CHAPTER IX 

 THE REJUVENATION OF VIRUSES 



1. The Transmission of Viruses by Insects 



However strong the virus structure may be, it will gradually be 

 assimilized by the host protoplasm if the virus continues to multiply 

 in the same kind of the host. Since the protoplasm in which the 

 virus multiplies is also assimilase similar to the virus, the protoplasm 

 may always be striving to assimilize the virus though it is weaker 

 than the virus in the assimilase action as to be assimilized, in the 

 main, by the virus. In addition, as the protoplasm structure involves 

 the reversibility, the protoplasm itself may be striving to recover its 

 original structure even when it has been changed into a virus structure; 

 that is to say, there must be a potential force in a virus to develop 

 by itself the original structure of the protoplasm. It will be expected 

 therefore that viruses themselves will be assimilized in the long run 

 by the protoplasm of the host cells if they always adhere to the same 

 kind of host. 



In order to avoid this fate, viruses have to change frequently 

 their host. Insects appear to be commonly utilized by viruses for 

 this purpose. Various animal and plant viruses are transmitted by 

 insects in which they can multiply, thus host change becoming possi- 

 ble. As for some plant viruses and their insect vectors, however, 

 many ca"ses are known in which virus multiplication in insects can 

 never be considered, although the viruses are undoubtedly transmitted 

 by insects. In such cases, insects can infect healthy plants immedi- 

 ately after they have acquired viruses, and they soon cease to be able 

 to do so, sometimes becoming non-infective within minutes and always 

 within hours after leaving the infected plants. 



In view of these facts it may be said that the relationship between 

 plant viruses and their insect vectors may be revealed in the following 

 two ways : In the first, the insect acquires infectivity after a cer- 

 tain incubation period and the infectivity continues for long periods, 

 frequently for the remainder of its life, as already described ; in the 

 second, the insect becomes infective immediately after leaving a 

 diseased plant and the infectivity is rapidly lost. 



Many workers may consider that in the latter case the insects 

 act simply mechanically, transferring the virus as a contaminant on 



