EVOLUTION AND VIRUSES 



it might be a "normal" component of this variety, a metabolic 

 product of its intrinsic metabolism that becomes a virus in 

 other plants. 



However, recent work suggests another explanation, again 

 based on the idea that viruses can lose and gain transmissibility 

 and that they tend to lose it when it is not continually selected 

 for. Although paracrinkle virus has not been found elsewhere 

 or to have any insect that transmits it, other viruses have been 

 found that resemble it very closely. One of these is common 

 in carnation plants; this shares antigens with paracrinkle virus, 

 indicating close relationship for it reflects common structures 

 in the proteins of the two and is a feature of related virus 

 strains, but the carnation virus is readily transmitted by aphids. 

 Another virus serologically related to both paracrinkle and 

 the carnation virus has been found in some other potato 

 varieties, but this one again is not aphid-transmitted. Hence, 

 it seems reasonable to assume one aphid-transmitted ancestor 

 for all three, but that only the carnation one now retains this 

 property, which has been lost during the years the others have 

 had their survival assured simply by the vegetative propaga- 

 tion of their hosts, without any selection for aphid-trans- 

 missibility. The other potato virus, like paracrinkle in King 

 Edward, causes no obvious symptoms in the varieties all stocks 

 of which are infected; this was an essential requirement, 

 because had it produced symptoms in a crop like the potato, a 

 virus unable to spread from plant to plant would long ago have 

 been eliminated by growers destroying obviously diseased 

 plants. 



Lysogenic bacteria also seemed for a time to offer evidence 

 for the endogenous origin of viruses in cells. Such bacteria 

 mostly grow normally, but a proportion produce bacterio- 

 phages and lyse. By appropriate stimuli, such as exposure to 

 ultraviolet radiation, they all will. That lysogenic bacteria 

 carry the potentialities of producing viruses is evident enough, 

 but is now seems probable that their ancestors contracted 

 these potentialities as a result of infection rather than that the 

 potentialities were originally intrinsic to their constitution. 

 Bacterial viruses can have two phases to their existence. First, 

 there is the long-known one whereby they appear as extra- 

 cellular units, the typical bacteriophages that dissolve their 



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