EVOLUTION AND VIRUSES 



one may be that some are so like "normal" cell components 

 that they cannot be readily separated from them. 



Changes in Transmissibility 



Few viruses have developed mechanisms comparable to 

 those that enable bacteriophages to move unaided from one 

 organism to another. Although some, such as those causing 

 the common cold and influenza, are air-borne and spread from 

 infected to uninfected people without the help of another 

 organism, most viruses of higher animals and plants depend 

 for their spread on insects or other kinds of arthropods. Active 

 insects that feed on many different individuals during their 

 lives are obviously excellent vehicles for moving entities from 

 one individual to another. Indeed, some viruses, such as tobacco 

 mosaic that now seem not to be insect-transmitted, may have 

 been so initially, but later developed other methods of spread 

 that have largely superseded transmission by insects. What 

 determines the ability of a given virus to be transmitted by a 

 given species of arthropod is not known, but it is clear that 

 no enormous changes in virus particles may be needed to 

 render them transmissible. There have been several examples 

 of viruses gaining or losing the ability to be transmitted by 

 a given species of insect while they have been studied experi- 

 mentally, and while retaining many of their other properties. 

 There is no reason to think that these changes ("mutations") 

 happen only during experimental work. On the contrary, 

 there is strong presumptive evidence that it has happened to 

 several viruses before anyone started to experiment with them. 



Potato virus Y, one of the most important potato viruses, 

 occurs in a range of forms distinguishable by the type and 

 severity of symptoms they cause in different potato varieties 

 and other plants such as tobacco. The strains common in 

 tobacco plants and in newly infected potato plants are usually 

 all readily transmitted by Myzus persicae and some other 

 aphids, but those in some old potato varieties are not. As 

 strains of virus Y are among those that have changed their 

 transmissibility during experiments, the difference can be 

 readily explained. A potato plant infected with virus Y sets 

 infected tubers which will in turn grow into infected plants, 

 and so on, indefinitely perpetuating the virus as long as the 



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