simplest situation, that, for example, of several 

 virus particles inside the same cell, which can multiply within the 

 irradiated cell as long as they are themselves active, and let us suppose 

 that the cell remains infective as long as it contains one active particle. 

 It has been found that the survival curve of the " infectivity " of such 

 multiply infected cells, early after infection, i.e. before the virus has 

 begun to multiply, fits in with the multiplicity of infection. The curve 

 agrees with the general equation : 



survival = 1 — (1 — e - aD)ri 



where n is the number of infecting particles, d the dose, and a the coeffic- 

 ient which characterizes the radiosensitivity of the individual particle. 

 When n increases, a does not change, all curves have parallel straight 

 parts. 



In the case of ploidy, however, a decreases when n increases. At the 

 same time, the morphology of the lesions may differ. In haploid yeast, 

 one observes only immediate death and double giant lethals. In 

 polyploid yeast, one observes in addition the many classical figures of 

 delayed recovery. I think that in the latter, some kind of multiplicity 

 reactivation takes place, but I really do not know what this means for 

 chromosomes. As a matter of fact, no simple hypothesis has given a 

 satisfactory account for the experimental results. 



Gray: With regard to Norman and At wood's results, I think these are 

 exactly the results to be expected because in a cell containing two 

 nuclei with nutritional deficiency you have two targets, the inactivation 

 of either of which is lethal if you plate on minimal medium. If you plate 

 on complete medium you must inactivate each of two targets. The 

 slope of the former inactivation curve being twice that of the latter is 

 thus in accordance with expectation. In fact, the slopes differ by a little 

 less than a factor of two, because there were recessive lethals to be taken 

 into consideration. I had not spotted this in the yeast. 



Swanson: In Tradescantia one can irradiate cells which are either 

 haploid or diploid, and the chromosome sensitivity is different on a 

 chromosome basis. Yet these presumably are the same types of chromo- 

 somes. We have done this fairly extensively on diploid and tetraploid 

 individuals of the same species of Tradescantia, but we found a con- 

 siderable difference in sensitivity in microspore chromosomes. 



Butler: Dr. Gray mentioned the effect of pressure; I don't think that 

 really works. The thermodynamic theory is based on equilibrium con- 

 siderations, there is an activated state through which the material is 

 passing. With ionizing particles you have got something equivalent 



