190 III. THE EVOLUTION OF VIRUSES 



the outsides of their stylets. Nevertheless. Bawden (39) claims that 

 there is much evidences showing that the vectors do not behave 

 merely mechanically. Thus, if insects acted solely by getting their 

 mouthparts contaminated externally with virus, it is difficult to see 

 why all those with similar feeding habits should not be equally effi- 

 cient vectors, except that those with larger stylets might perhaps be 

 expected to carry a little more virus. However, this is not so, and 

 insects with similar feeding habits Tvill transmit some viruses but not 

 others, and, with viruses that have a number of aphids as vectors, 

 one species is often much more efficient than the other. Bawden 

 states further that if transmission by insect were merely mechanical, 

 it might be expected that virus most readily transmitted by artificial 

 inoculation could also be most readily transmitted by insects. And, if 

 the viruses were merely acquired as a contaminant on the mouth 

 parts, those occurring in infected plants in the greatest concentration 

 might be expected to be most easily aquired and transmitted. But it 

 is not so. Tobacco mosaic virus and potato virus X are more easily 

 transmitted by rubbing or by needle inoculation, and occur in greater 

 concentration in extracts of infected plants, than any of the viruses 

 under; discussion. Neither of these normally appears to be aphid- 

 transmitted. 



Moreover, Bawden emphasizes the fact that insects can cause 

 infection with virus amounts much smaller than those needed by 

 ordinary inoculation method, and he describes further the remarkable 

 fact that the efficiency of insects as vectors is much increased if they 

 are prevented from feeding for some time before they are fed on the 

 source of the virus. 



2. The Mechanism of Rejuvenation 



There seems no doubt that plant viruses above mentioned which 

 are rapidly lost in the insect vectors fail to multiply in the insect 

 bodies. It seems, however, reasonable to consider that transitory 

 activation of the viruses may take place in the insect bodies. 



Prior to considering the mechanism of this virus activation, we 

 have to discuss the nature of protein denaturation. Proteins, in general, 

 when exposed to physical or chemical effects, may change their pro- 

 perties so as to become "denatured" through the alteration of their 

 structure. In this process the closely folded polypeptide chains in 

 protein molecules may unfold transitorily and subsequently again may 

 refold to give some other folding pattern different from the original 

 internal structure, as shown in Fig. 21 (102). 



