EVOLUTION AND VIRUSES 



clone continues in existence. Thus the future of a virus in such 

 a vegetatively propagated plant is secure, without need to be 

 transmitted to more plants. 



The position is very different with plants raised annually 

 from true seeds. Few viruses are seed-transmitted, and so 

 most seedlings start life virus-free even though their parents 

 were infected. Hence, to be common in annual plants, viruses 

 must have an efficient method of transmission, and selection 

 operates continually in favour of forms that are readily trans- 

 mitted. Similarly, a potato plant newly infected is likely to 

 have been infected by a readily transmissible form. But there 

 is no continuous selection inside this plant favouring the 

 survival of aphid-transmitted forms. Should other variants 

 arise that are better suited than the initial strain to survive, 

 perhaps better able to invade and permeate the tubers, then 

 these will predominate and the original form will disappear. 

 We can thus picture the old potato varieties, initially infected 

 by aphid-transmitted strains of virus Y, now containing dif- 

 ferent strains, strains that have lost this ability, which has no 

 survival value in vegetatively propagated plants, and strains 

 that would have originated only to disappear on the death of 

 their host plant had the plant not been vegetatively propagated. 



If, then, accepted viruses can lose and regain transmissibility 

 it seems reasonable to think that "normal" nucleoproteins 

 could also sometimes undergo analogous changes and achieve 

 one character of a virus by becoming transmissible. However, 

 it must be stressed that there is no substantial evidence of 

 viruses arising endogenously from normal cell components. 

 On occasions there has seemed to be some evidence, but when 

 critically examined it has proved valueless. One example, often 

 featured in arguments for the endogenous origins of viruses, 

 is potato paracrinkle virus, which is present in every plant of 

 the potato variety King Edward, one of the most widely grown 

 varieties in the United Kingdom. It does this variety little or 

 no harm, but cripples many others. However, it never occurs 

 in these other varieties naturally, and its effects on them are 

 known only from experimental transmissions. As paracrinkle 

 virus seemed to have no natural method of spread, how could 

 it have infected the first King Edward plant and so permeated 

 the whole variety? For a time, it was plausible to think that 



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