THE GENETICS OF VIRUSES 



commonly the activity of the virus is starved by its own success, 

 and equihbrium can often be restored to produce the kind of situation 

 we have in the broken tuhps. Plants can also be cured of viruses 

 by growing away from them under conditions favouring the host 

 at the expense of the parasite, just as the tissue of Scolopendrium or 

 the stock of Paramecium gets rid of the plasmagene it has carried. 

 In the extreme case, water at 45° C. will kill a Vinca virus in a few 

 hours without killing the plant, just as kappa can be killed in 

 Paramecium. 



The relations of viruses with one another, relations which are best 

 seen in plants, are not without genetic interest. Many pairs of viruses 

 have no effect on one another in the same host. They are neutral 

 or independent. A second type of reaction is co-operation. For example, 

 K. M. Smith has shown that Rosette Disease of tobacco is due to 

 two viruses. These can be separated by inoculation, and also by the 

 insect vector, which carries one of them only in the presence of the 

 other; but they act together like two complementary genes in 

 producing their effect, and in the self-multiplication on which this 

 effect depends. 



A third type of reaction is antagonism. This is commonly shown 

 more strongly between related or mutant strains of the same virus. 

 Thus an infection with a less virulent strain may enable a plant, 

 which does not develop anti-bodies, to repel an infection by a more 

 virulent strain. Such a situation is obviously due to the two strains 

 needing the same food— or, in terms of gene physiology, to their 

 having the same precursors. And finally, as Bawden and Kassanis 

 found. Severe Etch actively replaces Potato virus Y and Hyoscyamus 

 virus 3 in plants in which these viruses are already estabhshed. The 

 one virus, as it were, digests the other. Here we see an analogy with 

 suppressiveness as between plasmagenes, and also with the synthetic 

 sequences in Neurospora. 



The Genetics of Viruses 



Two aspects of the behaviour of viruses are of genetic importance. 

 The first is that of the virus itself Viruses that have been most 

 studied have been found, like all other pests and parasites, to exist 

 in strains of varying virulence just as their hosts are of var\'ing 



liUmeitl.s o/Geiictiti 200 O 



